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Managing, monitoring, and updating the kernel
A guide to managing the Linux kernel on Red Hat Enterprise Linux 9
Abstract
Making open source more inclusive
Red Hat is committed to replacing problematic language in our code, documentation, and web properties. We are beginning with these four terms: master, slave, blacklist, and whitelist. Because of the enormity of this endeavor, these changes will be implemented gradually over several upcoming releases. For more details, see our CTO Chris Wright’s message.
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Chapter 1. The Linux kernel RPM
The following sections describe the Linux kernel RPM package provided and maintained by Red Hat.
1.1. What an RPM is
An RPM package is a file containing other files and their metadata (information about the files that are needed by the system).
Specifically, an RPM package consists of the cpio
archive.
The cpio
archive contains:
- Files
RPM header (package metadata)
The
rpm
package manager uses this metadata to determine dependencies, where to install files, and other information.
Types of RPM packages
There are two types of RPM packages. Both types share the file format and tooling, but have different contents and serve different purposes:
Source RPM (SRPM)
An SRPM contains source code and a SPEC file, which describes how to build the source code into a binary RPM. Optionally, the patches to source code are included as well.
Binary RPM
A binary RPM contains the binaries built from the sources and patches.
1.2. The Linux kernel RPM package overview
The kernel
RPM is a meta package that does not contain any files, but rather ensures that the following required sub-packages are properly installed:
-
kernel-core
- contains the binary image of the kernel, all initramfs-related objects to bootstrap the system, and a minimal number of kernel modules to ensure core functionality. This sub-package alone could be used in virtualized and cloud environments to provide a Red Hat Enterprise Linux 9 kernel with a quick boot time and a small disk size footprint. -
kernel-modules
- contains the remaining kernel modules that are not present inkernel-core
.
The small set of kernel
sub-packages above aims to provide a reduced maintenance surface to system administrators especially in virtualized and cloud environments.
Optional kernel packages are for example:
-
kernel-modules-extra
- contains kernel modules for rare hardware and modules which loading is disabled by default. -
kernel-debug
— contains a kernel with numerous debugging options enabled for kernel diagnosis, at the expense of reduced performance. -
kernel-tools
— contains tools for manipulating the Linux kernel and supporting documentation. -
kernel-devel
— contains the kernel headers and makefiles sufficient to build modules against thekernel
package. -
kernel-abi-stablelists
— contains information pertaining to the RHEL kernel ABI, including a list of kernel symbols that are needed by external Linux kernel modules and adnf
plug-in to aid enforcement. -
kernel-headers
— includes the C header files that specify the interface between the Linux kernel and user-space libraries and programs. The header files define structures and constants that are needed for building most standard programs.
Additional resources
1.3. Displaying contents of the kernel package
The following procedure describes how to view the contents of the kernel package and its sub-packages without installing them using the rpm
command.
Prerequisites
-
Obtained
kernel
,kernel-core
,kernel-modules
,kernel-modules-extra
RPM packages for your CPU architecture
Procedure
List modules for
kernel
:$ rpm -qlp <kernel_rpm>
(contains no files) …-
List modules for
kernel-core
:
$ rpm -qlp <kernel-core_rpm>
…
/lib/modules/5.14.0-1.el9.x86_64/kernel/fs/udf/udf.ko.xz
/lib/modules/5.14.0-1.el9.x86_64/kernel/fs/xfs
/lib/modules/5.14.0-1.el9.x86_64/kernel/fs/xfs/xfs.ko.xz
/lib/modules/5.14.0-1.el9.x86_64/kernel/kernel
/lib/modules/5.14.0-1.el9.x86_64/kernel/kernel/trace
/lib/modules/5.14.0-1.el9.x86_64/kernel/kernel/trace/ring_buffer_benchmark.ko.xz
/lib/modules/5.14.0-1.el9.x86_64/kernel/lib
/lib/modules/5.14.0-1.el9.x86_64/kernel/lib/cordic.ko.xz
…
-
List modules for
kernel-modules
:
$ rpm -qlp <kernel-modules_rpm>
…
/lib/modules/5.14.0-1.el9.x86_64/kernel/drivers/infiniband/hw/mlx4/mlx4_ib.ko.xz
/lib/modules/5.14.0-1.el9.x86_64/kernel/drivers/infiniband/hw/mlx5/mlx5_ib.ko.xz
/lib/modules/5.14.0-1.el9.x86_64/kernel/drivers/infiniband/hw/qedr/qedr.ko.xz
/lib/modules/5.14.0-1.el9.x86_64/kernel/drivers/infiniband/hw/usnic/usnic_verbs.ko.xz
/lib/modules/5.14.0-1.el9.x86_64/kernel/drivers/infiniband/hw/vmw_pvrdma/vmw_pvrdma.ko.xz
…
-
List modules for
kernel-modules-extra
:
$ rpm -qlp <kernel-modules-extra_rpm>
…
/lib/modules/5.14.0-1.el9.x86_64/extra/net/sched/sch_cbq.ko.xz
/lib/modules/5.14.0-1.el9.x86_64/extra/net/sched/sch_choke.ko.xz
/lib/modules/5.14.0-1.el9.x86_64/extra/net/sched/sch_drr.ko.xz
/lib/modules/5.14.0-1.el9.x86_64/extra/net/sched/sch_dsmark.ko.xz
/lib/modules/5.14.0-1.el9.x86_64/extra/net/sched/sch_gred.ko.xz
…
Additional resources
-
rpm(8)
manual page - RPM packages
Chapter 2. Updating kernel with dnf
The following sections bring information about the Linux kernel provided and maintained by Red Hat (Red Hat kernel), and how to keep the Red Hat kernel updated. As a consequence, the operating system will have all the latest bug fixes, performance enhancements, and patches ensuring compatibility with new hardware.
2.1. What is the kernel
The kernel is a core part of a Linux operating system, which manages the system resources, and provides interface between hardware and software applications. The Red Hat kernel is a custom-built kernel based on the upstream Linux mainline kernel that Red Hat engineers further develop and harden with a focus on stability and compatibility with the latest technologies and hardware.
Before Red Hat releases a new kernel version, the kernel needs to pass a set of rigorous quality assurance tests.
The Red Hat kernels are packaged in the RPM format so that they are easy to upgrade and verify by the dnf package manager.
Kernels that have not been compiled by Red Hat are not supported by Red Hat.
2.2. What is dnf
This section refers to description of the dnf
package manager.
Additional resources
-
dnf(8)
manual page
2.3. Updating the kernel
The following procedure describes how to update the kernel using the dnf package manager.
Procedure
To update the kernel, use the following:
# dnf update kernel
This command updates the kernel along with all dependencies to the latest available version.
- Reboot your system for the changes to take effect.
2.4. Installing the kernel
The following procedure describes how to install new kernels using the dnf package manager.
Procedure
To install a specific kernel version, use the following:
# dnf install kernel-{version}
Additional resources
Chapter 3. Managing kernel modules
The following sections explain what kernel modules are, how to display their information, and how to perform basic administrative tasks with kernel modules.
3.1. Introduction to kernel modules
The Red Hat Enterprise Linux kernel can be extended with optional, additional pieces of functionality, called kernel modules, without having to reboot the system. On Red Hat Enterprise Linux 9, kernel modules are extra kernel code which is built into compressed <KERNEL_MODULE_NAME>.ko.xz
object files.
The most common functionality enabled by kernel modules are:
- Device driver which adds support for new hardware
-
Support for a file system such as
GFS2
orNFS
- System calls
On modern systems, kernel modules are automatically loaded when needed. However, in some cases it is necessary to load or unload modules manually.
Like the kernel itself, the modules can take parameters that customize their behavior if needed.
Tooling is provided to inspect which modules are currently running, which modules are available to load into the kernel and which parameters a module accepts. The tooling also provides a mechanism to load and unload kernel modules into the running kernel.
3.2. Kernel module dependencies
Certain kernel modules sometimes depend on one or more other kernel modules. The /lib/modules/<KERNEL_VERSION>/modules.dep
file contains a complete list of kernel module dependencies for the respective kernel version.
The dependency file is generated by the depmod
program, which is a part of the kmod
package. Many of the utilities provided by kmod
take module dependencies into account when performing operations so that manual dependency-tracking is rarely necessary.
The code of kernel modules is executed in kernel-space in the unrestricted mode. Because of this, you should be mindful of what modules you are loading.
Additional resources
-
modules.dep(5)
manual page -
depmod(8)
manual page
3.3. Listing currently loaded kernel modules
The following procedure describes how to view the currently loaded kernel modules.
Prerequisites
-
The
kmod
package is installed.
Procedure
To list all currently loaded kernel modules, execute:
$ lsmod Module Size Used by fuse 126976 3 uinput 20480 1 xt_CHECKSUM 16384 1 ipt_MASQUERADE 16384 1 xt_conntrack 16384 1 ipt_REJECT 16384 1 nft_counter 16384 16 nf_nat_tftp 16384 0 nf_conntrack_tftp 16384 1 nf_nat_tftp tun 49152 1 bridge 192512 0 stp 16384 1 bridge llc 16384 2 bridge,stp nf_tables_set 32768 5 nft_fib_inet 16384 1 …
In the example above:
- The first column provides the names of currently loaded modules.
- The second column displays the amount of memory per module in kilobytes.
- The last column shows the number, and optionally the names of modules that are dependent on a particular module.
Additional resources
-
/usr/share/doc/kmod/README
file -
lsmod(8)
manual page
3.4. Setting a kernel as default
The following procedure describes how to set a specific kernel as default using the grubby
command-line tool and GRUB2.
Procedure
Setting the kernel as default, using the
grubby
toolExecute the following command to set the kernel as default using the
grubby
tool:# grubby --set-default $kernel_path
The command uses a machine ID without the
.conf
suffix as an argument.NoteThe machine ID is located in the
/boot/loader/entries/
directory.
Setting the kernel as default, using the
id
argumentList the boot entries using the
id
argument and then set an intended kernel as default:# grubby --info ALL | grep id # grubby --set-default /boot/vmlinuz-<version>.<architecture>
NoteTo list the boot entries using the
title
argument, execute the# grubby --info=ALL | grep title
command.
Setting the default kernel for only the next boot
Execute the following command to set the default kernel for only the next reboot using the
grub2-reboot
command:# grub2-reboot <index|title|id>
WarningSet the default kernel for only the next boot with care. Installing new kernel RPM’s, self-built kernels, and manually adding the entries to the
/boot/loader/entries/
directory may change the index values.
3.5. Displaying information about kernel modules
When working with a kernel module, you may want to see further information about that module. This procedure describes how to display extra information about kernel modules.
Prerequisites
-
The
kmod
package is installed.
Procedure
- To display information about any kernel module, execute:
$ modinfo <KERNEL_MODULE_NAME> For example: $ modinfo virtio_net filename: /lib/modules/5.14.0-1.el9.x86_64/kernel/drivers/net/virtio_net.ko.xz license: GPL description: Virtio network driver rhelversion: 9.0 srcversion: 8809CDDBE7202A1B00B9F1C alias: virtio:d00000001v* depends: net_failover retpoline: Y intree: Y name: virtio_net vermagic: 5.14.0-1.el9.x86_64 SMP mod_unload modversions … parm: napi_weight:int parm: csum:bool parm: gso:bool parm: napi_tx:bool
+ The modinfo
command displays some detailed information about the specified kernel module. You can query information about all available modules, regardless of whether they are loaded or not. The parm
entries show parameters the user is able to set for the module, and what type of value they expect.
+
When entering the name of a kernel module, do not append the .ko.xz
extension to the end of the name. Kernel module names do not have extensions; their corresponding files do.
Additional resources
-
modinfo(8)
manual page
3.6. Loading kernel modules at system runtime
The optimal way to expand the functionality of the Linux kernel is by loading kernel modules. The following procedure describes how to use the modprobe
command to find and load a kernel module into the currently running kernel.
Prerequisites
- Root permissions
-
The
kmod
package is installed. - The respective kernel module is not loaded. To ensure this is the case, list the loaded kernel modules.
Procedure
Select a kernel module you want to load.
The modules are located in the
/lib/modules/$(uname -r)/kernel/<SUBSYSTEM>/
directory.Load the relevant kernel module:
# modprobe <MODULE_NAME>
NoteWhen entering the name of a kernel module, do not append the
.ko.xz
extension to the end of the name. Kernel module names do not have extensions; their corresponding files do.Optionally, verify the relevant module was loaded:
$ lsmod | grep <MODULE_NAME>
If the module was loaded correctly, this command displays the relevant kernel module. For example:
$ lsmod | grep serio_raw serio_raw 16384 0
The changes described in this procedure will not persist after rebooting the system. For information on how to load kernel modules to persist across system reboots, see Loading kernel modules automatically at system boot time.
Additional resources
-
modprobe(8)
manual page
3.7. Unloading kernel modules at system runtime
At times, you find that you need to unload certain kernel modules from the running kernel. The following procedure describes how to use the modprobe
command to find and unload a kernel module at system runtime from the currently loaded kernel.
Prerequisites
- Root permissions
-
The
kmod
package is installed.
Procedure
Execute the
lsmod
command and select a kernel module you want to unload.If a kernel module has dependencies, unload those prior to unloading the kernel module. For details on identifying modules with dependencies, see Listing currently loaded kernel modules and Kernel module dependencies.
Unload the relevant kernel module:
# modprobe -r <MODULE_NAME>
When entering the name of a kernel module, do not append the
.ko.xz
extension to the end of the name. Kernel module names do not have extensions; their corresponding files do.WarningDo not unload kernel modules when they are used by the running system. Doing so can lead to an unstable or non-operational system.
Optionally, verify the relevant module was unloaded:
$ lsmod | grep <MODULE_NAME>
If the module was unloaded successfully, this command does not display any output.
After finishing this procedure, the kernel modules that are defined to be automatically loaded on boot, will not stay unloaded after rebooting the system. For information on how to counter this outcome, see Preventing kernel modules from being automatically loaded at system boot time.
Additional resources
-
modprobe(8)
manual page
3.8. Unloading kernel modules at early stages of the boot process
In certain situations it is necessary to unload a kernel module very early in the booting process. For example, when the kernel module contains a code, which causes the system to become unresponsive, and the user is not able to reach the stage to permanently disable the rogue kernel module. In that case it is possible to temporarily block the loading of the kernel module using a bootloader.
The changes described in this procedure will not persist after the next reboot. For information on how to add a kernel module to a denylist so that it will not be automatically loaded during the boot process, see Preventing kernel modules from being automatically loaded at system boot time.
Prerequisites
- You have a loadable kernel module, which you want to prevent from loading for some reason.
Procedure
Edit the relevant bootloader entry to unload the desired kernel module before the booting sequence continues.
- Use the cursor keys to highlight the relevant bootloader entry.
Press e key to edit the entry.
Figure 3.1. Kernel boot menu
- Use the cursor keys to navigate to the line that starts with linux.
Append
modprobe.blacklist=module_name
to the end of the line.Figure 3.2. Kernel boot entry
The
serio_raw
kernel module illustrates a rogue module to be unloaded early in the boot process.- Press CTRL+x keys to boot using the modified configuration.
Verification
Once the system fully boots, verify that the relevant kernel module is not loaded.
# lsmod | grep serio_raw
Additional resources
3.9. Loading kernel modules automatically at system boot time
The following procedure describes how to configure a kernel module so that it is loaded automatically during the boot process.
Prerequisites
- Root permissions
-
The
kmod
package is installed.
Procedure
Select a kernel module you want to load during the boot process.
The modules are located in the
/lib/modules/$(uname -r)/kernel/<SUBSYSTEM>/
directory.Create a configuration file for the module:
# echo <MODULE_NAME> > /etc/modules-load.d/<MODULE_NAME>.conf
NoteWhen entering the name of a kernel module, do not append the
.ko.xz
extension to the end of the name. Kernel module names do not have extensions; their corresponding files do.Optionally, after reboot, verify the relevant module was loaded:
$ lsmod | grep <MODULE_NAME>
The example command above should succeed and display the relevant kernel module.
The changes described in this procedure will persist after rebooting the system.
Additional resources
-
modules-load.d(5)
manual page
3.10. Preventing kernel modules from being automatically loaded at system boot time
The following procedure describes how to add a kernel module to a denylist so that it will not be automatically loaded during the boot process.
Prerequisites
- Root permissions
-
The
kmod
package is installed. - Ensure that a kernel module in a denylist is not vital for your current system configuration.
Procedure
Select a kernel module that you want to put in a denylist:
$ lsmod Module Size Used by fuse 126976 3 xt_CHECKSUM 16384 1 ipt_MASQUERADE 16384 1 uinput 20480 1 xt_conntrack 16384 1 …
The
lsmod
command displays a list of modules loaded to the currently running kernel.Alternatively, identify an unloaded kernel module you want to prevent from potentially loading.
All kernel modules are located in the
/lib/modules/<KERNEL_VERSION>/kernel/<SUBSYSTEM>/
directory.
Create a configuration file for a denylist:
# vim /etc/modprobe.d/blacklist.conf # Blacklists <KERNEL_MODULE_1> blacklist <MODULE_NAME_1> install <MODULE_NAME_1> /bin/false # Blacklists <KERNEL_MODULE_2> blacklist <MODULE_NAME_2> install <MODULE_NAME_2> /bin/false # Blacklists <KERNEL_MODULE_n> blacklist <MODULE_NAME_n> install <MODULE_NAME_n> /bin/false …
The example shows the contents of the
blacklist.conf
file, edited by thevim
editor. Theblacklist
line ensures that the relevant kernel module will not be automatically loaded during the boot process. Theblacklist
command, however, does not prevent the module from being loaded as a dependency for another kernel module that is not in a denylist. Therefore theinstall
line causes the/bin/false
to run instead of installing a module.The lines starting with a hash sign are comments to make the file more readable.
NoteWhen entering the name of a kernel module, do not append the
.ko.xz
extension to the end of the name. Kernel module names do not have extensions; their corresponding files do.Create a backup copy of the current initial ramdisk image before rebuilding:
# cp /boot/initramfs-$(uname -r).img /boot/initramfs-$(uname -r).bak.$(date +%m-%d-%H%M%S).img
The command above creates a backup
initramfs
image in case the new version has an unexpected problem.Alternatively, create a backup copy of other initial ramdisk image which corresponds to the kernel version for which you want to put kernel modules in a denylist:
# cp /boot/initramfs-<SOME_VERSION>.img /boot/initramfs-<SOME_VERSION>.img.bak.$(date +%m-%d-%H%M%S)
Generate a new initial ramdisk image to reflect the changes:
# dracut -f -v
If you are building an initial ramdisk image for a different kernel version than you are currently booted into, specify both target
initramfs
and kernel version:# dracut -f -v /boot/initramfs-<TARGET_VERSION>.img <CORRESPONDING_TARGET_KERNEL_VERSION>
Reboot the system:
$ reboot
The changes described in this procedure will take effect and persist after rebooting the system. If you improperly put a key kernel module in a denylist, you can face an unstable or non-operational system.
Additional resources
- How do I prevent a kernel module from loading automatically?
-
dracut(8)
manual page
3.11. Compiling custom kernel modules
You can build a sampling kernel module as requested by various configurations at hardware and software level.
Prerequisites
-
You installed the
kernel-devel
,gcc
, andelfutils-libelf-devel
packages. - You have root permissions.
-
You created the
/root/testmodule/
directory where you compile the custom kernel module.
Procedure
Create the
/root/testmodule/test.c
file with the following content.#include <linux/module.h> #include <linux/kernel.h> int init_module(void) { printk("Hello World\n This is a test\n"); return 0; } void cleanup_module(void) { printk("Good Bye World"); } MODULE_LICENSE("GPL");
The
test.c
file is a source file that provides the main functionality to the kernel module. The file has been created in a dedicated/root/testmodule/
directory for organizational purposes. After the module compilation, the/root/testmodule/
directory will contain multiple files.The
test.c
file includes from the system libraries:-
The
linux/kernel.h
header file is necessary for theprintk()
function in the example code. The
linux/module.h
file contains function declarations and macro definitions to be shared between several source files written in C programming language.Next follow the
init_module()
andcleanup_module()
functions to start and end the kernel logging functionprintk()
, which prints text.
-
The
Create the
/root/testmodule/Makefile
file with the following content.obj-m := test.o
The Makefile contains instructions that the compiler has to produce an object file specifically named
test.o
. Theobj-m
directive specifies that the resultingtest.ko
file is going to be compiled as a loadable kernel module. Alternatively, theobj-y
directive would instruct to buildtest.ko
as a built-in kernel module.Compile the kernel module.
# make -C /lib/modules/$(uname -r)/build M=/root/testmodule modules make: Entering directory '/usr/src/kernels/5.14.0-70.17.1.el9_0.x86_64' CC [M] /root/testmodule/test.o MODPOST /root/testmodule/Module.symvers CC [M] /root/testmodule/test.mod.o LD [M] /root/testmodule/test.ko BTF [M] /root/testmodule/test.ko Skipping BTF generation for /root/testmodule/test.ko due to unavailability of vmlinux make: Leaving directory '/usr/src/kernels/5.14.0-70.17.1.el9_0.x86_64'
The compiler creates an object file (
test.o
) for each source file (test.c
) as an intermediate step before linking them together into the final kernel module (test.ko
).After a successful compilation,
/root/testmodule/
contains additional files that relate to the compiled custom kernel module. The compiled module itself is represented by thetest.ko
file.
Verification
Optional: check the contents of the
/root/testmodule/
directory:# ls -l /root/testmodule/ total 152 -rw-r—r--. 1 root root 16 Jul 26 08:19 Makefile -rw-r—r--. 1 root root 25 Jul 26 08:20 modules.order -rw-r—r--. 1 root root 0 Jul 26 08:20 Module.symvers -rw-r—r--. 1 root root 224 Jul 26 08:18 test.c -rw-r—r--. 1 root root 62176 Jul 26 08:20 test.ko -rw-r—r--. 1 root root 25 Jul 26 08:20 test.mod -rw-r—r--. 1 root root 849 Jul 26 08:20 test.mod.c -rw-r—r--. 1 root root 50936 Jul 26 08:20 test.mod.o -rw-r—r--. 1 root root 12912 Jul 26 08:20 test.o
Copy the kernel module to the
/lib/modules/$(uname -r)/
directory:# cp /root/testmodule/test.ko /lib/modules/$(uname -r)/
Update the modular dependency list:
# depmod -a
Load the kernel module:
# modprobe -v test insmod /lib/modules/5.14.0-1.el9.x86_64/test.ko
Verify that the kernel module was successfully loaded:
# lsmod | grep test test 16384 0
Read the latest messages from the kernel ring buffer:
# dmesg [74422.545004] Hello World This is a test
Additional resources
Chapter 4. Signing kernel modules for secure boot
You can enhance the security of your system by using signed kernel modules. The following sections describe how to self-sign privately built kernel modules for use with RHEL 9 on UEFI-based build systems where Secure Boot is enabled. These sections also provide an overview of available options for importing your public key into a target system where you want to deploy your kernel modules.
If Secure Boot is enabled, the UEFI operating system boot loaders, the Red Hat Enterprise Linux kernel, and all kernel modules have to be signed with a private key and authenticated with the corresponding public key. If they are not signed and authenticated, the system will not be allowed to finish the booting process.
The RHEL 9 distribution includes:
- Signed boot loaders
- Signed kernels
- Signed kernel modules
In addition, the signed first-stage boot loader and the signed kernel include embedded Red Hat public keys. These signed executable binaries and embedded keys enable RHEL 9 to install, boot, and run with the Microsoft UEFI Secure Boot Certification Authority keys that are provided by the UEFI firmware on systems that support UEFI Secure Boot. Note that not all UEFI-based systems include support for Secure Boot.
Prerequisites
To be able to sign externally built kernel modules, install the utilities listed in the following table on the build system.
Table 4.1. Required utilities
Utility | Provided by package | Used on | Purpose |
---|---|---|---|
|
| Build system | Generates public and private X.509 key pair |
|
| Build system | Executable file used to sign a kernel module with the private key |
|
| Target system | Optional utility used to manually enroll the public key |
|
| Target system | Optional utility used to display public keys in the system keyring |
The build system, where you build and sign your kernel module, does not need to have UEFI Secure Boot enabled and does not even need to be a UEFI-based system.
4.1. What is UEFI Secure Boot
With the Unified Extensible Firmware Interface (UEFI) Secure Boot technology you can ensure that the system boot loader is signed with a cryptographic key. The database of public keys which is contained in the firmware, authorizes the signing key. You can verify a signature in the next-stage boot loader and the kernel. As a result, you can prevent the execution of the kernel space code which has not been signed by a trusted key.
UEFI Secure Boot establishes a chain of trust from the firmware to the signed drivers and kernel modules as follows:
-
A UEFI private key signs, and a public key authenticates the
shim
first-stage boot loader. A certificate authority (CA) in turn signs the public key. The CA is stored in the firmware database. -
The
shim.efi
contains the Red Hat public key Red Hat Secure Boot (CA key 1) to authenticate the GRUB 2 boot loader, and the kernel. - The kernel in turn contains public keys to authenticate drivers and modules.
Secure Boot is the boot path validation component of the UEFI specification. The specification defines:
- Programming interface for cryptographically protected UEFI variables in non-volatile storage
- Storing the trusted X.509 root certificates in UEFI variables
- Validation of UEFI applications like boot loaders and drivers
- Procedures to revoke known-bad certificates and application hashes
UEFI Secure Boot does not prevent installation or removal of second-stage boot loaders. Also, it does not require explicit user confirmation of such changes. Signatures are verified during booting, not when the boot loader is installed or updated. Therefore, UEFI Secure Boot does not stop boot path manipulations. It helps in the detection of unauthorized changes.
The boot loader or the kernel work as long as a system trusted key signs them.
4.2. UEFI Secure Boot support
RHEL 9 includes support for the Unified Extensible Firmware Interface (UEFI) Secure Boot feature. It means that you can install and run RHEL 9 on machines with enabled UEFI Secure Boot. On these systems, you must sign the all loaded drivers with a trusted key. Otherwise the system will not accept them. Red Hat provides drivers which are signed and authenticated by the relevant Red Hat keys.
If you want to load externally built drivers you must sign them as well.
Restrictions Imposed by UEFI Secure Boot
- The system only runs the kernel mode code after its signature has been properly authenticated.
- GRUB 2 module loading is disabled because there is no infrastructure for signing and verification of GRUB 2 modules. Allowing them to be loaded constitutes execution of untrusted code inside the security perimeter that Secure Boot defines.
- Red Hat provides a signed GRUB 2 binary that contains all the supported modules on RHEL 9.
Additional resources
4.3. Requirements for authenticating kernel modules with X.509 keys
In RHEL 9, when a kernel module is loaded, the kernel checks the signature of the module against the public X.509 keys from the kernel system keyring (.builtin_trusted_keys
) and the kernel platform keyring (.platform
). The .platform
keyring contains keys from third-party platform providers and custom public keys. The keys from the kernel system .blacklist
keyring are excluded from verification. The following sections provide an overview of sources of keys, keyrings and examples of loaded keys from different sources in the system. Also, you can see how to authenticate a kernel module.
You need to meet certain conditions to load kernel modules on systems with enabled UEFI Secure Boot functionality.
If UEFI Secure Boot is enabled or if the module.sig_enforce
kernel parameter has been specified:
-
You can only load those signed kernel modules whose signatures were authenticated against keys from the system keyring (
.builtin_trusted_keys
) and the platform keyring (.platform
). -
The public key must not be on the system revoked keys keyring (
.blacklist
).
If UEFI Secure Boot is disabled and the module.sig_enforce
kernel parameter has not been specified:
- You can load unsigned kernel modules and signed kernel modules without a public key.
If the system is not UEFI-based or if UEFI Secure Boot is disabled:
-
Only the keys embedded in the kernel are loaded onto
.builtin_trusted_keys
and.platform
. - You have no ability to augment that set of keys without rebuilding the kernel.
Table 4.2. Kernel module authentication requirements for loading
Module signed | Public key found and signature valid | UEFI Secure Boot state | sig_enforce | Module load | Kernel tainted |
---|---|---|---|---|---|
Unsigned | - | Not enabled | Not enabled | Succeeds | Yes |
Not enabled | Enabled | Fails | - | ||
Enabled | - | Fails | - | ||
Signed | No | Not enabled | Not enabled | Succeeds | Yes |
Not enabled | Enabled | Fails | - | ||
Enabled | - | Fails | - | ||
Signed | Yes | Not enabled | Not enabled | Succeeds | No |
Not enabled | Enabled | Succeeds | No | ||
Enabled | - | Succeeds | No |
4.4. Sources for public keys
During boot, the kernel loads X.509 keys from a set of persistent key stores into the following keyrings:
-
The system keyring (
.builtin_trusted_keys
) -
The
.platform
keyring -
The system
.blacklist
keyring
Table 4.3. Sources for system keyrings
Source of X.509 keys | User can add keys | UEFI Secure Boot state | Keys loaded during boot |
---|---|---|---|
Embedded in kernel | No | - |
|
UEFI Secure Boot "db" | Limited | Not enabled | No |
Enabled |
| ||
Embedded in | No | Not enabled | No |
Enabled |
| ||
Machine Owner Key (MOK) list | Yes | Not enabled | No |
Enabled |
|
.builtin_trusted_keys
:
- a keyring that is built on boot
- contains trusted public keys
-
root
privileges are needed to view the keys
.platform
:
- a keyring that is built on boot
- contains keys from third-party platform providers and custom public keys
-
root
privileges are needed to view the keys
.blacklist
- a keyring with X.509 keys which have been revoked
-
a module signed by a key from
.blacklist
will fail authentication even if your public key is in.builtin_trusted_keys
UEFI Secure Boot db:
- a signature database
- stores keys (hashes) of UEFI applications, UEFI drivers, and bootloaders
- the keys can be loaded on the machine
UEFI Secure Boot dbx:
- a revoked signature database
- prevents keys from being loaded
-
the revoked keys from this database are added to the
.blacklist
keyring
4.5. Generating a public and private key pair
You need to generate a public and private X.509 key pair to succeed in your efforts of using kernel modules on a Secure Boot-enabled system. You will later use the private key to sign the kernel module. You will also have to add the corresponding public key to the Machine Owner Key (MOK) for Secure Boot to validate the signed module.
Some of the parameters for this key pair generation are best specified with a configuration file.
Procedure
Create a configuration file with parameters for the key pair generation:
# cat << EOF > configuration_file.config [ req ] default_bits = 4096 distinguished_name = req_distinguished_name prompt = no string_mask = utf8only x509_extensions = myexts [ req_distinguished_name ] O = Organization CN = Organization signing key emailAddress = E-mail address [ myexts ] basicConstraints=critical,CA:FALSE keyUsage=digitalSignature subjectKeyIdentifier=hash authorityKeyIdentifier=keyid EOF
Create an X.509 public and private key pair as shown in the following example:
# openssl req -x509 -new -nodes -utf8 -sha256 -days 36500 \ -batch -config configuration_file.config -outform DER \ -out my_signing_key_pub.der \ -keyout my_signing_key.priv
The public key will be written to the
my_signing_key_pub.der
file and the private key will be written to themy_signing_key.priv
file.ImportantIn RHEL 9, the validity dates of the key pair do not matter. The key does not expire, but it is recommended for good security practices that the kernel module be signed within the validity period of its signing key. However, the
sign-file
utility will not warn you and the key will be usable regardless of the validity dates.Optionally, you can review the validity dates of your public keys like in the example below:
# openssl x509 -inform der -text -noout -in my_signing_key_pub.der Validity Not Before: Feb 14 16:34:37 2019 GMT Not After : Feb 11 16:34:37 2029 GMT
- Enroll your public key on all systems where you want to authenticate and load your kernel module.
Apply strong security measures and access policies to guard the contents of your private key. In the wrong hands, the key could be used to compromise any system which is authenticated by the corresponding public key.
Additional resources
4.6. Example output of system keyrings
You can display information about the keys on the system keyrings using the keyctl
utility from the keyutils
package.
The following is a shortened example output of .builtin_trusted_keys
, .platform
, and .blacklist
keyrings from a RHEL 9 system where UEFI Secure Boot is enabled.
Prerequisites
- You have root permissions.
-
You installed the
keyctl
utility from thekeyutils
package.
# keyctl list %:.builtin_trusted_keys 6 keys in keyring: ...asymmetric: Red Hat Enterprise Linux Driver Update Program (key 3): bf57f3e87... ...asymmetric: Red Hat Secure Boot (CA key 1): 4016841644ce3a810408050766e8f8a29... ...asymmetric: Microsoft Corporation UEFI CA 2011: 13adbf4309bd82709c8cd54f316ed... ...asymmetric: Microsoft Windows Production PCA 2011: a92902398e16c49778cd90f99e... ...asymmetric: Red Hat Enterprise Linux kernel signing key: 4249689eefc77e95880b... ...asymmetric: Red Hat Enterprise Linux kpatch signing key: 4d38fd864ebe18c5f0b7... # keyctl list %:.platform 4 keys in keyring: ...asymmetric: VMware, Inc.: 4ad8da0472073... ...asymmetric: Red Hat Secure Boot CA 5: cc6fafe72... ...asymmetric: Microsoft Windows Production PCA 2011: a929f298e1... ...asymmetric: Microsoft Corporation UEFI CA 2011: 13adbf4e0bd82... # keyctl list %:.blacklist 4 keys in keyring: ...blacklist: bin:f5ff83a... ...blacklist: bin:0dfdbec... ...blacklist: bin:38f1d22... ...blacklist: bin:51f831f...
The .builtin_trusted_keys
keyring above shows the addition of two keys from the UEFI Secure Boot "db" keys as well as the Red Hat Secure Boot (CA key 1)
, which is embedded in the shim.efi
boot loader.
The following example shows the kernel console output. The messages identify the keys with a UEFI Secure Boot related source. These include UEFI Secure Boot db, embedded shim, and MOK list.
# dmesg | egrep 'integrity.*cert'
[1.512966] integrity: Loading X.509 certificate: UEFI:db
[1.513027] integrity: Loaded X.509 cert 'Microsoft Windows Production PCA 2011: a929023...
[1.513028] integrity: Loading X.509 certificate: UEFI:db
[1.513057] integrity: Loaded X.509 cert 'Microsoft Corporation UEFI CA 2011: 13adbf4309...
[1.513298] integrity: Loading X.509 certificate: UEFI:MokListRT (MOKvar table)
[1.513549] integrity: Loaded X.509 cert 'Red Hat Secure Boot CA 5: cc6fa5e72868ba494e93...
Additional resources
-
keyctl(1)
,dmesg(1)
manual pages
4.7. Enrolling public key on target system by adding the public key to the MOK list
When RHEL 9 boots on a UEFI-based system with Secure Boot enabled, the kernel loads onto the system keyring (.builtin_trusted_keys
) all public keys that are in the Secure Boot db key database. At the same time the kernel excludes the keys in the dbx database of revoked keys. The sections below describe different ways of importing a public key on a target system so that the system keyring (.builtin_trusted_keys
) is able to use the public key to authenticate a kernel module.
The Machine Owner Key (MOK) facility feature can be used to expand the UEFI Secure Boot key database. When RHEL 9 boots on a UEFI-enabled system with Secure Boot enabled, the keys on the MOK list are also added to the system keyring (.builtin_trusted_keys
) in addition to the keys from the key database. The MOK list keys are also stored persistently and securely in the same fashion as the Secure Boot database keys, but these are two separate facilities. The MOK facility is supported by shim.efi
, MokManager.efi
, grubx64.efi
, and the mokutil
utility.
Enrolling a MOK key requires manual interaction by a user at the UEFI system console on each target system. Nevertheless, the MOK facility provides a convenient method for testing newly generated key pairs and testing kernel modules signed with them.
Procedure
Request the addition of your public key to the MOK list:
# mokutil --import my_signing_key_pub.der
You will be asked to enter and confirm a password for this MOK enrollment request.
Reboot the machine.
The pending MOK key enrollment request will be noticed by
shim.efi
and it will launchMokManager.efi
to allow you to complete the enrollment from the UEFI console.Choose "Enroll MOK" and enter the password you previously associated with this request when prompted and confirm the enrollment.
Your public key is added to the MOK list, which is persistent.
Once a key is on the MOK list, it will be automatically propagated to the system keyring on this and subsequent boots when UEFI Secure Boot is enabled.
To facilitate authentication of your kernel module on your systems, consider requesting your system vendor to incorporate your public key into the UEFI Secure Boot key database in their factory firmware image.
4.8. Signing kernel modules with the private key
Users are able to obtain enhanced security benefits on their systems by loading signed kernel modules if the UEFI Secure Boot mechanism is enabled. The following sections describe how to sign kernel modules with the private key.
Prerequisites
- You generated a public and private key pair and know the validity dates of your public keys. For details, see Generating a public and private key pair.
- You enrolled your public key on the target system. For details, see Enrolling public key on target system by adding the public key to the MOK list.
- You have a kernel module in ELF image format available for signing.
Procedure
Execute the
sign-file
utility with parameters as shown in the example below:# /usr/src/kernels/$(uname -r)/scripts/sign-file sha256 my_signing_key.priv my_signing_key_pub.der my_module.ko
sign-file
computes and appends the signature directly to the ELF image in your kernel module file. Themodinfo
utility can be used to display information about the kernel module’s signature, if it is present.NoteThe appended signature is not contained in an ELF image section and is not a formal part of the ELF image. Therefore, utilities such as
readelf
will not be able to display the signature on your kernel module.Your kernel module is now ready for loading. Note that your signed kernel module is also loadable on systems where UEFI Secure Boot is disabled or on a non-UEFI system. That means you do not need to provide both a signed and unsigned version of your kernel module.
ImportantIn RHEL 9, the validity dates of the key pair matter. The key does not expire, but the kernel module must be signed within the validity period of its signing key. The
sign-file
utility will not warn you of this. For example, a key that is only valid in 2021 can be used to authenticate a kernel module signed in 2021 with that key. However, users cannot use that key to sign a kernel module in 2022.
Additional resources
4.9. Loading signed kernel modules
Once your public key is enrolled in the system keyring (.builtin_trusted_keys
) and the MOK list, and after you have signed the respective kernel module with your private key, you can finally load your signed kernel module with the the modprobe
command as described in the following section.
Prerequisites
- You have generated the public and private keypair. For details, see Generating a public and private key pair.
- You have enrolled the public key into the system keyring. For details, see Enrolling public key on target system by adding the public key to the MOK list.
- You have signed a kernel module with the private key. For details, see Signing kernel modules with the private key.
Procedure
Verify that your public keys are on the system keyring:
# keyctl list %:.platform
Install the
kernel-modules-extra
package which will create the/lib/modules/$(uname -r)/extra/
directory:# dnf -y install kernel-modules-extra
Copy the kernel module into the
/extra/
directory of the kernel you want:# cp my_module.ko /lib/modules/$(uname -r)/extra/
Update the modular dependency list:
# depmod -a
Load the kernel module and verify that it was successfully loaded:
# modprobe -v my_module # lsmod | grep my_module
Optionally, to load the module on boot, add it to the
/etc/modules-loaded.d/my_module.conf
file:# echo "my_module" > /etc/modules-load.d/my_module.conf
Additional resources
Chapter 5. Configuring kernel command-line parameters
Kernel command-line parameters are a way to change the behavior of certain aspects of the Red Hat Enterprise Linux kernel at boot time. As a system administrator, you have full control over what options get set at boot. Certain kernel behaviors are only able to be set at boot time, so understanding how to make these changes is a key administration skill.
Opting to change the behavior of the system by modifying kernel command-line parameters may have negative effects on your system. You should therefore test changes prior to deploying them in production. For further guidance, contact Red Hat Support.
5.1. Understanding kernel command-line parameters
Kernel command-line parameters are used for boot time configuration of:
- The Red Hat Enterprise Linux kernel
- The initial RAM disk
- The user space features
Kernel boot time parameters are often used to overwrite default values and for setting specific hardware settings.
By default, the kernel command-line parameters for systems using the GRUB2 bootloader are defined in the boot entry configuration file for each kernel boot entry.
Additional resources
-
kernel-command-line(7)
,bootparam(7)
anddracut.cmdline(7)
manual pages - How to install and boot custom kernels in Red Hat Enterprise Linux 8
5.2. What grubby is
grubby
is a utility for manipulating bootloader-specific configuration files.
You can use grubby
also for changing the default boot entry, and for adding/removing arguments from a GRUB2 menu entry.
For more details see the grubby(8)
manual page.
5.3. What boot entries are
A boot entry is a collection of options which are stored in a configuration file and tied to a particular kernel version. In practice, you have at least as many boot entries as your system has installed kernels. The boot entry configuration file is located in the /boot/loader/entries/
directory and can look like this:
d8712ab6d4f14683c5625e87b52b6b6e-5.14.0-1.el9.x86_64.conf
The file name above consists of a machine ID stored in the /etc/machine-id
file, and a kernel version.
The boot entry configuration file contains information about the kernel version, the initial ramdisk image, and the kernel command-line parameters. The example contents of a boot entry config can be seen below:
title Red Hat Enterprise Linux (5.14.0-1.el9.x86_64) 9.0 (Plow) version 5.14.0-1.el9.x86_64 linux /vmlinuz-5.14.0-1.el9.x86_64 initrd /initramfs-5.14.0-1.el9.x86_64.img options root=/dev/mapper/rhel_kvm--02--guest08-root ro crashkernel=1G-4G:192M,4G-64G:256M,64G-:512M resume=/dev/mapper/rhel_kvm--02--guest08-swap rd.lvm.lv=rhel_kvm-02-guest08/root rd.lvm.lv=rhel_kvm-02-guest08/swap console=ttyS0,115200 grub_users $grub_users grub_arg --unrestricted grub_class kernel
Additional resources
5.4. Changing kernel command-line parameters for all boot entries
This procedure describes how to change kernel command-line parameters for all boot entries on your system.
Prerequisites
-
Verify that the
grubby
utility is installed on your system. -
Verify that the
zipl
utility is installed on your IBM Z system.
Procedure
To add a parameter:
# grubby --update-kernel=ALL --args="<NEW_PARAMETER>"
For systems that use the GRUB2 bootloader and, on IBM Z that use the zIPL bootloader, the command adds a new kernel parameter to each
/boot/loader/entries/<ENTRY>.conf
file.On IBM Z that use the zIPL bootloader, the command adds a new kernel parameter to each
/boot/loader/entries/<ENTRY>.conf
file.-
On IBM Z, execute the
zipl
command with no options to update the boot menu.
-
On IBM Z, execute the
To remove a parameter:
# grubby --update-kernel=ALL --remove-args="<PARAMETER_TO_REMOVE>"
-
On IBM Z, execute the
zipl
command with no options to update the boot menu.
-
On IBM Z, execute the
After each update of your kernel package, propagate the configured kernel options to the new kernels:
# grub2-mkconfig -o /etc/grub2.cfg
ImportantNewly installed kernels do not inherit the kernel command-line parameters from your previously configured kernels. You must run the
grub2-mkconfig
command on the newly installed kernel to propagate the needed parameters to your new kernel.
Additional resources
- Understanding kernel command-line parameters
-
grubby(8)
andzipl(8)
manual pages - grubby tool
5.5. Changing kernel command-line parameters for a single boot entry
This procedure describes how to change kernel command-line parameters for a single boot entry on your system.
Prerequisites
-
Verify that the
grubby
andzipl
utilities are installed on your system.
Procedure
To add a parameter:
# grubby --update-kernel=/boot/vmlinuz-$(uname -r) --args="<NEW_PARAMETER>"
-
On IBM Z, execute the
zipl
command with no options to update the boot menu.
-
On IBM Z, execute the
To remove a parameter use the following:
# grubby --update-kernel=/boot/vmlinuz-$(uname -r) --remove-args="<PARAMETER_TO_REMOVE>"
-
On IBM Z, execute the
zipl
command with no options to update the boot menu.
-
On IBM Z, execute the
-
grubby
modifies and stores the kernel command-line parameters of an individual kernel boot entry in the/boot/loader/entries/<ENTRY>.conf
file.
Additional resources
- Understanding kernel command-line parameters
-
grubby(8)
andzipl(8)
manual pages - grubby tool
5.6. Changing kernel command-line parameters temporarily at boot time
The following procedure allows you to make temporary changes to a Kernel Menu Entry by changing the kernel parameters only during a single boot process.
Procedure
- Select the kernel you want to start when the GRUB 2 boot menu appears and press the e key to edit the kernel parameters.
-
Find the kernel command line by moving the cursor down. The kernel command line starts with
linux
on 64-Bit IBM Power Series and x86-64 BIOS-based systems, orlinuxefi
on UEFI systems. Move the cursor to the end of the line.
NotePress Ctrl+a to jump to the start of the line and Ctrl+e to jump to the end of the line. On some systems, Home and End keys might also work.
-
Edit the kernel parameters as required. For example, to run the system in emergency mode, add the emergency parameter at the end of the
linux
line:
linux ($root)/vmlinuz-5.14.0-63.el9.x86_64 root=/dev/mapper/rhel-root ro crashkernel=1G-4G:192M,4G-64G:256M,64G-:512M resume=/dev/mapper/rhel-swap rd.lvm.lv=rhel/root rd.lvm.lv=rhel/swap rhgb quiet pass:quotes[_emergency_]
+ To enable the system messages, remove the rhgb
and quiet
parameters.
- Press Ctrl+x to boot with the selected kernel and the modified command line parameters.
Press Esc key to leave command line editing and it will drop all the user made changes.
This procedure applies only for a single boot and does not persistently make the changes.
5.7. Configuring GRUB 2 settings to enable serial console connection
The serial console is beneficial when you need to connect to a headless server or an embedded system and the network is down. Or when you need to avoid security rules and obtain login access on a different system.
You need to configure some default GRUB 2 settings to use the serial console connection.
Prerequisites
- You have root permissions.
Procedure
Add the following two lines to the
/etc/default/grub
file:GRUB_TERMINAL="serial" GRUB_SERIAL_COMMAND="serial --speed=9600 --unit=0 --word=8 --parity=no --stop=1"
The first line disables the graphical terminal. The
GRUB_TERMINAL
key overrides values ofGRUB_TERMINAL_INPUT
andGRUB_TERMINAL_OUTPUT
keys.The second line adjusts the baud rate (
--speed
), parity and other values to fit your environment and hardware. Note that a much higher baud rate, for example 115200, is preferable for tasks such as following log files.Update the GRUB 2 configuration file.
On BIOS-based machines:
# grub2-mkconfig -o /boot/grub2/grub.cfg
On UEFI-based machines:
# grub2-mkconfig -o /boot/grub2/grub.cfg
- Reboot the system for the changes to take effect.
Chapter 6. Configuring kernel parameters at runtime
As a system administrator, you can modify many facets of the Red Hat Enterprise Linux kernel’s behavior at runtime. This section describes how to configure kernel parameters at runtime by using the sysctl
command and by modifying the configuration files in the /etc/sysctl.d/
and /proc/sys/
directories.
6.1. What are kernel parameters
Kernel parameters are tunable values which you can adjust while the system is running. There is no requirement to reboot or recompile the kernel for changes to take effect.
It is possible to address the kernel parameters through:
-
The
sysctl
command -
The virtual file system mounted at the
/proc/sys/
directory -
The configuration files in the
/etc/sysctl.d/
directory
Tunables are divided into classes by the kernel subsystem. Red Hat Enterprise Linux has the following tunable classes:
Table 6.1. Table of sysctl classes
Tunable class | Subsystem |
---|---|
abi | Execution domains and personalities |
crypto | Cryptographic interfaces |
debug | Kernel debugging interfaces |
dev | Device-specific information |
fs | Global and specific file system tunables |
kernel | Global kernel tunables |
net | Network tunables |
sunrpc | Sun Remote Procedure Call (NFS) |
user | User Namespace limits |
vm | Tuning and management of memory, buffers, and cache |
Configuring kernel parameters on a production system requires careful planning. Unplanned changes may render the kernel unstable, requiring a system reboot. Verify that you are using valid options before changing any kernel values.
Additional resources
-
sysctl(8)
, andsysctl.d(5)
manual pages
6.2. Configuring kernel parameters temporarily with sysctl
The following procedure describes how to use the sysctl
command to temporarily set kernel parameters at runtime. The command is also useful for listing and filtering tunables.
Prerequisites
- Root permissions
Procedure
To list all parameters and their values, use the following:
# sysctl -a
NoteThe
# sysctl -a
command displays kernel parameters, which can be adjusted at runtime and at boot time.To configure a parameter temporarily, use the command as in the following example:
# sysctl <TUNABLE_CLASS>.<PARAMETER>=<TARGET_VALUE>
The sample command above changes the parameter value while the system is running. The changes take effect immediately, without a need for restart.
NoteThe changes return back to default after your system reboots.
Additional resources
6.3. Configuring kernel parameters permanently with sysctl
The following procedure describes how to use the sysctl
command to permanently set kernel parameters.
Prerequisites
- Root permissions
Procedure
To list all parameters, use the following:
# sysctl -a
The command displays all kernel parameters that can be configured at runtime.
To configure a parameter permanently:
# sysctl -w <TUNABLE_CLASS>.<PARAMETER>=<TARGET_VALUE> >> /etc/sysctl.conf
The sample command changes the tunable value and writes it to the
/etc/sysctl.conf
file, which overrides the default values of kernel parameters. The changes take effect immediately and persistently, without a need for restart.
To permanently modify kernel parameters you can also make manual changes to the configuration files in the /etc/sysctl.d/
directory.
Additional resources
-
sysctl(8)
andsysctl.conf(5)
manual pages - Using configuration files in /etc/sysctl.d/ to adjust kernel parameters
6.4. Using configuration files in /etc/sysctl.d/ to adjust kernel parameters
The following procedure describes how to manually modify configuration files in the /etc/sysctl.d/
directory to permanently set kernel parameters.
Prerequisites
- Root permissions
Procedure
Create a new configuration file in
/etc/sysctl.d/
:# vim /etc/sysctl.d/<some_file.conf>
Include kernel parameters, one per line, as follows:
<TUNABLE_CLASS>.<PARAMETER>=<TARGET_VALUE>
<TUNABLE_CLASS>.<PARAMETER>=<TARGET_VALUE>
- Save the configuration file.
Reboot the machine for the changes to take effect.
Alternatively, to apply changes without rebooting, execute:
# sysctl -p /etc/sysctl.d/<some_file.conf>
The command enables you to read values from the configuration file, which you created earlier.
Additional resources
-
sysctl(8)
,sysctl.d(5)
manual pages
6.5. Configuring kernel parameters temporarily through /proc/sys/
The following procedure describes how to set kernel parameters temporarily through the files in the virtual file system /proc/sys/
directory.
Prerequisites
- Root permissions
Procedure
Identify a kernel parameter you want to configure:
# ls -l /proc/sys/<TUNABLE_CLASS>/
The writable files returned by the command can be used to configure the kernel. The files with read-only permissions provide feedback on the current settings.
Assign a target value to the kernel parameter:
# echo <TARGET_VALUE> > /proc/sys/<TUNABLE_CLASS>/<PARAMETER>
The command makes configuration changes that will disappear once the system is restarted.
Optionally, verify the value of the newly set kernel parameter:
# cat /proc/sys/<TUNABLE_CLASS>/<PARAMETER>
Chapter 7. Keeping kernel panic parameters disabled in virtualized environments
When configuring a virtualized environment in RHEL 9, you should not enable the softlockup_panic
and nmi_watchdog
kernel parameters, as the virtualized environment may trigger a spurious soft lockup that should not require a system panic.
The following sections explain the reasons behind this advice by summarizing:
- What causes a soft lockup.
- Describing the kernel parameters that control a system’s behavior on a soft lockup.
- Explaining how soft lockups may be triggered in a virtualized environment.
7.1. What is a soft lockup
A soft lockup is a situation usually caused by a bug, when a task is executing in kernel space on a CPU without rescheduling. The task also does not allow any other task to execute on that particular CPU. As a result, a warning is displayed to a user through the system console. This problem is also referred to as the soft lockup firing.
Additional resources
7.2. Parameters controlling kernel panic
The following kernel parameters can be set to control a system’s behavior when a soft lockup is detected.
- softlockup_panic
Controls whether or not the kernel will panic when a soft lockup is detected.
Type Value Effect Integer
0
kernel does not panic on soft lockup
Integer
1
kernel panics on soft lockup
By default, on RHEL8 this value is 0.
In order to panic, the system needs to detect a hard lockup first. The detection is controlled by the
nmi_watchdog
parameter.- nmi_watchdog
Controls whether lockup detection mechanisms (
watchdogs
) are active or not. This parameter is of integer type.Value Effect 0
disables lockup detector
1
enables lockup detector
The hard lockup detector monitors each CPU for its ability to respond to interrupts.
- watchdog_thresh
Controls frequency of watchdog
hrtimer
, NMI events, and soft/hard lockup thresholds.Default threshold Soft lockup threshold 10 seconds
2 *
watchdog_thresh
Setting this parameter to zero disables lockup detection altogether.
Additional resources
7.3. Spurious soft lockups in virtualized environments
The soft lockup firing on physical hosts, as described in What is a soft lockup, usually represents a kernel or hardware bug. The same phenomenon happening on guest operating systems in virtualized environments may represent a false warning.
Heavy work-load on a host or high contention over some specific resource such as memory, usually causes a spurious soft lockup firing. This is because the host may schedule out the guest CPU for a period longer than 20 seconds. Then when the guest CPU is again scheduled to run on the host, it experiences a time jump which triggers due timers. The timers include also watchdog hrtimer
, which can consequently report a soft lockup on the guest CPU.
Because a soft lockup in a virtualized environment may be spurious, you should not enable the kernel parameters that would cause a system panic when a soft lockup is reported on a guest CPU.
To understand soft lockups in guests, it is essential to know that the host schedules the guest as a task, and the guest then schedules its own tasks.
Chapter 8. Adjusting kernel parameters for database servers
There are different sets of kernel parameters which can affect performance of specific database applications. The following sections explain what kernel parameters to configure to secure efficient operation of database servers and databases.
8.1. Introduction to database servers
A database server is a service that provides features of a database management system (DBMS). DBMS provides utilities for database administration and interacts with end users, applications, and databases.
Red Hat Enterprise Linux 9 provides the following database management systems:
- MariaDB 10.5
- MySQL 8.0
- PostgreSQL 13
- Redis 6
8.2. Parameters affecting performance of database applications
The following kernel parameters affect performance of database applications.
- fs.aio-max-nr
Defines the maximum number of asynchronous I/O operations the system can handle on the server.
NoteRaising the
fs.aio-max-nr
parameter produces no additional changes beyond increasing the aio limit.- fs.file-max
Defines the maximum number of file handles (temporary file names or IDs assigned to open files) the system supports at any instance.
The kernel dynamically allocates file handles whenever a file handle is requested by an application. The kernel however does not free these file handles when they are released by the application. The kernel recycles these file handles instead. This means that over time the total number of allocated file handles will increase even though the number of currently used file handles may be low.
- kernel.shmall
-
Defines the total number of shared memory pages that can be used system-wide. To use the entire main memory, the value of the
kernel.shmall
parameter should be ≤ total main memory size. - kernel.shmmax
- Defines the maximum size in bytes of a single shared memory segment that a Linux process can allocate in its virtual address space.
- kernel.shmmni
- Defines the maximum number of shared memory segments the database server is able to handle.
- net.ipv4.ip_local_port_range
- Defines the port range the system can use for programs which want to connect to a database server without a specific port number.
- net.core.rmem_default
- Defines the default receive socket memory through Transmission Control Protocol (TCP).
- net.core.rmem_max
- Defines the maximum receive socket memory through Transmission Control Protocol (TCP).
- net.core.wmem_default
- Defines the default send socket memory through Transmission Control Protocol (TCP).
- net.core.wmem_max
- Defines the maximum send socket memory through Transmission Control Protocol (TCP).
- vm.dirty_bytes / vm.dirty_ratio
-
Defines a threshold in bytes / in percentage of dirty-able memory at which a process generating dirty data is started in the
write()
function.
Either vm.dirty_bytes
or vm.dirty_ratio
can be specified at a time.
- vm.dirty_background_bytes / vm.dirty_background_ratio
- Defines a threshold in bytes / in percentage of dirty-able memory at which the kernel tries to actively write dirty data to hard-disk.
Either vm.dirty_background_bytes
or vm.dirty_background_ratio
can be specified at a time.
- vm.dirty_writeback_centisecs
Defines a time interval between periodic wake-ups of the kernel threads responsible for writing dirty data to hard-disk.
This kernel parameters measures in 100th’s of a second.
- vm.dirty_expire_centisecs
Defines the time after which dirty data is old enough to be written to hard-disk.
This kernel parameters measures in 100th’s of a second.
Additional resources
Chapter 9. Getting started with kernel logging
Log files are files that contain messages about the system, including the kernel, services, and applications running on it. The logging system in Red Hat Enterprise Linux is based on the built-in syslog protocol. Various utilities use this system to record events and organize them into log files. These files are useful when auditing the operating system or troubleshooting problems.
9.1. What is the kernel ring buffer
During the boot process, the console provides a lot of important information about the initial phase of the system startup. To avoid loss of the early messages the kernel utilizes what is called a ring buffer. This buffer stores all messages, including boot messages, generated by the printk()
function within the kernel code. The messages from the kernel ring buffer are then read and stored in log files on permanent storage, for example, by the syslog
service.
The buffer mentioned above is a cyclic data structure which has a fixed size, and is hard-coded into the kernel. Users can display data stored in the kernel ring buffer through the dmesg
command or the /var/log/boot.log
file. When the ring buffer is full, the new data overwrites the old.
Additional resources
-
syslog(2)
anddmesg(1)
manual page
9.2. Role of printk on log-levels and kernel logging
Each message the kernel reports has a log-level associated with it that defines the importance of the message. The kernel ring buffer, as described in What is the kernel ring buffer, collects kernel messages of all log-levels. It is the kernel.printk
parameter that defines what messages from the buffer are printed to the console.
The log-level values break down in this order:
- 0 — Kernel emergency. The system is unusable.
- 1 — Kernel alert. Action must be taken immediately.
- 2 — Condition of the kernel is considered critical.
- 3 — General kernel error condition.
- 4 — General kernel warning condition.
- 5 — Kernel notice of a normal but significant condition.
- 6 — Kernel informational message.
- 7 — Kernel debug-level messages.
By default, kernel.printk
in RHEL 9 contains the following four values:
# sysctl kernel.printk
kernel.printk = 7 4 1 7
The four values define the following:
- value. Console log-level, defines the lowest priority of messages printed to the console.
- value. Default log-level for messages without an explicit log-level attached to them.
- value. Sets the lowest possible log-level configuration for the console log-level.
value. Sets default value for the console log-level at boot time.
Each of these values above defines a different rule for handling error messages.
The default 7 4 1 7 printk
value allows for better debugging of kernel activity. However, when coupled with a serial console, this printk
setting is able to cause intense I/O bursts that could lead to a RHEL system becoming temporarily unresponsive. To avoid these situations, setting a printk
value of 4 4 1 7 typically works, but at the expense of losing the extra debugging information.
Also note that certain kernel command line parameters, such as quiet
or debug
, change the default kernel.printk
values.
Additional resources
-
syslog(2)
manual page
Chapter 10. Installing kdump
In many cases, the kdump
service is installed and activated by default on the new Red Hat Enterprise Linux installations. This section includes information about kdump
.
10.1. What is kdump
kdump
is a service providing a crash dumping mechanism. The service enables you to save the contents of the system’s memory for later analysis. kdump
uses the kexec
system call to boot into the second kernel (a capture kernel) without rebooting; and then captures the contents of the crashed kernel’s memory (a crash dump or a vmcore) and saves it. The second kernel resides in a reserved part of the system memory.
A kernel crash dump can be the only information available in the event of a system failure (a critical bug). Therefore, ensuring that kdump
is operational is important in mission-critical environments. Red Hat advise that system administrators regularly update and test kexec-tools
in your normal kernel update cycle. This is especially important when new kernel features are implemented.
10.2. Performing kdump installation
In many cases, the kdump
service is installed and activated by default on new Red Hat Enterprise Linux installations. This procedure provides information on how to install kdump
when it is not enabled by default in some cases.
The Anaconda installer includes a screen for kdump
configuration when performing an interactive installation using the graphical or text interface. The installer screen is titled as KDUMP
and is available from the main Installation Summary screen and only allows limited configuration. You can only enable KDUMP and reserve the required amount of memory.

Some installation options, for example custom Kickstart installations, do not support installing or enabling kdump
by default. In such scenarios, you can install kdump
using the procedure described in this section.
Prerequisites
- An active RHEL subscription
- A repository containing the kexec-tools package for your system CPU architecture
-
Fulfilled requirements for
kdump
configurations and targets
Procedure
Check the status of
kdump
installation on your system:# rpm -q kexec-tools
Output if the package is installed:
# kexec-tools-2.0.22-13.el9.x86_64
Output if the package is not installed:
# package kexec-tools is not installed
Install
kdump
and other necessary packages:# yum install kexec-tools
Chapter 11. Configuring kdump on the command line
The memory for kdump
is reserved during the system boot. The memory size is configured in the system’s Grand Unified Bootloader (GRUB) 2 configuration file. The memory size depends on the crashkernel=
value specified in the configuration file and the size of the system’s physical memory.
11.1. Configuring kdump memory usage
The kexec-tools
package maintains the default crashkernel
memory reservation values. The kdump
service uses the default value to reserve the crashkernel
memory for each kernel.
The automatic memory allocation for kdump
varies based on the system hardware architecture and available memory size. For example, on AMD and Intel 64-bit architectures, the crashkernel
default parameters work only when the available memory is more than 1 GB. By default, kexec-tools
configures the following memory reserves on AMD64 and Intel 64-bit architectures:
crashkernel=1G-4G:192M,4G-64G:256M,64G-:512M
The memory requirement of the crash kernel may vary depending on the hardware and machine specifications. If the default crashkernel
value does not work on your system, you can run the kdumpctl estimate
command and query a rough estimate value without triggering a crash. The estimated crashkernel
value may not be accurate and can serve as a reference to set an appropriate crashkernel
value.
The crashkernel=auto
option in the boot command line is no longer supported on RHEL 9 and later releases.
Prerequisites
- Root privileges
-
Fulfilled
kdump
requirements for configurations and targets. -
On IBM Z systems, ensure the
zipl
utility is installed.
Procedure
Configure the default value for
crashkernel
:# kdumpctl reset-crashkernel --kernel=ALL
(Optional) To use a custom
crashkernel
value:Configure the required memory reserve:
crashkernel=192M
The example reserves 192 MB of memory if the total amount of system memory is 1 GB or higher and lower than 4 GB. If the total amount of memory is more than 4 GB, 256 MB is reserved for
kdump
instead.(Optional) Offset the reserved memory:
Some systems require to reserve memory with a certain fixed offset since crash kernel reservation is very early, and it wants to reserve some area for special usage. If the offset is set, the reserved memory begins there. To offset the reserved memory, use the following syntax:
crashkernel=192M@16M
The example above reserves 192 MB of memory starting at 16 MB (physical address 0x01000000). If the offset parameter is set to 0 or omitted entirely,
kdump
offsets the reserved memory automatically. You can also offset memory when setting a variable memory reservation by specifying the offset as the last value. For example,crashkernel=1G-4G:192M,2G-64G:256M@16M
.
Update the bootloader configuration:
# grubby --update-kernel ALL --args "crashkernel=<CUSTOM-VALUE>”
On IBM Z systems that use the zIPL bootloader, the command adds a new kernel parameter to each
/boot/loader/entries/<ENTRY>.conf
file.On IBM Z systems, to update the boot menu, execute the
zipl
command with no options specified:# zipl
Reboot for changes to take effect:
# reboot
Verification
Activate the
sysrq
key to boot into thekdump
kernel:# echo 1 > /proc/sys/kernel/sysrq # echo c > /proc/sysrq-trigger
This forces the Linux kernel to crash and copy the
address-YYYY-MM-DD-HH:MM:SS/vmcore
file to the target location specified in the configuration file.Verify that the
vmcore
file is dumped in the target as specified in the/etc/kdump.conf
file:$ ls /var/crash/127.0.0.1-2022-01-18-0 /var/crash/127.0.0.1-2022-01-18-05:23:10': kexec-dmesg.log vmcore vmcore-dmesg.txt
In this example, the kernel saves the
vmcore
in the default target directory,/var/crash/
.
11.2. Configuring the kdump target
When a kernel crash is captured, the core dump can be either stored as a file in a local file system, written directly to a device, or sent over a network using the NFS
(Network File System) or SSH
(Secure Shell) protocol. Only one of these options can be set at a time, and the default behavior is to store the vmcore file in the /var/crash/
directory of the local file system.
Prerequisites
-
Fulfilled
kdump
requirements for configurations and targets.
Procedure
To store the
vmcore
file in/var/crash/
directory of the local file system, edit the/etc/kdump.conf
file and specify the path:# path /var/crash
The option
path /var/crash
represents the path to the file system in whichkdump
saves thevmcore
file. When you specify a dump target in the/etc/kdump.conf
file, then thepath
is relative to the specified dump target.If you do not specify a dump target in the
/etc/kdump.conf
file, then thepath
represents the absolute path from the root directory. Depending on what is mounted in the current system, the dump target and the adjusted dump path are taken automatically.
kdump
saves the vmcore
file in /var/crash/var/crash
directory, when the dump target is mounted at /var/crash
and the option path
is also set as /var/crash
in the /etc/kdump.conf
file. For example, in the following instance, the ext4
file system is already mounted at /var/crash
and the path
are set as /var/crash
:
grep -v ^# etc/kdump.conf | grep -v ^$ ext4 /dev/mapper/vg00-varcrashvol path /var/crash core_collector makedumpfile -c --message-level 1 -d 31
This results in the /var/crash/var/crash
path. To solve this problem, use the option path /
instead of path /var/crash
To change the local directory in which the core dump is to be saved, as
root
, edit the/etc/kdump.conf
configuration file as described below.-
Remove the hash sign ("#") from the beginning of the
#path /var/crash
line. Replace the value with the intended directory path. For example:
path /usr/local/cores
ImportantIn RHEL 9, the directory defined as the kdump target using the
path
directive must exist when thekdump
systemd service is started - otherwise the service fails. This behavior is different from earlier releases of RHEL, where the directory was being created automatically if it did not exist when starting the service.
-
Remove the hash sign ("#") from the beginning of the
To write the file to a different partition, as
root
, edit the/etc/kdump.conf
configuration file as described below.Remove the hash sign ("#") from the beginning of the
#ext4
line, depending on your choice.-
device name (the
#ext4 /dev/vg/lv_kdump
line) -
file system label (the
#ext4 LABEL=/boot
line) -
UUID (the
#ext4 UUID=03138356-5e61-4ab3-b58e-27507ac41937
line)
-
device name (the
Change the file system type as well as the device name, label or UUID to the desired values. For example:
ext4 UUID=03138356-5e61-4ab3-b58e-27507ac41937
ImportantIt is recommended to specify storage devices using a
LABEL=
orUUID=
. Disk device names such as/dev/sda3
are not guaranteed to be consistent across reboot.ImportantWhen dumping to Direct Access Storage Device (DASD) on IBM Z hardware, it is essential that the dump devices are correctly specified in
/etc/dasd.conf
before proceeding.
To write the dump directly to a device:
-
Remove the hash sign ("#") from the beginning of the
#raw /dev/vg/lv_kdump
line. Replace the value with the intended device name. For example:
raw /dev/sdb1
-
Remove the hash sign ("#") from the beginning of the
To store the dump to a remote machine using the
NFS
protocol:-
Remove the hash sign ("#") from the beginning of the
#nfs my.server.com:/export/tmp
line. Replace the value with a valid hostname and directory path. For example:
nfs penguin.example.com:/export/cores
-
Remove the hash sign ("#") from the beginning of the
To store the dump to a remote machine using the
SSH
protocol:-
Remove the hash sign ("#") from the beginning of the
#ssh user@my.server.com
line. - Replace the value with a valid username and hostname.
Include your
SSH
key in the configuration.-
Remove the hash sign from the beginning of the
#sshkey /root/.ssh/kdump_id_rsa
line. Change the value to the location of a key valid on the server you are trying to dump to. For example:
ssh john@penguin.example.com sshkey /root/.ssh/mykey
-
Remove the hash sign from the beginning of the
-
Remove the hash sign ("#") from the beginning of the
11.3. Configuring the core collector
The kdump
service uses the core_collector
program to capture the vmcore
image. In RHEL, the makedumpfile
utility is the default core collector.
makedumpfile
is a dump program that helps to copy only the required pages using various dump levels and compress the size of a dump file.
makedumpfile is a dump program that helps to copy only necessary pages using various dump levels and compress the size of a dump file.
Using makedumpfile
, you can create a small size dump file either by compressing dump data or by excluding pages or both. It needs the first kernel debug information to distinguish the not necessary pages by analyzing how the first kernel uses the memory.
Syntax
core_collector makedumpfile -z -d 31 --message-level 1
Options
-
-c
,-l
,-z
, or-p
: specifies the compress dump file format by each page when you use one of these options:-c
forzlib
,-l
forlzo
,-z
forzstd
, or-p
forsnappy
. -
-d
(dump_level)
: excludes pages so that they do not get copied to the dump file. --message-level
: specifies the message types.Using
--message-level
, you can restrict the outputs to print. For example, specifying 7 as the message level prints common messages and error messages. The maximum value for--message_level
is 31.
Prerequisites
-
Fulfilled
kdump
requirements for configurations and targets.
Procedure
As
root
user, edit the/etc/kdump.conf
configuration file to remove the hash sign ("#") from the beginning of the following command:core_collector makedumpfile -z -d 31 --message-level 1
To enable dump file compression, specify one of the
makedumpfile
options:core_collector makedumpfile -z -d 31 --message-level 1
where,
-
-z
specifies thedump
compressed file format. -
-d
specifies dump level as 31. -
--message-level
specifies message level as 1.
-
Also, consider the following example that uses -l
:
To compress a dump file using
-l
:core_collector makedumpfile -l -d 31 --message-level 1
Additional resources
-
makedumpfile(8)
manual page
11.4. Configuring the kdump default failure responses
By default, when kdump
fails to create a crash dump file at the configured target location, the system reboots and the dump is lost in the process. To change this behavior, follow the procedure below.
Prerequisites
- Root permissions.
-
Fulfilled requirements for
kdump
configurations and targets.
Procedure
-
As
root
, remove the hash sign ("#") from the beginning of the#failure_action
line in the/etc/kdump.conf
configuration file. Replace the value with a desired action.
failure_action poweroff
11.5. Configuration file for kdump
The configuration file for kdump
kernel is /etc/sysconfig/kdump
. This file controls the kdump
kernel command line parameters.
For most configurations, use the default options. However, in some scenarios you might need to modify certain parameters to control the kdump
kernel behavior. For example, modifying to append the kdump
kernel command-line to obtain a detailed debugging output.
This section covers information on modifying the KDUMP_COMMANDLINE_REMOVE
and KDUMP_COMMANDLINE_APPEND
options for kdump
. For information on additional configuration options refer to Documentation/admin-guide/kernel-parameters.txt
or the /etc/sysconfig/kdump
file.
KDUMP_COMMANDLINE_REMOVE
This option removes arguments from the current
kdump
command line. It removes parameters that may causekdump
errors orkdump
kernel boot failures. These parameters may have been parsed from the previousKDUMP_COMMANDLINE
process or inherited from the/proc/cmdline
file. When this variable is not configured, it inherits all values from the/proc/cmdline
file. Configuring this option also provides information that is helpful in debugging an issue.Example
To remove certain arguments, add them to
KDUMP_COMMANDLINE_REMOVE
as follows:# KDUMP_COMMANDLINE_REMOVE="hugepages hugepagesz slub_debug quiet log_buf_len swiotlb"
KDUMP_COMMANDLINE_APPEND
This option appends arguments to the current command line. These arguments may have been parsed by the previous
KDUMP_COMMANDLINE_REMOVE
variable.For the
kdump
kernel, disabling certain modules such asmce
,cgroup
,numa
,hest_disable
can help prevent kernel errors. These modules may consume a significant portion of the kernel memory reserved forkdump
or causekdump
kernel boot failures.Example
To disable memory
cgroups
on thekdump
kernel command line, run the command as follows:# KDUMP_COMMANDLINE_APPEND="cgroup_disable=memory"
Additional resources
-
Documentation/admin-guide/kernel-parameters.txt
file -
/etc/sysconfig/kdump
file
11.6. Enabling and disabling the kdump service
To start the kdump
service at boot time, follow the procedure below.
Prerequisites
-
Fulfilled
kdump
requirements for configurations and targets. -
All configurations for installing
kdump
are set up according to your needs.
Procedure
To enable the
kdump
service, use the following command:# systemctl enable kdump.service
This enables the service for
multi-user.target
.To start the service in the current session, use the following command:
# systemctl start kdump.service
To stop the
kdump
service, type the following command:# systemctl stop kdump.service
To disable the
kdump
service, execute the following command:# systemctl disable kdump.service
It is recommended to set kptr_restrict=1
as default. When kptr_restrict
is set to (1) as default, the kdumpctl
service loads the crash kernel even if Kernel Address Space Layout (KASLR) is enabled or not enabled.
Troubleshooting step
When kptr_restrict
is not set to (1), and if KASLR is enabled, the contents of /proc/kore
file are generated as all zeros. Consequently, the kdumpctl
service fails to access the /proc/kcore
and load the crash kernel.
To work around this problem, the kexec-kdump-howto.txt
file displays a warning message, which specifies to keep the recommended setting as kptr_restrict=1
.
To ensure that kdumpctl
service loads the crash kernel, verify that:
-
Kernel
kptr_restrict=1
in thesysctl.conf
file.
11.7. Testing the kdump configuration
You can test that the crash dump process works and is valid before the machine enters production.
The commands below cause the kernel to crash. Use caution when following these steps, and never carelessly use them on active production system.
Procedure
-
Reboot the system with
kdump
enabled. Make sure that
kdump
is running:# systemctl is-active kdump active
Force the Linux kernel to crash:
# echo 1 > /proc/sys/kernel/sysrq # echo c > /proc/sysrq-trigger
WarningThe command above crashes the kernel, and a reboot is required.
Once booted again, the
address-YYYY-MM-DD-HH:MM:SS/vmcore
file is created at the location you have specified in the/etc/kdump.conf
file (by default to/var/crash/
).NoteThis action confirms the validity of the configuration. Also it is possible to use this action to record how long it takes for a crash dump to complete with a representative work-load.
11.8. Preventing kernel drivers from loading for kdump
This section explains how to prevent the capture kernel from loading certain kernel drivers using the /etc/sysconfig/kdump
configuration file. You can prevent the kdump
initramfs from loading the specified kernel module. To achieve this, you need to put the KDUMP_COMMANDLINE_APPEND=
variable in the /etc/sysconfig/kdump
file. This helps to prevent the out-of-memory (oom) killer or other crash kernel failures.
You can append the KDUMP_COMMANDLINE_APPEND=
variable using one of the following configuration options:
-
rd.driver.blacklist=<modules>
-
modprobe.blacklist=<modules>
Procedure
Select a kernel module that you intend to block from loading.
$ lsmod Module Size Used by fuse 126976 3 xt_CHECKSUM 16384 1 ipt_MASQUERADE 16384 1 uinput 20480 1 xt_conntrack 16384 1
The
lsmod
command displays a list of modules that are loaded to the currently running kernel.Update the
KDUMP_COMMANDLINE_APPEND=
variable in the/etc/sysconfig/kdump
file.# KDUMP_COMMANDLINE_APPEND="rd.driver.blacklist=hv_vmbus,hv_storvsc,hv_utils,hv_netvsc,hid-hyperv"
Also,consider the following example using the
modprobe.blacklist=<modules>
configuration option.# KDUMP_COMMANDLINE_APPEND="modprobe.blacklist=emcp modprobe.blacklist=bnx2fc modprobe.blacklist=libfcoe modprobe.blacklist=fcoe"
Restart the
kdump
service.# systemctl restart kdump
Additional resources
-
dracut.cmdline
manual page
11.9. Running kdump on systems with encrypted disk
When you run a Linux Unified Key Setup (LUKS) encrypted partition, the system require certain amount of available memory. If the system has less than the required amount of available memory, the systemd-cryptsetup
service fails to mount the partition. As a result, capturing the vmcore
file to an encrypted target location fails in the second kernel (capture kernel).
The kdumpctl estimate
command helps you estimate the amount of memory you need for kdump
. It prints the recommended crashkernel
value, which is the most suitable memory size required for kdump
.
The recommended crashkernel
value is calculated based on the current kernel size, kernel modules, initramfs
, and the LUKS encrypted target memory requirement.
In case you use the custom crashkernel
option, kdumpctl estimate
prints the LUKS required size
value. The value is the memory size required for LUKS encrypted target.
Procedure
Print the estimate
crashkernel
value:# kdumpctl estimate Encrypted kdump target requires extra memory, assuming using the keyslot with minimum memory requirement Reserved crashkernel: 256M Recommended crashkernel: 652M Kernel image size: 47M Kernel modules size: 8M Initramfs size: 20M Runtime reservation: 64M LUKS required size: 512M Large modules: none WARNING: Current crashkernel size is lower than recommended size 652M.
Configure the amount of required memory by increasing
crashkernel
to the desired value.# grubby –args=”crashkernel=652M” --update-kernel=ALL
Reboot for changes to take effect.
# reboot
If the kdump
service still fails to save the dump file to the encrypted target, increase the crashkernel
value gradually to configure an appropriate amount of memory.
Chapter 12. Supported kdump configurations and targets
12.1. Memory requirements for kdump
In order for kdump
to be able to capture a kernel crash dump and save it for further analysis, a part of the system memory has to be permanently reserved for the capture kernel. When reserved, this part of the system memory is not available to the main kernel.
The memory requirements vary based on certain system parameters. One of the major factors is the system’s hardware architecture. To find out the exact machine architecture (such as Intel 64 and AMD64, also known as x86_64) and print it to standard output, use the following command:
$ uname -m
The following table lists the minimum memory requirements to automatically reserve a memory size for kdump
on the latest available versions. The size changes according to the system’s architecture and total available physical memory.
Table 12.1. Minimum amount of reserved memory required for kdump
Architecture | Available Memory | Minimum Reserved Memory |
---|---|---|
AMD64 and Intel 64 ( | 1 GB to 4 GB | 160 MB of RAM. |
4 GB to 64 GB | 192 MB of RAM. | |
64 GB to 1 TB | 256 MB of RAM. | |
1 TB and more | 512 MB of RAM. | |
64-bit ARM architecture ( | 2 GB and more | 448 MB of RAM. |
IBM Power Systems ( | 2 GB to 4 GB | 384 MB of RAM. |
4 GB to 16 GB | 512 MB of RAM. | |
16 GB to 64 GB | 1 GB of RAM. | |
64 GB to 128 GB | 2 GB of RAM. | |
128 GB and more | 4 GB of RAM. | |
IBM Z ( | 1 GB to 4 GB | 160 MB of RAM. |
4 GB to 64 GB | 192 MB of RAM. | |
64 GB to 1 TB | 256 MB of RAM. | |
1 TB and more | 512 MB of RAM. |
On many systems, kdump
is able to estimate the amount of required memory and reserve it automatically. This behavior is enabled by default, but only works on systems that have more than a certain amount of total available memory, which varies based on the system architecture.
The configuration of reserved memory based on the total amount of memory in the system is a best effort estimation. The actual required memory may vary due to other factors such as I/O devices. Using not enough of memory might cause that a debug kernel is not able to boot as a capture kernel in case of a kernel panic. To avoid this problem, sufficiently increase the crash kernel memory.
12.2. Minimum threshold for memory reservation
The kexec-tools
utility, by default, configures the crashkernel
command line parameter and reserves a certain amount of memory for kdump
. For the default memory reservation to work, a certain amount of total memory must be available in the system. The amount of memory required differs based on the system’s architecture.
The following table lists the minimum threshold values for memory allocation. If the system has memory less than the specified threshold value, you must configure the memory manually.
Table 12.2. Minimum amount of memory required for memory reservation
Architecture | Required Memory |
---|---|
AMD64 and Intel 64 ( | 1 GB |
IBM Power Systems ( | 2 GB |
IBM Z ( | 1 GB |
ARM (aarch64) | 2GB |
12.3. Supported kdump targets
When a kernel crash is captured, the vmcore dump file can be either written directly to a device, stored as a file on a local file system, or sent over a network. The table below contains a complete list of dump targets that are currently supported or explicitly unsupported by kdump
.
Type | Supported Targets | Unsupported Targets |
---|---|---|
Raw device | All locally attached raw disks and partitions. | |
Local file system |
|
Any local file system not explicitly listed as supported in this table, including the |
Remote directory |
Remote directories accessed using the |
Remote directories on the |
Remote directories accessed using the |
Remote directories accessed using the | Multipath-based storages. |
Remote directories accessed over | ||
Remote directories accessed using the | ||
Remote directories accessed using the | ||
Remote directories accessed using wireless network interfaces. |
Utilizing firmware assisted dump (fadump
) to capture a vmcore and store it to a remote machine using SSH or NFS protocol causes renaming of the network interface to kdump-<interface-name>
. The renaming happens if the <interface-name>
is generic, for example *eth#
, net#
, and so on. This problem occurs because the vmcore capture scripts in the initial RAM disk (initrd
) add the kdump- prefix to the network interface name to secure persistent naming. Since the same initrd
is used also for a regular boot, the interface name is changed for the production kernel too.
12.4. Supported kdump filtering levels
To reduce the size of the dump file, kdump
uses the makedumpfile
core collector to compress the data and optionally to omit unwanted information. The table below contains a complete list of filtering levels that are currently supported by the makedumpfile
utility.
Option | Description |
---|---|
| Zero pages |
| Cache pages |
| Cache private |
| User pages |
| Free pages |
The makedumpfile
command supports removal of transparent huge pages and hugetlbfs pages. Consider both these types of hugepages User Pages and remove them using the -8
level.
12.5. Supported default failure responses
By default, when kdump
fails to create a core dump, the operating system reboots. You can, however, configure kdump
to perform a different operation in case it fails to save the core dump to the primary target. The table below lists all default actions that are currently supported.
Option | Description |
---|---|
| Attempt to save the core dump to the root file system. This option is especially useful in combination with a network target: if the network target is unreachable, this option configures kdump to save the core dump locally. The system is rebooted afterwards. |
| Reboot the system, losing the core dump in the process. |
| Halt the system, losing the core dump in the process. |
| Power off the system, losing the core dump in the process. |
| Run a shell session from within the initramfs, allowing the user to record the core dump manually. |
|
Enable additional operations such as |
|
Specifies the action to perform when a dump might fail in the event of a kernel crash. The default |
12.6. Using final_action parameter
The final_action
parameter enables you to use certain additional operations such as reboot
, halt
, and poweroff
actions after a successful kdump
or when an invoked failure_action
mechanism using shell
or dump_to_rootfs
completes. If the final_action
option is not specified, it defaults to reboot
.
Procedure
To configure
final_action
, edit the/etc/kdump.conf
file and add one of the following options:# final_action <reboot | halt | poweroff>
Restart the
kdump
service for the changes to take effect:# kdumpctl restart
12.7. Using failure_action parameter
The failure_action
parameter specifies the action to perform when a dump fails in the event of a kernel crash. The default action for failure_action
is reboot
, which reboots the system.
failure_action
specifies one of the following actions to take:
-
reboot
: reboots the system after a dump failure. -
dump_to_rootfs
: saves the dump file on a root file system when a non-root dump target is configured. -
halt
: halts the system. -
poweroff
: stops the running operations on the system. -
shell
: starts a shell session insideinitramfs
, from which you can manually perform additional recovery actions.
Procedure:
To configure an action to take if the dump fails, edit the
/etc/kdump.conf
file and specify one of thefailure_action
options:# failure_action <reboot | halt | poweroff | shell | dump_to_rootfs>
Restart the
kdump
service for the changes to take effect:# kdumpctl restart
Chapter 13. Firmware assisted dump mechanisms
Firmware assisted dump (fadump) is a dump capturing mechanism, provided as an alternative to the kdump
mechanism on IBM POWER systems. The kexec
and kdump
mechanisms are useful for capturing core dumps on AMD64 and Intel 64 systems. However, some hardware such as mini systems and mainframe computers, leverage the onboard firmware to isolate regions of memory and prevent any accidental overwriting of data that is important to the crash analysis. This section covers fadump
mechanisms and how they integrate with RHEL. The fadump
utility is optimized for these expanded dumping features on IBM POWER systems.
13.1. Firmware assisted dump on IBM PowerPC hardware
The fadump
utility captures the vmcore
file from a fully-reset system with PCI and I/O devices. This mechanism uses firmware to preserve memory regions during a crash and then reuses the kdump
userspace scripts to save the vmcore
file. The memory regions consist of all system memory contents, except the boot memory, system registers, and hardware Page Table Entries (PTEs).
The fadump
mechanism offers improved reliability over the traditional dump type, by rebooting the partition and using a new kernel to dump the data from the previous kernel crash. The fadump
requires an IBM POWER6 processor-based or later version hardware platform.
For further details about the fadump
mechanism, including PowerPC specific methods of resetting hardware, see the /usr/share/doc/kexec-tools/fadump-howto.txt
file.
The area of memory that is not preserved, known as boot memory, is the amount of RAM required to successfully boot the kernel after a crash event. By default, the boot memory size is 256MB or 5% of total system RAM, whichever is larger.
Unlike kexec-initiated
event, the fadump
mechanism uses the production kernel to recover a crash dump. When booting after a crash, PowerPC hardware makes the device node /proc/device-tree/rtas/ibm.kernel-dump
available to the proc
filesystem (procfs
). The fadump-aware kdump
scripts, check for the stored vmcore
, and then complete the system reboot cleanly.
13.2. Enabling firmware assisted dump mechanism
You can enhance the crash dumping capabilities of IBM POWER systems by enabling the firmware assisted dump (fadump) mechanism
Prerequisites
- Root access
Procedure
-
Install the
kexec-tools
package. Configure the default value for
crashkernel
.# kdumpctl reset-crashkernel –fadump=on --kernel=ALL
(Optional) Reserve boot memory instead of the default value.
# grubby --update-kernel ALL --args=”fadump=on crashkernel=xxM"
where,
xx
is the required memory size in megabytes.NoteWhen specifying boot configuration options, test the configuration by rebooting the kernel with
kdump
enabled. If thekdump
kernel fails to boot, increase thecrashkernel
value gradually to set an appropriate value.Reboot for changes to take effect.
# reboot
13.3. Firmware assisted dump mechanisms on IBM Z hardware
IBM Z systems supports two firmware assisted dump mechanisms: Stand-alone dump (sadump
) and VMDUMP
dump file.
The kdump
infrastructure is supported and utilized on IBM Z systems. However, using one of the firmware assisted dump (fadump
) methods for IBM Z can provide various benefits:
-
The
sadump
mechanism is initiated and controlled from the system console, and is stored on anIPL
bootable device. -
The
VMDUMP
mechanism is similar tosadump
. This tool is initiated from the system console, but retrieves the resulting dump from hardware and copies it to the system for analysis. -
These methods, similar to other hardware-based dump mechanisms, have the ability to capture the state of a machine in the early boot phase, before the
kdump
service starts. -
Although
VMDUMP
contains a mechanism to receive the dump file into a Red Hat Enterprise Linux system, the configuration and control ofVMDUMP
is managed from the IBM Z Hardware console.
13.4. Using sadump on Fujitsu PRIMEQUEST systems
The Fujitsu sadump
mechanism is designed to provide a fallback
dump capture in an event when kdump
is unable to complete successfully. The sadump
mechanism is invoked manually from the system Management Board (MMB) interface. Using MMB, configure kdump
like for an Intel 64 or AMD 64 server and then perform the following additional steps to enable sadump
.
Procedure
Add or edit the following lines in the
/etc/sysctl.conf
file to ensure thatkdump
starts as expected forsadump
:# kernel.panic=0 kernel.unknown_nmi_panic=1
WarningIn particular, ensure that after
kdump
, the system does not reboot. If the system reboots afterkdump
has fails to save thevmcore
file, then it is not possible to invoke thesadump
.Set the
failure_action
parameter in/etc/kdump.conf
appropriately ashalt
orshell
.# failure_action shell
Additional resources
- The FUJITSU Server PRIMEQUEST 2000 Series Installation Manual
Chapter 14. Analyzing a core dump
To determine the cause of the system crash, you can use the crash utility, which provides an interactive prompt very similar to the GNU Debugger (GDB). This utility allows you to interactively analyze a core dump created by kdump
, netdump
, diskdump
or xendump
as well as a running Linux system. Alternatively, you have the option to use Kernel Oops Analyzer or the Kdump Helper tool.
14.1. Installing the crash utility
The following procedure describes how to install the crash analyzing tool.
Procedure
Enable the relevant repositories:
# subscription-manager repos --enable pass:quotes[baseos repository]
# subscription-manager repos --enable pass:quotes[appstream repository]
# subscription-manager repos --enable pass:quotes[rhel-9-for-x86_64-baseos-debug-rpms]
Install the
crash
package:# yum install crash
Install the
kernel-debuginfo
package:# yum install kernel-debuginfo
The package corresponds to your running kernel and provides the data necessary for the dump analysis.
Additional resources
14.2. Running and exiting the crash utility
The following procedure describes how to start the crash utility for analyzing the cause of the system crash.
Prerequisites
-
Identify the currently running kernel (for example
4.18.0-5.el9.x86_64
).
Procedure
To start the
crash
utility, two necessary parameters need to be passed to the command:-
The debug-info (a decompressed vmlinuz image), for example
/usr/lib/debug/lib/modules/4.18.0-5.el9.x86_64/vmlinux
provided through a specifickernel-debuginfo
package. The actual vmcore file, for example
/var/crash/127.0.0.1-2018-10-06-14:05:33/vmcore
The resulting
crash
command then looks like this:# crash /usr/lib/debug/lib/modules/4.18.0-5.el9.x86_64/vmlinux /var/crash/127.0.0.1-2018-10-06-14:05:33/vmcore
Use the same <kernel> version that was captured by
kdump
.Running the crash utility
The following example shows analyzing a core dump created on October 6 2018 at 14:05 PM, using the 4.18.0-5.el8.x86_64 kernel.
WARNING: kernel relocated [202MB]: patching 90160 gdb minimal_symbol values KERNEL: /usr/lib/debug/lib/modules/4.18.0-5.el8.x86_64/vmlinux DUMPFILE: /var/crash/127.0.0.1-2018-10-06-14:05:33/vmcore [PARTIAL DUMP] CPUS: 2 DATE: Sat Oct 6 14:05:16 2018 UPTIME: 01:03:57 LOAD AVERAGE: 0.00, 0.00, 0.00 TASKS: 586 NODENAME: localhost.localdomain RELEASE: 4.18.0-5.el8.x86_64 VERSION: #1 SMP Wed Aug 29 11:51:55 UTC 2018 MACHINE: x86_64 (2904 Mhz) MEMORY: 2.9 GB PANIC: "sysrq: SysRq : Trigger a crash" PID: 10635 COMMAND: "bash" TASK: ffff8d6c84271800 [THREAD_INFO: ffff8d6c84271800] CPU: 1 STATE: TASK_RUNNING (SYSRQ) crash>
-
The debug-info (a decompressed vmlinuz image), for example
To exit the interactive prompt and terminate crash, type
exit
orq
.crash> exit ~]#
Additional resources
14.3. Displaying various indicators in the crash utility
The following procedures describe how to use the crash utility and display various indicators, such as a kernel message buffer, a backtrace, a process status, virtual memory information and open files.
- Displaying the message buffer
-
To display the kernel message buffer, type the
log
command at the interactive prompt as displayed in the example below:
crash> log ... several lines omitted ... EIP: 0060:[<c068124f>] EFLAGS: 00010096 CPU: 2 EIP is at sysrq_handle_crash+0xf/0x20 EAX: 00000063 EBX: 00000063 ECX: c09e1c8c EDX: 00000000 ESI: c0a09ca0 EDI: 00000286 EBP: 00000000 ESP: ef4dbf24 DS: 007b ES: 007b FS: 00d8 GS: 00e0 SS: 0068 Process bash (pid: 5591, ti=ef4da000 task=f196d560 task.ti=ef4da000) Stack: c068146b c0960891 c0968653 00000003 00000000 00000002 efade5c0 c06814d0 <0> fffffffb c068150f b7776000 f2600c40 c0569ec4 ef4dbf9c 00000002 b7776000 <0> efade5c0 00000002 b7776000 c0569e60 c051de50 ef4dbf9c f196d560 ef4dbfb4 Call Trace: [<c068146b>] ? __handle_sysrq+0xfb/0x160 [<c06814d0>] ? write_sysrq_trigger+0x0/0x50 [<c068150f>] ? write_sysrq_trigger+0x3f/0x50 [<c0569ec4>] ? proc_reg_write+0x64/0xa0 [<c0569e60>] ? proc_reg_write+0x0/0xa0 [<c051de50>] ? vfs_write+0xa0/0x190 [<c051e8d1>] ? sys_write+0x41/0x70 [<c0409adc>] ? syscall_call+0x7/0xb Code: a0 c0 01 0f b6 41 03 19 d2 f7 d2 83 e2 03 83 e0 cf c1 e2 04 09 d0 88 41 03 f3 c3 90 c7 05 c8 1b 9e c0 01 00 00 00 0f ae f8 89 f6 <c6> 05 00 00 00 00 01 c3 89 f6 8d bc 27 00 00 00 00 8d 50 d0 83 EIP: [<c068124f>] sysrq_handle_crash+0xf/0x20 SS:ESP 0068:ef4dbf24 CR2: 0000000000000000
Type
help log
for more information on the command usage.NoteThe kernel message buffer includes the most essential information about the system crash and, as such, it is always dumped first in to the
vmcore-dmesg.txt
file. This is useful when an attempt to get the fullvmcore
file failed, for example because of lack of space on the target location. By default,vmcore-dmesg.txt
is located in the/var/crash/
directory.-
To display the kernel message buffer, type the
- Displaying a backtrace
-
To display the kernel stack trace, use the
bt
command.
crash> bt PID: 5591 TASK: f196d560 CPU: 2 COMMAND: "bash" #0 [ef4dbdcc] crash_kexec at c0494922 #1 [ef4dbe20] oops_end at c080e402 #2 [ef4dbe34] no_context at c043089d #3 [ef4dbe58] bad_area at c0430b26 #4 [ef4dbe6c] do_page_fault at c080fb9b #5 [ef4dbee4] error_code (via page_fault) at c080d809 EAX: 00000063 EBX: 00000063 ECX: c09e1c8c EDX: 00000000 EBP: 00000000 DS: 007b ESI: c0a09ca0 ES: 007b EDI: 00000286 GS: 00e0 CS: 0060 EIP: c068124f ERR: ffffffff EFLAGS: 00010096 #6 [ef4dbf18] sysrq_handle_crash at c068124f #7 [ef4dbf24] __handle_sysrq at c0681469 #8 [ef4dbf48] write_sysrq_trigger at c068150a #9 [ef4dbf54] proc_reg_write at c0569ec2 #10 [ef4dbf74] vfs_write at c051de4e #11 [ef4dbf94] sys_write at c051e8cc #12 [ef4dbfb0] system_call at c0409ad5 EAX: ffffffda EBX: 00000001 ECX: b7776000 EDX: 00000002 DS: 007b ESI: 00000002 ES: 007b EDI: b7776000 SS: 007b ESP: bfcb2088 EBP: bfcb20b4 GS: 0033 CS: 0073 EIP: 00edc416 ERR: 00000004 EFLAGS: 00000246
Type
bt <pid>
to display the backtrace of a specific process or typehelp bt
for more information onbt
usage.-
To display the kernel stack trace, use the
- Displaying a process status
-
To display the status of processes in the system, use the
ps
command.
crash>
ps
PID PPID CPU TASK ST %MEM VSZ RSS COMM > 0 0 0 c09dc560 RU 0.0 0 0 [swapper] > 0 0 1 f7072030 RU 0.0 0 0 [swapper] 0 0 2 f70a3a90 RU 0.0 0 0 [swapper] > 0 0 3 f70ac560 RU 0.0 0 0 [swapper] 1 0 1 f705ba90 IN 0.0 2828 1424 init ... several lines omitted ... 5566 1 1 f2592560 IN 0.0 12876 784 auditd 5567 1 2 ef427560 IN 0.0 12876 784 auditd 5587 5132 0 f196d030 IN 0.0 11064 3184 sshd > 5591 5587 2 f196d560 RU 0.0 5084 1648 bashUse
ps <pid>
to display the status of a single specific process. Use help ps for more information onps
usage.-
To display the status of processes in the system, use the
- Displaying virtual memory information
-
To display basic virtual memory information, type the
vm
command at the interactive prompt.
crash> vm PID: 5591 TASK: f196d560 CPU: 2 COMMAND: "bash" MM PGD RSS TOTAL_VM f19b5900 ef9c6000 1648k 5084k VMA START END FLAGS FILE f1bb0310 242000 260000 8000875 /lib/ld-2.12.so f26af0b8 260000 261000 8100871 /lib/ld-2.12.so efbc275c 261000 262000 8100873 /lib/ld-2.12.so efbc2a18 268000 3ed000 8000075 /lib/libc-2.12.so efbc23d8 3ed000 3ee000 8000070 /lib/libc-2.12.so efbc2888 3ee000 3f0000 8100071 /lib/libc-2.12.so efbc2cd4 3f0000 3f1000 8100073 /lib/libc-2.12.so efbc243c 3f1000 3f4000 100073 efbc28ec 3f6000 3f9000 8000075 /lib/libdl-2.12.so efbc2568 3f9000 3fa000 8100071 /lib/libdl-2.12.so efbc2f2c 3fa000 3fb000 8100073 /lib/libdl-2.12.so f26af888 7e6000 7fc000 8000075 /lib/libtinfo.so.5.7 f26aff2c 7fc000 7ff000 8100073 /lib/libtinfo.so.5.7 efbc211c d83000 d8f000 8000075 /lib/libnss_files-2.12.so efbc2504 d8f000 d90000 8100071 /lib/libnss_files-2.12.so efbc2950 d90000 d91000 8100073 /lib/libnss_files-2.12.so f26afe00 edc000 edd000 4040075 f1bb0a18 8047000 8118000 8001875 /bin/bash f1bb01e4 8118000 811d000 8101873 /bin/bash f1bb0c70 811d000 8122000 100073 f26afae0 9fd9000 9ffa000 100073 ... several lines omitted ...
Use
vm <pid>
to display information on a single specific process, or usehelp vm
for more information onvm
usage.-
To display basic virtual memory information, type the
- Displaying open files
-
To display information about open files, use the
files
command.
crash>
files
PID: 5591 TASK: f196d560 CPU: 2 COMMAND: "bash" ROOT: / CWD: /root FD FILE DENTRY INODE TYPE PATH 0 f734f640 eedc2c6c eecd6048 CHR /pts/0 1 efade5c0 eee14090 f00431d4 REG /proc/sysrq-trigger 2 f734f640 eedc2c6c eecd6048 CHR /pts/0 10 f734f640 eedc2c6c eecd6048 CHR /pts/0 255 f734f640 eedc2c6c eecd6048 CHR /pts/0Use
files <pid>
to display files opened by only one selected process, or usehelp files
for more information onfiles
usage.-
To display information about open files, use the
14.4. Using Kernel Oops Analyzer
The Kernel Oops Analyzer tool analyzes the crash dump by comparing the oops messages with known issues in the knowledge base.
Prerequisites
- Secure an oops message to feed the Kernel Oops Analyzer.
Procedure
- Access the Kernel Oops Analyzer tool.
To diagnose a kernel crash issue, upload a kernel oops log generated in
vmcore
.Alternatively you can also diagnose a kernel crash issue by providing a text message or a
vmcore-dmesg.txt
as an input.
-
Click
DETECT
to compare the oops message based on information from themakedumpfile
against known solutions.
Additional resources
14.5. The Kdump Helper tool
The Kdump Helper tool helps to set up the kdump
using the provided information. Kdump Helper generates a configuration script based on your preferences. Initiating and running the script on your server sets up the kdump
service.
Additional resources
Chapter 15. Applying patches with kernel live patching
You can use the Red Hat Enterprise Linux kernel live patching solution to patch a running kernel without rebooting or restarting any processes.
With this solution, system administrators:
- Can immediately apply critical security patches to the kernel.
- Do not have to wait for long-running tasks to complete, for users to log off, or for scheduled downtime.
- Control the system’s uptime more and do not sacrifice security or stability.
Note that not every critical or important CVE will be resolved using the kernel live patching solution. Our goal is to reduce the required reboots for security-related patches, not to eliminate them entirely. For more details about the scope of live patching, see the Customer Portal Solutions article.
Some incompatibilities exist between kernel live patching and other kernel subcomponents. Read the Limitations of kpatch carefully before using kernel live patching.
15.1. Limitations of kpatch
-
The
kpatch
feature is not a general-purpose kernel upgrade mechanism. It is used for applying simple security and bug fix updates when rebooting the system is not immediately possible. -
Do not use the
SystemTap
orkprobe
tools during or after loading a patch. The patch could fail to take effect until after such probes have been removed.
15.2. Support for third-party live patching
The kpatch
utility is the only kernel live patching utility supported by Red Hat with the RPM modules provided by Red Hat repositories. Red Hat will not support any live patches which were not provided by Red Hat itself.
If you require support for an issue that arises with a third-party live patch, Red Hat recommends that you open a case with the live patching vendor at the outset of any investigation in which a root cause determination is necessary. This allows the source code to be supplied if the vendor allows, and for their support organization to provide assistance in root cause determination prior to escalating the investigation to Red Hat Support.
For any system running with third-party live patches, Red Hat reserves the right to ask for reproduction with Red Hat shipped and supported software. In the event that this is not possible, we require a similar system and workload be deployed on your test environment without live patches applied, to confirm if the same behavior is observed.
For more information about third-party software support policies, see How does Red Hat Global Support Services handle third-party software, drivers, and/or uncertified hardware/hypervisors or guest operating systems?
15.3. Access to kernel live patches
Kernel live patching capability is implemented as a kernel module (kmod
) that is delivered as an RPM package.
All customers have access to kernel live patches, which are delivered through the usual channels. However, customers who do not subscribe to an extended support offering will lose access to new patches for the current minor release once the next minor release becomes available. For example, customers with standard subscriptions will only be able to live patch RHEL 9.1 kernel until the RHEL 9.2 kernel is released.
15.4. Components of kernel live patching
The components of kernel live patching are as follows:
- Kernel patch module
- The delivery mechanism for kernel live patches.
- A kernel module which is built specifically for the kernel being patched.
- The patch module contains the code of the desired fixes for the kernel.
-
The patch modules register with the
livepatch
kernel subsystem and provide information about original functions to be replaced, with corresponding pointers to the replacement functions. Kernel patch modules are delivered as RPMs. -
The naming convention is
kpatch_<kernel version>_<kpatch version>_<kpatch release>
. The "kernel version" part of the name has dots replaced with underscores.
- The
kpatch
utility - A command-line utility for managing patch modules.
- The
kpatch
service -
A
systemd
service required bymultiuser.target
. This target loads the kernel patch module at boot time. - The
kpatch-dnf
package - A DNF plugin delivered in the form of an RPM package. This plugin manages automatic subscription to kernel live patches.
15.5. How kernel live patching works
The kpatch
kernel patching solution uses the livepatch
kernel subsystem to redirect old functions to new ones. When a live kernel patch is applied to a system, the following things happen:
-
The kernel patch module is copied to the
/var/lib/kpatch/
directory and registered for re-application to the kernel bysystemd
on next boot. -
The kpatch module is loaded into the running kernel and the new functions are registered to the
ftrace
mechanism with a pointer to the location in memory of the new code. -
When the kernel accesses the patched function, it is redirected by the
ftrace
mechanism which bypasses the original functions and redirects the kernel to patched version of the function.
Figure 15.1. How kernel live patching works

15.6. Subscribing the currently installed kernels to the live patching stream
A kernel patch module is delivered in an RPM package, specific to the version of the kernel being patched. Each RPM package will be cumulatively updated over time.
The following procedure explains how to subscribe to all future cumulative live patching updates for a given kernel. Because live patches are cumulative, you cannot select which individual patches are deployed for a given kernel.
Red Hat does not support any third party live patches applied to a Red Hat supported system.
Prerequisites
- Root permissions
Procedure
Optionally, check your kernel version:
# uname -r 5.14.0-1.el9.x86_64
Search for a live patching package that corresponds to the version of your kernel:
# dnf search $(uname -r)
Install the live patching package:
# dnf install "kpatch-patch = $(uname -r)"
The command above installs and applies the latest cumulative live patches for that specific kernel only.
If the version of a live patching package is 1-1 or higher, the package will contain a patch module. In that case the kernel will be automatically patched during the installation of the live patching package.
The kernel patch module is also installed into the
/var/lib/kpatch/
directory to be loaded by thesystemd
system and service manager during the future reboots.NoteAn empty live patching package will be installed when there are no live patches available for a given kernel. An empty live patching package will have a kpatch_version-kpatch_release of 0-0, for example
kpatch-patch-5_14_0-1-0-0.x86_64.rpm
. The installation of the empty RPM subscribes the system to all future live patches for the given kernel.Optionally, verify that the kernel is patched:
# kpatch list Loaded patch modules: kpatch_5_14_0_1_0_1 [enabled] Installed patch modules: kpatch_5_14_0_1_0_1 (5.14.0-1.el9.x86_64) …
The output shows that the kernel patch module has been loaded into the kernel, which is now patched with the latest fixes from the
kpatch-patch-5_14_0-1-0-1.el9.x86_64.rpm
package.
Additional resources
-
kpatch(1)
manual page - Configuring basic system settings in RHEL
15.7. Automatically subscribing any future kernel to the live patching stream
You can use the kpatch-dnf
DNF plugin to subscribe your system to fixes delivered by the kernel patch module, also known as kernel live patches. The plugin enables automatic subscription for any kernel the system currently uses, and also for kernels to-be-installed in the future.
Prerequisites
- You have root permissions.
Procedure
Optionally, check all installed kernels and the kernel you are currently running:
# dnf list installed | grep kernel Updating Subscription Management repositories. Installed Packages ... kernel-core.x86_64 5.14.0-1.el9 @beaker-BaseOS kernel-core.x86_64 5.14.0-2.el9 @@commandline ... # uname -r 5.14.0-2.el9.x86_64
Install the
kpatch-dnf
plugin:# dnf install kpatch-dnf
Enable automatic subscription to kernel live patches:
# dnf kpatch auto Updating Subscription Management repositories. Last metadata expiration check: 1:38:21 ago on Fri 17 Sep 2021 07:29:53 AM EDT. Dependencies resolved. ================================================== Package Architecture ================================================== Installing: kpatch-patch-5_14_0-1 x86_64 kpatch-patch-5_14_0-2 x86_64 Transaction Summary =================================================== Install 2 Packages …
This command subscribes all currently installed kernels to receiving kernel live patches. The command also installs and applies the latest cumulative live patches, if any, for all installed kernels.
In the future, when you update the kernel, live patches will automatically be installed during the new kernel installation process.
The kernel patch module is also installed into the
/var/lib/kpatch/
directory to be loaded by thesystemd
system and service manager during future reboots.NoteAn empty live patching package will be installed when there are no live patches available for a given kernel. An empty live patching package will have a kpatch_version-kpatch_release of 0-0, for example
kpatch-patch-5_14_0-1-0-0.el9.x86_64.rpm
.The installation of the empty RPM subscribes the system to all future live patches for the given kernel.
Verification step
Verify that all installed kernels have been patched:
# kpatch list Loaded patch modules: kpatch_5_14_0_2_0_1 [enabled] Installed patch modules: kpatch_5_14_0_1_0_1 (5.14.0-1.el9.x86_64) kpatch_5_14_0_2_0_1 (5.14.0-2.el9.x86_64)
The output shows that both the kernel you are running, and the other installed kernel have been patched with fixes from
kpatch-patch-5_14_0-1-0-1.el9.x86_64.rpm
andkpatch-patch-5_14_0-2-0-1.el9.x86_64.rpm
packages respectively.
Additional resources
-
kpatch(1)
anddnf-kpatch(8)
manual pages - Configuring basic system settings in RHEL
15.8. Disabling automatic subscription to the live patching stream
When you subscribe your system to fixes delivered by the kernel patch module, your subscription is automatic. You can disable this feature, and thus disable automatic installation of kpatch-patch
packages.
Prerequisites
- You have root permissions.
Procedure
Optionally, check all installed kernels and the kernel you are currently running:
# dnf list installed | grep kernel Updating Subscription Management repositories. Installed Packages ... kernel-core.x86_64 5.14.0-1.el9 @beaker-BaseOS kernel-core.x86_64 5.14.0-2.el9 @@commandline ... # uname -r 5.14.0-2.el9.x86_64
Disable automatic subscription to kernel live patches:
# dnf kpatch manual Updating Subscription Management repositories.
Verification step
You can check for the successful outcome:
# yum kpatch status ... Updating Subscription Management repositories. Last metadata expiration check: 0:30:41 ago on Tue Jun 14 15:59:26 2022. Kpatch update setting: manual
Additional resources
-
kpatch(1)
anddnf-kpatch(8)
manual pages
15.9. Updating kernel patch modules
Since kernel patch modules are delivered and applied through RPM packages, updating a cumulative kernel patch module is like updating any other RPM package.
Prerequisites
- The system is subscribed to the live patching stream, as described in Subscribing the currently installed kernels to the live patching stream.
Procedure
Update to a new cumulative version for the current kernel:
# dnf update "kpatch-patch = $(uname -r)"
The command above automatically installs and applies any updates that are available for the currently running kernel. Including any future released cumulative live patches.
Alternatively, update all installed kernel patch modules:
# dnf update "kpatch-patch"
When the system reboots into the same kernel, the kernel is automatically live patched again by the kpatch.service
systemd service.
Additional resources
15.10. Removing the live patching package
The following procedure describes how to disable the Red Hat Enterprise Linux kernel live patching solution by removing the live patching package.
Prerequisites
- Root permissions
- The live patching package is installed.
Procedure
Select the live patching package:
# dnf list installed | grep kpatch-patch kpatch-patch-5_14_0-1.x86_64 0-1.el9 @@commandline …
The example output above lists live patching packages that you installed.
Remove the live patching package:
# dnf remove kpatch-patch-5_14_0-1.x86_64
When a live patching package is removed, the kernel remains patched until the next reboot, but the kernel patch module is removed from disk. On future reboot, the corresponding kernel will no longer be patched.
- Reboot your system.
Verify that the live patching package has been removed:
# dnf list installed | grep kpatch-patch
The command displays no output if the package has been successfully removed.
Optionally, verify that the kernel live patching solution is disabled:
# kpatch list Loaded patch modules:
The example output shows that the kernel is not patched and the live patching solution is not active because there are no patch modules that are currently loaded.
Currently, Red Hat does not support reverting live patches without rebooting your system. In case of any issues, contact our support team.
Additional resources
-
kpatch(1)
manual page - Configuring basic system settings in RHEL
15.11. Uninstalling the kernel patch module
The following procedure describes how to prevent the Red Hat Enterprise Linux kernel live patching solution from applying a kernel patch module on subsequent boots.
Prerequisites
- Root permissions
- A live patching package is installed.
- A kernel patch module is installed and loaded.
Procedure
Select a kernel patch module:
# kpatch list Loaded patch modules: kpatch_5_14_0_1_0_1 [enabled] Installed patch modules: kpatch_5_14_0_1_0_1 (5.14.0-1.el9.x86_64) …
Uninstall the selected kernel patch module:
# kpatch uninstall kpatch_5_14_0_1_0_1 uninstalling kpatch_5_14_0_1_0_1 (5.14.0-1.el9.x86_64)
Note that the uninstalled kernel patch module is still loaded:
# kpatch list Loaded patch modules: kpatch_5_14_0_1_0_1 [enabled] Installed patch modules: <NO_RESULT>
When the selected module is uninstalled, the kernel remains patched until the next reboot, but the kernel patch module is removed from disk.
- Reboot your system.
Optionally, verify that the kernel patch module has been uninstalled:
# kpatch list Loaded patch modules: …
The example output above shows no loaded or installed kernel patch modules, therefore the kernel is not patched and the kernel live patching solution is not active.
Currently, Red Hat does not support reverting live patches without rebooting your system. In case of any issues, contact our support team.
Additional resources
-
kpatch(1)
manual page
15.12. Disabling kpatch.service
The following procedure describes how to prevent the Red Hat Enterprise Linux kernel live patching solution from applying all kernel patch modules globally on subsequent boots.
Prerequisites
- Root permissions
- A live patching package is installed.
- A kernel patch module is installed and loaded.
Procedure
Verify
kpatch.service
is enabled:# systemctl is-enabled kpatch.service enabled
Disable
kpatch.service
:# systemctl disable kpatch.service Removed /etc/systemd/system/multi-user.target.wants/kpatch.service.
Note that the applied kernel patch module is still loaded:
# kpatch list Loaded patch modules: kpatch_5_14_0_1_0_1 [enabled] Installed patch modules: kpatch_5_14_0_1_0_1 (5.14.0-1.el9.x86_64)
- Reboot your system.
Optionally, verify the status of
kpatch.service
:# systemctl status kpatch.service ● kpatch.service - "Apply kpatch kernel patches" Loaded: loaded (/usr/lib/systemd/system/kpatch.service; disabled; vendor preset: disabled) Active: inactive (dead)
The example output testifies that
kpatch.service
has been disabled and is not running. Thereby, the kernel live patching solution is not active.Verify that the kernel patch module has been unloaded:
# kpatch list Loaded patch modules: Installed patch modules: kpatch_5_14_0_1_0_1 (5.14.0-1.el9.x86_64)
The example output above shows that a kernel patch module is still installed but the kernel is not patched.
Currently, Red Hat does not support reverting live patches without rebooting your system. In case of any issues, contact our support team.
Additional resources
-
kpatch(1)
manual page - Configuring basic system settings in RHEL
Chapter 16. Using systemd to manage resources used by applications
RHEL 9 moves the resource management settings from the process level to the application level by binding the system of cgroup
hierarchies with the systemd
unit tree. Therefore, you can manage the system resources with the systemctl
command, or by modifying the systemd
unit files.
To achieve this, systemd
takes various configuration options from the unit files or directly via the systemctl
command. Then systemd
applies those options to specific process groups by utilizing the Linux kernel system calls and features like cgroups
and namespaces
.
You can review the full set of configuration options for systemd
in the following manual pages:
-
systemd.resource-control(5)
-
systemd.exec(5)
16.1. Allocating system resources using systemd
To modify the distribution of system resources, you can apply one or more of the following distribution models:
- Weights
You can distribute the resource by adding up the weights of all sub-groups and giving each sub-group the fraction matching its ratio against the sum.
For example, if you have 10 cgroups, each with weight of value 100, the sum is 1000. Each cgroup receives one tenth of the resource.
Weight is usually used to distribute stateless resources. For example the CPUWeight= option is an implementation of this resource distribution model.
- Limits
A cgroup can consume up to the configured amount of the resource. The sum of sub-group limits can exceed the limit of the parent cgroup. Therefore it is possible to overcommit resources in this model.
For example the MemoryMax= option is an implementation of this resource distribution model.
- Protections
You can set up a protected amount of a resource for a cgroup. If the resource usage is below the protection boundary, the kernel will try not to penalize this cgroup in favor of other cgroups that compete for the same resource. An overcommit is also possible.
For example the MemoryLow= option is an implementation of this resource distribution model.
- Allocations
- Exclusive allocations of an absolute amount of a finite resource. An overcommit is not possible. An example of this resource type in Linux is the real-time budget.
- unit file option
A setting for resource control configuration.
For example, you can configure CPU resource with options like CPUAccounting=, or CPUQuota=. Similarly, you can configure memory or I/O resources with options like AllowedMemoryNodes= and IOAccounting=.
Procedure
To change the required value of the unit file option of your service, you can adjust the value in the unit file, or use systemctl
command:
Check the assigned values for the service of your choice.
# systemctl show --property <unit file option> <service name>
Set the required value of the CPU time allocation policy option:
# systemctl set-property <service name> <unit file option>=<value>
Verification steps
Check the newly assigned values for the service of your choice.
# systemctl show --property <unit file option> <service name>
Additional resources
-
systemd.resource-control(5)
,systemd.exec(5)
manual pages
16.2. Role of systemd in resource management
The core function of systemd
is service management and supervision. The systemd
system and service manager ensures that managed services start at the right time and in the correct order during the boot process. The services have to run smoothly to use the underlying hardware platform optimally. Therefore, systemd
also provides capabilities to define resource management policies, and to tune various options, which can improve the performance of the service.
In general, Red Hat recommends you use systemd
for controlling the usage of system resources. You should manually configure the cgroups
virtual file system only in special cases. For example, when you need to use cgroup-v1
controllers that have no equivalents in cgroup-v2
hierarchy.
16.3. Overview of systemd hierarchy for cgroups
On the backend, the systemd
system and service manager makes use of the slice
, the scope
and the service
units to organize and structure processes in the control groups. You can further modify this hierarchy by creating custom unit files or using the systemctl
command. Also, systemd
automatically mounts hierarchies for important kernel resource controllers at the /sys/fs/cgroup/
directory.
Three systemd
unit types are used for resource control:
Service - A process or a group of processes, which
systemd
started according to a unit configuration file. Services encapsulate the specified processes so that they can be started and stopped as one set. Services are named in the following way:<name>.service
Scope - A group of externally created processes. Scopes encapsulate processes that are started and stopped by the arbitrary processes through the
fork()
function and then registered bysystemd
at runtime. For example, user sessions, containers, and virtual machines are treated as scopes. Scopes are named as follows:<name>.scope
Slice - A group of hierarchically organized units. Slices organize a hierarchy in which scopes and services are placed. The actual processes are contained in scopes or in services. Every name of a slice unit corresponds to the path to a location in the hierarchy. The dash ("-") character acts as a separator of the path components to a slice from the
-.slice
root slice. In the following example:<parent-name>.slice
parent-name.slice
is a sub-slice ofparent.slice
, which is a sub-slice of the-.slice
root slice.parent-name.slice
can have its own sub-slice namedparent-name-name2.slice
, and so on.
The service
, the scope
, and the slice
units directly map to objects in the control group hierarchy. When these units are activated, they map directly to control group paths built from the unit names.
The following is an abbreviated example of a control group hierarchy:
Control group /: -.slice ├─user.slice │ ├─user-42.slice │ │ ├─session-c1.scope │ │ │ ├─ 967 gdm-session-worker [pam/gdm-launch-environment] │ │ │ ├─1035 /usr/libexec/gdm-x-session gnome-session --autostart /usr/share/gdm/greeter/autostart │ │ │ ├─1054 /usr/libexec/Xorg vt1 -displayfd 3 -auth /run/user/42/gdm/Xauthority -background none -noreset -keeptty -verbose 3 │ │ │ ├─1212 /usr/libexec/gnome-session-binary --autostart /usr/share/gdm/greeter/autostart │ │ │ ├─1369 /usr/bin/gnome-shell │ │ │ ├─1732 ibus-daemon --xim --panel disable │ │ │ ├─1752 /usr/libexec/ibus-dconf │ │ │ ├─1762 /usr/libexec/ibus-x11 --kill-daemon │ │ │ ├─1912 /usr/libexec/gsd-xsettings │ │ │ ├─1917 /usr/libexec/gsd-a11y-settings │ │ │ ├─1920 /usr/libexec/gsd-clipboard … ├─init.scope │ └─1 /usr/lib/systemd/systemd --switched-root --system --deserialize 18 └─system.slice ├─rngd.service │ └─800 /sbin/rngd -f ├─systemd-udevd.service │ └─659 /usr/lib/systemd/systemd-udevd ├─chronyd.service │ └─823 /usr/sbin/chronyd ├─auditd.service │ ├─761 /sbin/auditd │ └─763 /usr/sbin/sedispatch ├─accounts-daemon.service │ └─876 /usr/libexec/accounts-daemon ├─example.service │ ├─ 929 /bin/bash /home/jdoe/example.sh │ └─4902 sleep 1 …
The example above shows that services and scopes contain processes and are placed in slices that do not contain processes of their own.
Additional resources
- Configuring basic system settings in Red Hat Enterprise Linux
- What are kernel resource controllers
-
systemd.resource-control(5)
,systemd.exec(5)
,cgroups(7)
,fork()
,fork(2)
manual pages - Understanding cgroups
16.4. Listing systemd units
The following procedure describes how to use the systemd
system and service manager to list its units.
Procedure
To list all active units on the system, execute the
# systemctl
command and the terminal will return an output similar to the following example:# systemctl UNIT LOAD ACTIVE SUB DESCRIPTION … init.scope loaded active running System and Service Manager session-2.scope loaded active running Session 2 of user jdoe abrt-ccpp.service loaded active exited Install ABRT coredump hook abrt-oops.service loaded active running ABRT kernel log watcher abrt-vmcore.service loaded active exited Harvest vmcores for ABRT abrt-xorg.service loaded active running ABRT Xorg log watcher … -.slice loaded active active Root Slice machine.slice loaded active active Virtual Machine and Container Slice system-getty.slice loaded active active system-getty.slice system-lvm2\x2dpvscan.slice loaded active active system-lvm2\x2dpvscan.slice system-sshd\x2dkeygen.slice loaded active active system-sshd\x2dkeygen.slice system-systemd\x2dhibernate\x2dresume.slice loaded active active system-systemd\x2dhibernate\x2dresume> system-user\x2druntime\x2ddir.slice loaded active active system-user\x2druntime\x2ddir.slice system.slice loaded active active System Slice user-1000.slice loaded active active User Slice of UID 1000 user-42.slice loaded active active User Slice of UID 42 user.slice loaded active active User and Session Slice …
-
UNIT
- a name of a unit that also reflects the unit position in a control group hierarchy. The units relevant for resource control are a slice, a scope, and a service. -
LOAD
- indicates whether the unit configuration file was properly loaded. If the unit file failed to load, the field contains the state error instead of loaded. Other unit load states are: stub, merged, and masked. -
ACTIVE
- the high-level unit activation state, which is a generalization ofSUB
. -
SUB
- the low-level unit activation state. The range of possible values depends on the unit type. -
DESCRIPTION
- the description of the unit content and functionality.
-
To list inactive units, execute:
# systemctl --all
To limit the amount of information in the output, execute:
# systemctl --type service,masked
The
--type
option requires a comma-separated list of unit types such as a service and a slice, or unit load states such as loaded and masked.
Additional resources
- Configuring basic system settings in RHEL
-
systemd.resource-control(5)
,systemd.exec(5)
manual pages
16.5. Viewing systemd control group hierarchy
The following procedure describes how to display control groups (cgroups
) hierarchy and processes running in specific cgroups
.
Procedure
To display the whole
cgroups
hierarchy on your system, execute# systemd-cgls
:# systemd-cgls Control group /: -.slice ├─user.slice │ ├─user-42.slice │ │ ├─session-c1.scope │ │ │ ├─ 965 gdm-session-worker [pam/gdm-launch-environment] │ │ │ ├─1040 /usr/libexec/gdm-x-session gnome-session --autostart /usr/share/gdm/greeter/autostart … ├─init.scope │ └─1 /usr/lib/systemd/systemd --switched-root --system --deserialize 18 └─system.slice … ├─example.service │ ├─6882 /bin/bash /home/jdoe/example.sh │ └─6902 sleep 1 ├─systemd-journald.service └─629 /usr/lib/systemd/systemd-journald …
The example output returns the entire
cgroups
hierarchy, where the highest level is formed by slices.To display the
cgroups
hierarchy filtered by a resource controller, execute# systemd-cgls <resource_controller>
:# systemd-cgls memory Controller memory; Control group /: ├─1 /usr/lib/systemd/systemd --switched-root --system --deserialize 18 ├─user.slice │ ├─user-42.slice │ │ ├─session-c1.scope │ │ │ ├─ 965 gdm-session-worker [pam/gdm-launch-environment] … └─system.slice | … ├─chronyd.service │ └─844 /usr/sbin/chronyd ├─example.service │ ├─8914 /bin/bash /home/jdoe/example.sh │ └─8916 sleep 1 …
The example output of the above command lists the services that interact with the selected controller.
To display detailed information about a certain unit and its part of the
cgroups
hierarchy, execute# systemctl status <system_unit>
:# systemctl status example.service ● example.service - My example service Loaded: loaded (/usr/lib/systemd/system/example.service; enabled; vendor preset: disabled) Active: active (running) since Tue 2019-04-16 12:12:39 CEST; 3s ago Main PID: 17737 (bash) Tasks: 2 (limit: 11522) Memory: 496.0K (limit: 1.5M) CGroup: /system.slice/example.service ├─17737 /bin/bash /home/jdoe/example.sh └─17743 sleep 1 Apr 16 12:12:39 redhat systemd[1]: Started My example service. Apr 16 12:12:39 redhat bash[17737]: The current time is Tue Apr 16 12:12:39 CEST 2019 Apr 16 12:12:40 redhat bash[17737]: The current time is Tue Apr 16 12:12:40 CEST 2019
Additional resources
- What are kernel resource controllers
-
systemd.resource-control(5)
,cgroups(7)
manual pages
16.6. Viewing cgroups of processes
The following procedure describes how to learn which control group (cgroup
) a process belongs to. Then you can check the cgroup
to learn which controllers and controller-specific configurations it uses.
Procedure
To view which
cgroup
a process belongs to, run the# cat proc/<PID>/cgroup
command:# cat /proc/2467/cgroup 0::/system.slice/example.service
The example output relates to a process of interest. In this case, it is a process identified by
PID 2467
, which belongs to theexample.service
unit. You can determine whether the process was placed in a correct control group as defined by thesystemd
unit file specifications.To display what controllers the
cgroup
utilizes and the respective configuration files, check thecgroup
directory:# cat /sys/fs/cgroup/system.slice/example.service/cgroup.controllers memory pids # ls /sys/fs/cgroup/system.slice/example.service/ cgroup.controllers cgroup.events … cpu.pressure cpu.stat io.pressure memory.current memory.events … pids.current pids.events pids.max
The version 1 hierarchy of cgroups
uses a per-controller model. Therefore the output from the /proc/PID/cgroup
file shows, which cgroups
under each controller the PID belongs to. You can find the respective cgroups
under the controller directories at /sys/fs/cgroup/<controller_name>/
.
Additional resources
-
cgroups(7)
manual page - What are kernel resource controllers
-
Documentation in the
/usr/share/doc/kernel-doc-<kernel_version>/Documentation/admin-guide/cgroup-v2.rst
file (after installing thekernel-doc
package)
16.7. Monitoring resource consumption
The following procedure describes how to view a list of currently running control groups (cgroups
) and their resource consumption in real-time.
Procedure
To see a dynamic account of currently running
cgroups
, execute the# systemd-cgtop
command:# systemd-cgtop Control Group Tasks %CPU Memory Input/s Output/s / 607 29.8 1.5G - - /system.slice 125 - 428.7M - - /system.slice/ModemManager.service 3 - 8.6M - - /system.slice/NetworkManager.service 3 - 12.8M - - /system.slice/accounts-daemon.service 3 - 1.8M - - /system.slice/boot.mount - - 48.0K - - /system.slice/chronyd.service 1 - 2.0M - - /system.slice/cockpit.socket - - 1.3M - - /system.slice/colord.service 3 - 3.5M - - /system.slice/crond.service 1 - 1.8M - - /system.slice/cups.service 1 - 3.1M - - /system.slice/dev-hugepages.mount - - 244.0K - - /system.slice/dev-mapper-rhel\x2dswap.swap - - 912.0K - - /system.slice/dev-mqueue.mount - - 48.0K - - /system.slice/example.service 2 - 2.0M - - /system.slice/firewalld.service 2 - 28.8M - - ...
The example output displays currently running
cgroups
ordered by their resource usage (CPU, memory, disk I/O load). The list refreshes every 1 second by default. Therefore, it offers a dynamic insight into the actual resource usage of each control group.
Additional resources
-
systemd-cgtop(1)
manual page
16.8. Using systemd unit files to set limits for applications
Each existing or running unit is supervised by the systemd
, which also creates control groups for them. The units have configuration files in the /usr/lib/systemd/system/
directory. You can manually modify the unit files to set limits, prioritize, or control access to hardware resources for groups of processes.
Prerequisites
-
You have the
root
privileges.
Procedure
Modify the
/usr/lib/systemd/system/example.service
file to limit the memory usage of a service:… [Service] MemoryMax=1500K …
The configuration above places a maximum memory limit, which the processes in a control group cannot exceed. The
example.service
service is part of such a control group which has imposed limitations. You can use suffixes K, M, G, or T to identify Kilobyte, Megabyte, Gigabyte, or Terabyte as a unit of measurement.Reload all unit configuration files:
# systemctl daemon-reload
Restart the service:
# systemctl restart example.service
You can review the full set of configuration options for systemd
in the following manual pages:
-
systemd.resource-control(5)
-
systemd.exec(5)
Verification
Check that the changes took effect:
# cat /sys/fs/cgroup/system.slice/example.service/memory.max 1536000
The example output shows that the memory consumption was limited at around 1,500 KB.
Additional resources
- Understanding cgroups
- Configuring basic system settings in Red Hat Enterprise Linux
-
systemd.resource-control(5)
,systemd.exec(5)
,cgroups(7)
manual pages
16.9. Using systemctl command to set limits to applications
CPU affinity settings help you restrict the access of a particular process to some CPUs. Effectively, the CPU scheduler never schedules the process to run on the CPU that is not in the affinity mask of the process.
The default CPU affinity mask applies to all services managed by systemd
.
To configure CPU affinity mask for a particular systemd
service, systemd
provides CPUAffinity=
both as a unit file option and a manager configuration option in the /etc/systemd/system.conf
file.
The CPUAffinity=
unit file option sets a list of CPUs or CPU ranges that are merged and used as the affinity mask.
After configuring CPU affinity mask for a particular systemd
service, you must restart the service to apply the changes.
Procedure
To set CPU affinity mask for a particular systemd
service using the CPUAffinity
unit file option:
Check the values of the
CPUAffinity
unit file option in the service of your choice:$ systemctl show --property <CPU affinity configuration option> <service name>
As a root, set the required value of the
CPUAffinity
unit file option for the CPU ranges used as the affinity mask:# systemctl set-property <service name> CPUAffinity=<value>
Restart the service to apply the changes.
# systemctl restart <service name>
You can review the full set of configuration options for systemd
in the following manual pages:
-
systemd.resource-control(5)
-
systemd.exec(5)
16.10. Setting global default CPU affinity through manager configuration
The CPUAffinity
option in the /etc/systemd/system.conf
file defines an affinity mask for the process identification number (PID) 1 and all processes forked off of PID1. You can then override the CPUAffinity
on a per-service basis.
To set default CPU affinity mask for all systemd services using the manager configuration option:
-
Set the CPU numbers for the
CPUAffinity=
option in the/etc/systemd/system.conf
file. Save the edited file and reload the
systemd
service:# systemctl daemon-reload
- Reboot the server to apply the changes.
You can review the full set of configuration options for systemd
in the following manual pages:
-
systemd.resource-control(5)
-
systemd.exec(5)
16.11. Configuring NUMA policies using systemd
Non-uniform memory access (NUMA) is a computer memory subsystem design, in which the memory access time depends on the physical memory location relative to the processor.
Memory close to the CPU has lower latency (local memory) than memory that is local for a different CPU (foreign memory) or is shared between a set of CPUs.
In terms of the Linux kernel, NUMA policy governs where (for example, on which NUMA nodes) the kernel allocates physical memory pages for the process.
systemd
provides unit file options NUMAPolicy
and NUMAMask
to control memory allocation policies for services.
Procedure
To set the NUMA memory policy through the NUMAPolicy
unit file option:
Check the values of the
NUMAPolicy
unit file option in the service of your choice:$ systemctl show --property <NUMA policy configuration option> <service name>
As a root, set the required policy type of the
NUMAPolicy
unit file option:# systemctl set-property <service name> NUMAPolicy=<value>
Restart the service to apply the changes.
# systemctl restart <service name>
To set a global NUMAPolicy
setting through the manager configuration option:
-
Search in the
/etc/systemd/system.conf
file for theNUMAPolicy
option. - Edit the policy type and save the file.
Reload the
systemd
configuration:# systemd daemon-reload
- Reboot the server.
When you configure a strict NUMA policy, for example bind
, make sure that you also appropriately set the CPUAffinity=
unit file option.
Additional resources
- Using systemctl command to set limits to applications
-
systemd.resource-control(5)
,systemd.exec(5)
,set_mempolicy(2)
manual pages.
16.12. NUMA policy configuration options for systemd
Systemd provides the following options to configure the NUMA policy:
NUMAPolicy
Controls the NUMA memory policy of the executed processes. The following policy types are possible:
- default
- preferred
- bind
- interleave
- local
NUMAMask
Controls the NUMA node list which is associated with the selected NUMA policy.
Note that the
NUMAMask
option is not required to be specified for the following policies:- default
- local
For the preferred policy, the list specifies only a single NUMA node.
Additional resources
-
systemd.resource-control(5)
,systemd.exec(5)
, andset_mempolicy(2)
manual pages
16.13. Creating transient cgroups using systemd-run command
The transient cgroups
set limits on resources consumed by a unit (service or scope) during its runtime.
Procedure
To create a transient control group, use the
systemd-run
command in the following format:# systemd-run --unit=<name> --slice=<name>.slice <command>
This command creates and starts a transient service or a scope unit and runs a custom command in such a unit.
-
The
--unit=<name>
option gives a name to the unit. If--unit
is not specified, the name is generated automatically. -
The
--slice=<name>.slice
option makes your service or scope unit a member of a specified slice. Replace<name>.slice
with the name of an existing slice (as shown in the output ofsystemctl -t slice
), or create a new slice by passing a unique name. By default, services and scopes are created as members of thesystem.slice
. Replace
<command>
with the command you wish to execute in the service or the scope unit.The following message is displayed to confirm that you created and started the service or the scope successfully:
# Running as unit <name>.service
-
The
Optionally, keep the unit running after its processes finished to collect run-time information:
# systemd-run --unit=<name> --slice=<name>.slice --remain-after-exit <command>
The command creates and starts a transient service unit and runs a custom command in such a unit. The
--remain-after-exit
option ensures that the service keeps running after its processes have finished.
Additional resources
- Understanding control groups
- Configuring basic system settings in RHEL
-
systemd-run(1)
manual page
16.14. Removing transient control groups
You can use the systemd
system and service manager to remove transient control groups (cgroups
) if you no longer need to limit, prioritize, or control access to hardware resources for groups of processes.
Transient cgroups
are automatically released once all the processes that a service or a scope unit contains, finish.
Procedure
To stop the service unit with all its processes, execute:
# systemctl stop name.service
To terminate one or more of the unit processes, execute:
# systemctl kill name.service --kill-who=PID,... --signal=<signal>
The command above uses the
--kill-who
option to select process(es) from the control group you wish to terminate. To kill multiple processes at the same time, pass a comma-separated list of PIDs. The--signal
option determines the type of POSIX signal to be sent to the specified processes. The default signal is SIGTERM.
Additional resources
- Understanding control groups
- What are kernel resource controllers
-
systemd.resource-control(5)
,cgroups(7)
manual pages - Configuring basic system settings in RHEL
Chapter 17. Understanding cgroups
You can use the control groups (cgroups
) kernel functionality to set limits, prioritize or isolate the hardware resources of processes. This allows you to granularly control resource usage of applications to utilize them more efficiently.
17.1. Understanding control groups
Control groups is a Linux kernel feature that enables you to organize processes into hierarchically ordered groups - cgroups
. The hierarchy (control groups tree) is defined by providing structure to cgroups
virtual file system, mounted by default on the /sys/fs/cgroup/
directory. The systemd
system and service manager utilizes cgroups
to organize all units and services that it governs. Alternatively, you can manage cgroups
hierarchies manually by creating and removing sub-directories in the /sys/fs/cgroup/
directory.
The resource controllers (a kernel component) then modify the behavior of processes in cgroups
by limiting, prioritizing or allocating system resources, (such as CPU time, memory, network bandwidth, or various combinations) of those processes.
The added value of cgroups
is process aggregation which enables division of hardware resources among applications and users. Thereby an increase in overall efficiency, stability and security of users' environment can be achieved.
- Control groups version 1
Control groups version 1 (
cgroups-v1
) provide a per-resource controller hierarchy. It means that each resource, such as CPU, memory, I/O, and so on, has its own control group hierarchy. It is possible to combine different control group hierarchies in a way that one controller can coordinate with another one in managing their respective resources. However, the two controllers may belong to different process hierarchies, which does not permit their proper coordination.The
cgroups-v1
controllers were developed across a large time span and as a result, the behavior and naming of their control files is not uniform.- Control groups version 2
The problems with controller coordination, which stemmed from hierarchy flexibility, led to the development of control groups version 2.
Control groups version 2 (
cgroups-v2
) provides a single control group hierarchy against which all resource controllers are mounted.The control file behavior and naming is consistent among different controllers.
RHEL 9, by default, mounts and utilizes cgroups-v2
.
This sub-section was based on a Devconf.cz 2019 presentation.[1]
Additional resources
- What are kernel resource controllers
-
cgroups(7)
manual page - cgroups-v1
- cgroups-v2
17.2. What are kernel resource controllers
The functionality of control groups is enabled by kernel resource controllers. RHEL 9 supports various controllers for control groups version 1 (cgroups-v1
) and control groups version 2 (cgroups-v2
).
A resource controller, also called a control group subsystem, is a kernel subsystem that represents a single resource, such as CPU time, memory, network bandwidth or disk I/O. The Linux kernel provides a range of resource controllers that are mounted automatically by the systemd
system and service manager. Find a list of currently mounted resource controllers in the /proc/cgroups
file.
The following controllers are available for cgroups-v1
:
-
blkio
- can set limits on input/output access to and from block devices. -
cpu
- can adjust the parameters of the Completely Fair Scheduler (CFS) scheduler for control group’s tasks. It is mounted together with thecpuacct
controller on the same mount. -
cpuacct
- creates automatic reports on CPU resources used by tasks in a control group. It is mounted together with thecpu
controller on the same mount. -
cpuset
- can be used to restrict control group tasks to run only on a specified subset of CPUs and to direct the tasks to use memory only on specified memory nodes. -
devices
- can control access to devices for tasks in a control group. -
freezer
- can be used to suspend or resume tasks in a control group. -
memory
- can be used to set limits on memory use by tasks in a control group and generates automatic reports on memory resources used by those tasks. -
net_cls
- tags network packets with a class identifier (classid
) that enables the Linux traffic controller (thetc
command) to identify packets that originate from a particular control group task. A subsystem ofnet_cls
, thenet_filter
(iptables), can also use this tag to perform actions on such packets. Thenet_filter
tags network sockets with a firewall identifier (fwid
) that allows the Linux firewall (throughiptables
command) to identify packets originating from a particular control group task. -
net_prio
- sets the priority of network traffic. -
pids
- can set limits for a number of processes and their children in a control group. -
perf_event
- can group tasks for monitoring by theperf
performance monitoring and reporting utility. -
rdma
- can set limits on Remote Direct Memory Access/InfiniBand specific resources in a control group. -
hugetlb
- can be used to limit the usage of large size virtual memory pages by tasks in a control group.
The following controllers are available for cgroups-v2
:
-
io
- A follow-up toblkio
ofcgroups-v1
. -
memory
- A follow-up tomemory
ofcgroups-v1
. -
pids
- Same aspids
incgroups-v1
. -
rdma
- Same asrdma
incgroups-v1
. -
cpu
- A follow-up tocpu
andcpuacct
ofcgroups-v1
. -
cpuset
- Supports only the core functionality (cpus{,.effective}
,mems{,.effective}
) with a new partition feature. -
perf_event
- Support is inherent, no explicit control file. You can specify av2 cgroup
as a parameter to theperf
command that will profile all the tasks within thatcgroup
.
A resource controller can be used either in a cgroups-v1
hierarchy or a cgroups-v2
hierarchy, not simultaneously in both.
Additional resources
-
cgroups(7)
manual page -
Documentation in
/usr/share/doc/kernel-doc-<kernel_version>/Documentation/cgroups-v1/
directory (after installing thekernel-doc
package).
17.3. What are namespaces
Namespaces are one of the most important methods for organizing and identifying software objects.
A namespace wraps a global system resource (for example a mount point, a network device, or a hostname) in an abstraction that makes it appear to processes within the namespace that they have their own isolated instance of the global resource. One of the most common technologies that utilize namespaces are containers.
Changes to a particular global resource are visible only to processes in that namespace and do not affect the rest of the system or other namespaces.
To inspect which namespaces a process is a member of, you can check the symbolic links in the /proc/<PID>/ns/
directory.
The following table shows supported namespaces and resources which they isolate:
Namespace | Isolates |
---|---|
Mount | Mount points |
UTS | Hostname and NIS domain name |
IPC | System V IPC, POSIX message queues |
PID | Process IDs |
Network | Network devices, stacks, ports, etc |
User | User and group IDs |
Control groups | Control group root directory |
Additional resources
-
namespaces(7)
andcgroup_namespaces(7)
manual pages - Understanding control groups
Chapter 18. Improving system performance with zswap
You can improve system performance by enabling the zswap
kernel feature.
18.1. What is zswap
This section explains what zswap
is and how it can lead to system performance improvement.
zswap
is a kernel feature that provides a compressed RAM cache for swap pages. The mechanism works as follows: zswap
takes pages that are in the process of being swapped out and attempts to compress them into a dynamically allocated RAM-based memory pool. When the pool becomes full or the RAM becomes exhausted, zswap
evicts pages from compressed cache on an LRU basis (least recently used) to the backing swap device. After the page has been decompressed into the swap cache, zswap
frees the compressed version in the pool.
- The benefits of
zswap
- significant I/O reduction
- significant improvement of workload performance
In Red Hat Enterprise Linux 9, zswap
is enabled by default.
Additional resources
18.2. Enabling zswap at runtime
You can enable the zswap
feature at system runtime using the sysfs
interface.
Prerequisites
- You have root permissions.
Procedure
Enable
zswap
:# echo 1 > /sys/module/zswap/parameters/enabled
Verification step
Verify that
zswap
is enabled:# grep -r . /sys/kernel/debug/zswap duplicate_entry:0 pool_limit_hit:13422200 pool_total_size:6184960 (pool size in total in pages) reject_alloc_fail:5 reject_compress_poor:0 reject_kmemcache_fail:0 reject_reclaim_fail:13422200 stored_pages:4251 (pool size after compression) written_back_pages:0
Additional resources
18.3. Enabling zswap permanently
You can enable the zswap
feature permanently by providing the zswap.enabled=1
kernel command-line parameter.
Prerequisites
- You have root permissions.
-
The
grubby
orzipl
utility is installed on your system.
Procedure
Enable
zswap
permanently:# grubby --update-kernel=/boot/vmlinuz-$(uname -r) --args="zswap.enabled=1"
- Reboot the system for the changes to take effect.
Verification steps
Verify that
zswap
is enabled:# cat /proc/cmdline BOOT_IMAGE=(hd0,msdos1)/vmlinuz-5.14.0-70.5.1.el9_0.x86_64 root=/dev/mapper/rhel-root ro crashkernel=1G-4G:192M,4G-64G:256M,64G-:512M resume=/dev/mapper/rhel-swap rd.lvm.lv=rhel/root rd.lvm.lv=rhel/swap rhgb quiet zswap.enabled=1
Additional resources
Chapter 19. Using cgroupfs to manually manage cgroups
You can manage cgroup
hierarchies on your system by creating directories on the cgroupfs
virtual file system. The file system is mounted by default on the /sys/fs/cgroup/
directory and you can specify desired configurations in dedicated control files.
In general, Red Hat recommends you use systemd
for controlling the usage of system resources. You should manually configure the cgroups
virtual file system only in special cases. For example, when you need to use cgroup-v1
controllers that have no equivalents in cgroup-v2
hierarchy.
19.1. Creating cgroups and enabling controllers in cgroups-v2 file system
You can manage the control groups (cgroups
) by creating or removing directories and by writing to files in the cgroups
virtual file system. The file system is by default mounted on the /sys/fs/cgroup/
directory. To use settings from the cgroups
controllers, you also need to enable the desired controllers for child cgroups
. The root cgroup
has, by default, enabled the memory
and pids
controllers for its child cgroups
. Therefore, Red Hat recommends to create at least two levels of child cgroups
inside the /sys/fs/cgroup/
root cgroup
. This way you optionally remove the memory
and pids
controllers from the child cgroups
and maintain better organizational clarity of cgroup
files.
Prerequisites
- You have root permissions.
Procedure
Create the
/sys/fs/cgroup/Example/
directory:# mkdir /sys/fs/cgroup/Example/
The
/sys/fs/cgroup/Example/
directory defines a child group. When you create the/sys/fs/cgroup/Example/
directory, somecgroups-v2
interface files are automatically created in the directory. The/sys/fs/cgroup/Example/
directory contains also controller-specific files for thememory
andpids
controllers.Optionally, inspect the newly created child control group:
# ll /sys/fs/cgroup/Example/ -r—r—r--. 1 root root 0 Jun 1 10:33 cgroup.controllers -r—r—r--. 1 root root 0 Jun 1 10:33 cgroup.events -rw-r—r--. 1 root root 0 Jun 1 10:33 cgroup.freeze -rw-r—r--. 1 root root 0 Jun 1 10:33 cgroup.procs … -rw-r—r--. 1 root root 0 Jun 1 10:33 cgroup.subtree_control -r—r—r--. 1 root root 0 Jun 1 10:33 memory.events.local -rw-r—r--. 1 root root 0 Jun 1 10:33 memory.high -rw-r—r--. 1 root root 0 Jun 1 10:33 memory.low … -r—r—r--. 1 root root 0 Jun 1 10:33 pids.current -r—r—r--. 1 root root 0 Jun 1 10:33 pids.events -rw-r—r--. 1 root root 0 Jun 1 10:33 pids.max
The example output shows general
cgroup
control interface files such ascgroup.procs
orcgroup.controllers
. These files are common to all control groups, regardless of enabled controllers.The files such as
memory.high
andpids.max
relate to thememory
andpids
controllers, which are in the root control group (/sys/fs/cgroup/
), and are enabled by default bysystemd
.By default, the newly created child group inherits all settings from the parent
cgroup
. In this case, there are no limits from the rootcgroup
.Verify that the desired controllers are available in the
/sys/fs/cgroup/cgroup.controllers
file:# cat /sys/fs/cgroup/cgroup.controllers cpuset cpu io memory hugetlb pids rdma
Enable the desired controllers. In this example it is
cpu
andcpuset
controllers:# echo "+cpu" >> /sys/fs/cgroup/cgroup.subtree_control # echo "+cpuset" >> /sys/fs/cgroup/cgroup.subtree_control
These commands enable the
cpu
andcpuset
controllers for the immediate child groups of the/sys/fs/cgroup/
root control group. Including the newly createdExample
control group. A child group is where you can specify processes and apply control checks to each of the processes based on your criteria.Users can read the contents of the
cgroup.subtree_control
file at any level to get an idea of what controllers are going to be available for enablement in the immediate child group.NoteBy default, the
/sys/fs/cgroup/cgroup.subtree_control
file in the root control group containsmemory
andpids
controllers.Enable the desired controllers for child
cgroups
of theExample
control group:# echo "+cpu +cpuset" >> /sys/fs/cgroup/Example/cgroup.subtree_control
This command ensures that the immediate child control group will only have controllers relevant to regulate the CPU time distribution - not to
memory
orpids
controllers.Create the
/sys/fs/cgroup/Example/tasks/
directory:# mkdir /sys/fs/cgroup/Example/tasks/
The
/sys/fs/cgroup/Example/tasks/
directory defines a child group with files that relate purely tocpu
andcpuset
controllers. You can now assign processes to this control group and utilizecpu
andcpuset
controller options for your processes.Optionally, inspect the child control group:
# ll /sys/fs/cgroup/Example/tasks -r—r—r--. 1 root root 0 Jun 1 11:45 cgroup.controllers -r—r—r--. 1 root root 0 Jun 1 11:45 cgroup.events -rw-r—r--. 1 root root 0 Jun 1 11:45 cgroup.freeze -rw-r—r--. 1 root root 0 Jun 1 11:45 cgroup.max.depth -rw-r—r--. 1 root root 0 Jun 1 11:45 cgroup.max.descendants -rw-r—r--. 1 root root 0 Jun 1 11:45 cgroup.procs -r—r—r--. 1 root root 0 Jun 1 11:45 cgroup.stat -rw-r—r--. 1 root root 0 Jun 1 11:45 cgroup.subtree_control -rw-r—r--. 1 root root 0 Jun 1 11:45 cgroup.threads -rw-r—r--. 1 root root 0 Jun 1 11:45 cgroup.type -rw-r—r--. 1 root root 0 Jun 1 11:45 cpu.max -rw-r—r--. 1 root root 0 Jun 1 11:45 cpu.pressure -rw-r—r--. 1 root root 0 Jun 1 11:45 cpuset.cpus -r—r—r--. 1 root root 0 Jun 1 11:45 cpuset.cpus.effective -rw-r—r--. 1 root root 0 Jun 1 11:45 cpuset.cpus.partition -rw-r—r--. 1 root root 0 Jun 1 11:45 cpuset.mems -r—r—r--. 1 root root 0 Jun 1 11:45 cpuset.mems.effective -r—r—r--. 1 root root 0 Jun 1 11:45 cpu.stat -rw-r—r--. 1 root root 0 Jun 1 11:45 cpu.weight -rw-r—r--. 1 root root 0 Jun 1 11:45 cpu.weight.nice -rw-r—r--. 1 root root 0 Jun 1 11:45 io.pressure -rw-r—r--. 1 root root 0 Jun 1 11:45 memory.pressure
The cpu
controller is only activated if the relevant child control group has at least 2 processes which compete for time on a single CPU.
Verification steps
Optional: confirm that you have created a new
cgroup
with only the desired controllers active:# cat /sys/fs/cgroup/Example/tasks/cgroup.controllers cpuset cpu
Additional resources
- Understanding control groups
- What are kernel resource controllers
- Mounting cgroups-v1
-
cgroups(7)
,sysfs(5)
manual pages
19.2. Controlling distribution of CPU time for applications by adjusting CPU weight
You need to assign values to the relevant files of the cpu
controller to regulate distribution of the CPU time to applications under the specific cgroup tree.
Prerequisites
- You have root permissions.
- You have applications for which you want to control distribution of CPU time.
You created a two level hierarchy of child control groups inside the
/sys/fs/cgroup/
root control group as in the following example:… ├── Example │ ├── g1 │ ├── g2 │ └── g3 …
-
You enabled the
cpu
controller in the parent control group and in child control groups similarly as described in Creating cgroups and enabling controllers in cgroups-v2 file system.
Procedure
Configure desired CPU weights to achieve resource restrictions within the control groups:
# echo "150" > /sys/fs/cgroup/Example/g1/cpu.weight # echo "100" > /sys/fs/cgroup/Example/g2/cpu.weight # echo "50" > /sys/fs/cgroup/Example/g3/cpu.weight
Add the applications' PIDs to the
g1
,g2
, andg3
child groups:# echo "33373" > /sys/fs/cgroup/Example/g1/cgroup.procs # echo "33374" > /sys/fs/cgroup/Example/g2/cgroup.procs # echo "33377" > /sys/fs/cgroup/Example/g3/cgroup.procs
The example commands ensure that desired applications become members of the
Example/g*/
child cgroups and will get their CPU time distributed as per the configuration of those cgroups.The weights of the children cgroups (
g1
,g2
,g3
) that have running processes are summed up at the level of the parent cgroup (Example
). The CPU resource is then distributed proportionally based on the respective weights.As a result, when all processes run at the same time, the kernel allocates to each of them the proportionate CPU time based on their respective cgroup’s
cpu.weight
file:Child cgroup cpu.weight
fileCPU time allocation g1
150
~50% (150/300)
g2
100
~33% (100/300)
g3
50
~16% (50/300)
The value of the
cpu.weight
controller file is not a percentage.If one process stopped running, leaving cgroup
g2
with no running processes, the calculation would omit the cgroupg2
and only account weights of cgroupsg1
andg3
:Child cgroup cpu.weight
fileCPU time allocation g1
150
~75% (150/200)
g3
50
~25% (50/200)
ImportantIf a child cgroup had multiple running processes, the CPU time allocated to the respective cgroup would be distributed equally to the member processes of that cgroup.
Verification
Verify that the applications run in the specified control groups:
# cat /proc/33373/cgroup /proc/33374/cgroup /proc/33377/cgroup 0::/Example/g1 0::/Example/g2 0::/Example/g3
The command output shows the processes of the specified applications that run in the
Example/g*/
child cgroups.Inspect the current CPU consumption of the throttled applications:
# top top - 05:17:18 up 1 day, 18:25, 1 user, load average: 3.03, 3.03, 3.00 Tasks: 95 total, 4 running, 91 sleeping, 0 stopped, 0 zombie %Cpu(s): 18.1 us, 81.6 sy, 0.0 ni, 0.0 id, 0.0 wa, 0.3 hi, 0.0 si, 0.0 st MiB Mem : 3737.0 total, 3233.7 free, 132.8 used, 370.5 buff/cache MiB Swap: 4060.0 total, 4060.0 free, 0.0 used. 3373.1 avail Mem PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND 33373 root 20 0 18720 1748 1460 R 49.5 0.0 415:05.87 sha1sum 33374 root 20 0 18720 1756 1464 R 32.9 0.0 412:58.33 sha1sum 33377 root 20 0 18720 1860 1568 R 16.3 0.0 411:03.12 sha1sum 760 root 20 0 416620 28540 15296 S 0.3 0.7 0:10.23 tuned 1 root 20 0 186328 14108 9484 S 0.0 0.4 0:02.00 systemd 2 root 20 0 0 0 0 S 0.0 0.0 0:00.01 kthread ...
NoteWe forced all the example processes to run on a single CPU for clearer illustration. The CPU weight applies the same principles also when used on multiple CPUs.
Notice that the CPU resource for the
PID 33373
,PID 33374
, andPID 33377
was allocated based on the weights, 150, 100, 50, you assigned to the respective child cgroups. The weights correspond to around 50%, 33%, and 16% allocation of CPU time for each application.
Additional resources
19.3. Mounting cgroups-v1
During the boot process, RHEL 9 mounts the cgroup-v2
virtual filesystem by default. To utilize cgroup-v1
functionality in limiting resources for your applications, manually configure the system.
Both cgroup-v1
and cgroup-v2
are fully enabled in the kernel. There is no default control group version from the kernel point of view, and is decided by systemd
to mount at startup.
Prerequisites
- You have root permissions.
Procedure
Configure the system to mount
cgroups-v1
by default during system boot by thesystemd
system and service manager:# grubby --update-kernel=/boot/vmlinuz-$(uname -r) --args="systemd.unified_cgroup_hierarchy=0 systemd.legacy_systemd_cgroup_controller"
This adds the necessary kernel command-line parameters to the current boot entry.
To add the same parameters to all kernel boot entries:
# grubby --update-kernel=ALL --args="systemd.unified_cgroup_hierarchy=0 systemd.legacy_systemd_cgroup_controller"
- Reboot the system for the changes to take effect.
Verification
Optionally, verify that the
cgroups-v1
filesystem was mounted:# mount -l | grep cgroup tmpfs on /sys/fs/cgroup type tmpfs (ro,nosuid,nodev,noexec,seclabel,size=4096k,nr_inodes=1024,mode=755,inode64) cgroup on /sys/fs/cgroup/systemd type cgroup (rw,nosuid,nodev,noexec,relatime,seclabel,xattr,release_agent=/usr/lib/systemd/systemd-cgroups-agent,name=systemd) cgroup on /sys/fs/cgroup/perf_event type cgroup (rw,nosuid,nodev,noexec,relatime,seclabel,perf_event) cgroup on /sys/fs/cgroup/cpu,cpuacct type cgroup (rw,nosuid,nodev,noexec,relatime,seclabel,cpu,cpuacct) cgroup on /sys/fs/cgroup/pids type cgroup (rw,nosuid,nodev,noexec,relatime,seclabel,pids) cgroup on /sys/fs/cgroup/cpuset type cgroup (rw,nosuid,nodev,noexec,relatime,seclabel,cpuset) cgroup on /sys/fs/cgroup/net_cls,net_prio type cgroup (rw,nosuid,nodev,noexec,relatime,seclabel,net_cls,net_prio) cgroup on /sys/fs/cgroup/hugetlb type cgroup (rw,nosuid,nodev,noexec,relatime,seclabel,hugetlb) cgroup on /sys/fs/cgroup/memory type cgroup (rw,nosuid,nodev,noexec,relatime,seclabel,memory) cgroup on /sys/fs/cgroup/blkio type cgroup (rw,nosuid,nodev,noexec,relatime,seclabel,blkio) cgroup on /sys/fs/cgroup/devices type cgroup (rw,nosuid,nodev,noexec,relatime,seclabel,devices) cgroup on /sys/fs/cgroup/misc type cgroup (rw,nosuid,nodev,noexec,relatime,seclabel,misc) cgroup on /sys/fs/cgroup/freezer type cgroup (rw,nosuid,nodev,noexec,relatime,seclabel,freezer) cgroup on /sys/fs/cgroup/rdma type cgroup (rw,nosuid,nodev,noexec,relatime,seclabel,rdma)
The
cgroups-v1
filesystems that correspond to variouscgroup-v1
controllers, were successfully mounted on the/sys/fs/cgroup/
directory.Optionally, inspect the contents of the
/sys/fs/cgroup/
directory:# ll /sys/fs/cgroup/ dr-xr-xr-x. 10 root root 0 Mar 16 09:34 blkio lrwxrwxrwx. 1 root root 11 Mar 16 09:34 cpu → cpu,cpuacct lrwxrwxrwx. 1 root root 11 Mar 16 09:34 cpuacct → cpu,cpuacct dr-xr-xr-x. 10 root root 0 Mar 16 09:34 cpu,cpuacct dr-xr-xr-x. 2 root root 0 Mar 16 09:34 cpuset dr-xr-xr-x. 10 root root 0 Mar 16 09:34 devices dr-xr-xr-x. 2 root root 0 Mar 16 09:34 freezer dr-xr-xr-x. 2 root root 0 Mar 16 09:34 hugetlb dr-xr-xr-x. 10 root root 0 Mar 16 09:34 memory dr-xr-xr-x. 2 root root 0 Mar 16 09:34 misc lrwxrwxrwx. 1 root root 16 Mar 16 09:34 net_cls → net_cls,net_prio dr-xr-xr-x. 2 root root 0 Mar 16 09:34 net_cls,net_prio lrwxrwxrwx. 1 root root 16 Mar 16 09:34 net_prio → net_cls,net_prio dr-xr-xr-x. 2 root root 0 Mar 16 09:34 perf_event dr-xr-xr-x. 10 root root 0 Mar 16 09:34 pids dr-xr-xr-x. 2 root root 0 Mar 16 09:34 rdma dr-xr-xr-x. 11 root root 0 Mar 16 09:34 systemd
The
/sys/fs/cgroup/
directory, also called the root control group, by default, contains controller-specific directories such ascpuset
. In addition, there are some directories related tosystemd
.
Additional resources
- Understanding control groups
- What are kernel resource controllers
-
cgroups(7)
,sysfs(5)
manual pages - cgroup-v2 enabled by default in RHEL 9
19.4. Setting CPU limits to applications using cgroups-v1
Sometimes an application consumes a lot of CPU time, which may negatively impact the overall health of your environment. Use the /sys/fs/
virtual file system to configure CPU limits to an application using control groups version 1 (cgroups-v1
).
Prerequisites
- You have root permissions.
- You have an application whose CPU consumption you want to restrict.
You configured the system to mount
cgroups-v1
by default during system boot by thesystemd
system and service manager:# grubby --update-kernel=/boot/vmlinuz-$(uname -r) --args="systemd.unified_cgroup_hierarchy=0 systemd.legacy_systemd_cgroup_controller"
This adds the necessary kernel command-line parameters to the current boot entry.
Procedure
Identify the process ID (PID) of the application you want to restrict in CPU consumption:
# top top - 11:34:09 up 11 min, 1 user, load average: 0.51, 0.27, 0.22 Tasks: 267 total, 3 running, 264 sleeping, 0 stopped, 0 zombie %Cpu(s): 49.0 us, 3.3 sy, 0.0 ni, 47.5 id, 0.0 wa, 0.2 hi, 0.0 si, 0.0 st MiB Mem : 1826.8 total, 303.4 free, 1046.8 used, 476.5 buff/cache MiB Swap: 1536.0 total, 1396.0 free, 140.0 used. 616.4 avail Mem PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND 6955 root 20 0 228440 1752 1472 R 99.3 0.1 0:32.71 sha1sum 5760 jdoe 20 0 3603868 205188 64196 S 3.7 11.0 0:17.19 gnome-shell 6448 jdoe 20 0 743648 30640 19488 S 0.7 1.6 0:02.73 gnome-terminal- 1 root 20 0 245300 6568 4116 S 0.3 0.4 0:01.87 systemd 505 root 20 0 0 0 0 I 0.3 0.0 0:00.75 kworker/u4:4-events_unbound ...
The example output of the
top
program reveals thatPID 6955
(illustrative applicationsha1sum
) consumes a lot of CPU resources.Create a sub-directory in the
cpu
resource controller directory:# mkdir /sys/fs/cgroup/cpu/Example/
The directory above represents a control group, where you can place specific processes and apply certain CPU limits to the processes. At the same time, some
cgroups-v1
interface files andcpu
controller-specific files will be created in the directory.Optionally, inspect the newly created control group:
# ll /sys/fs/cgroup/cpu/Example/ -rw-r—r--. 1 root root 0 Mar 11 11:42 cgroup.clone_children -rw-r—r--. 1 root root 0 Mar 11 11:42 cgroup.procs -r—r—r--. 1 root root 0 Mar 11 11:42 cpuacct.stat -rw-r—r--. 1 root root 0 Mar 11 11:42 cpuacct.usage -r—r—r--. 1 root root 0 Mar 11 11:42 cpuacct.usage_all -r—r—r--. 1 root root 0 Mar 11 11:42 cpuacct.usage_percpu -r—r—r--. 1 root root 0 Mar 11 11:42 cpuacct.usage_percpu_sys -r—r—r--. 1 root root 0 Mar 11 11:42 cpuacct.usage_percpu_user -r—r—r--. 1 root root 0 Mar 11 11:42 cpuacct.usage_sys -r—r—r--. 1 root root 0 Mar 11 11:42 cpuacct.usage_user -rw-r—r--. 1 root root 0 Mar 11 11:42 cpu.cfs_period_us -rw-r—r--. 1 root root 0 Mar 11 11:42 cpu.cfs_quota_us -rw-r—r--. 1 root root 0 Mar 11 11:42 cpu.rt_period_us -rw-r—r--. 1 root root 0 Mar 11 11:42 cpu.rt_runtime_us -rw-r—r--. 1 root root 0 Mar 11 11:42 cpu.shares -r—r—r--. 1 root root 0 Mar 11 11:42 cpu.stat -rw-r—r--. 1 root root 0 Mar 11 11:42 notify_on_release -rw-r—r--. 1 root root 0 Mar 11 11:42 tasks
The example output shows files, such as
cpuacct.usage
,cpu.cfs._period_us
, that represent specific configurations and/or limits, which can be set for processes in theExample
control group. Notice that the respective file names are prefixed with the name of the control group controller to which they belong.By default, the newly created control group inherits access to the system’s entire CPU resources without a limit.
Configure CPU limits for the control group:
# echo "1000000" > /sys/fs/cgroup/cpu/Example/cpu.cfs_period_us # echo "200000" > /sys/fs/cgroup/cpu/Example/cpu.cfs_quota_us
The
cpu.cfs_period_us
file represents a period of time in microseconds (µs, represented here as "us") for how frequently a control group’s access to CPU resources should be reallocated. The upper limit is 1 second and the lower limit is 1000 microseconds.The
cpu.cfs_quota_us
file represents the total amount of time in microseconds for which all processes collectively in a control group can run during one period (as defined bycpu.cfs_period_us
). As soon as processes in a control group, during a single period, use up all the time specified by the quota, they are throttled for the remainder of the period and not allowed to run until the next period. The lower limit is 1000 microseconds.The example commands above set the CPU time limits so that all processes collectively in the
Example
control group will be able to run only for 0.2 seconds (defined bycpu.cfs_quota_us
) out of every 1 second (defined bycpu.cfs_period_us
).Optionally, verify the limits:
# cat /sys/fs/cgroup/cpu/Example/cpu.cfs_period_us /sys/fs/cgroup/cpu/Example/cpu.cfs_quota_us 1000000 200000
Add the application’s PID to the
Example
control group:# echo "6955" > /sys/fs/cgroup/cpu/Example/cgroup.procs or # echo "6955" > /sys/fs/cgroup/cpu/Example/tasks
The previous command ensures that a desired application becomes a member of the
Example
control group and hence does not exceed the CPU limits configured for theExample
control group. The PID should represent an existing process in the system. ThePID 6955
here was assigned to processsha1sum /dev/zero &
, used to illustrate the use-case of thecpu
controller.Verify that the application runs in the specified control group:
# cat /proc/6955/cgroup 12:cpuset:/ 11:hugetlb:/ 10:net_cls,net_prio:/ 9:memory:/user.slice/user-1000.slice/user@1000.service 8:devices:/user.slice 7:blkio:/ 6:freezer:/ 5:rdma:/ 4:pids:/user.slice/user-1000.slice/user@1000.service 3:perf_event:/ 2:cpu,cpuacct:/Example 1:name=systemd:/user.slice/user-1000.slice/user@1000.service/gnome-terminal-server.service
The example output above shows that the process of the desired application runs in the
Example
control group, which applies CPU limits to the application’s process.Identify the current CPU consumption of your throttled application:
# top top - 12:28:42 up 1:06, 1 user, load average: 1.02, 1.02, 1.00 Tasks: 266 total, 6 running, 260 sleeping, 0 stopped, 0 zombie %Cpu(s): 11.0 us, 1.2 sy, 0.0 ni, 87.5 id, 0.0 wa, 0.2 hi, 0.0 si, 0.2 st MiB Mem : 1826.8 total, 287.1 free, 1054.4 used, 485.3 buff/cache MiB Swap: 1536.0 total, 1396.7 free, 139.2 used. 608.3 avail Mem PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND 6955 root 20 0 228440 1752 1472 R 20.6 0.1 47:11.43 sha1sum 5760 jdoe 20 0 3604956 208832 65316 R 2.3 11.2 0:43.50 gnome-shell 6448 jdoe 20 0 743836 31736 19488 S 0.7 1.7 0:08.25 gnome-terminal- 505 root 20 0 0 0 0 I 0.3 0.0 0:03.39 kworker/u4:4-events_unbound 4217 root 20 0 74192 1612 1320 S 0.3 0.1 0:01.19 spice-vdagentd ...
Notice that the CPU consumption of the
PID 6955
has decreased from 99% to 20%.
The cgroups-v2
counterpart for cpu.cfs_period_us
and cpu.cfs_quota_us
is the cpu.max
file. The cpu.max
file is available through the cpu
controller.
Additional resources
- Understanding control groups
- What kernel resource controllers are
-
cgroups(7)
,sysfs(5)
manual pages
Chapter 20. Analyzing system performance with BPF Compiler Collection
As a system administrator, you can use the BPF Compiler Collection (BCC) library to create tools for analyzing the performance of your Linux operating system and gathering information, which could be difficult to obtain through other interfaces.
20.1. An introduction to BCC
BPF Compiler Collection (BCC) is a library, which facilitates the creation of the extended Berkeley Packet Filter (eBPF) programs. The main utility of eBPF programs is analyzing OS performance and network performance without experiencing overhead or security issues.
BCC removes the need for users to know deep technical details of eBPF, and provides many out-of-the-box starting points, such as the bcc-tools
package with pre-created eBPF programs.
The eBPF programs are triggered on events, such as disk I/O, TCP connections, and process creations. It is unlikely that the programs should cause the kernel to crash, loop or become unresponsive because they run in a safe virtual machine in the kernel.
20.2. Installing the bcc-tools package
This section describes how to install the bcc-tools
package, which also installs the BPF Compiler Collection (BCC) library as a dependency.
Procedure
Install
bcc-tools
:#
dnf install bcc-tools
The BCC tools are installed in the
/usr/share/bcc/tools/
directory.Optionally, inspect the tools:
#
ll /usr/share/bcc/tools/
... -rwxr-xr-x. 1 root root 4198 Dec 14 17:53 dcsnoop -rwxr-xr-x. 1 root root 3931 Dec 14 17:53 dcstat -rwxr-xr-x. 1 root root 20040 Dec 14 17:53 deadlock_detector -rw-r--r--. 1 root root 7105 Dec 14 17:53 deadlock_detector.c drwxr-xr-x. 3 root root 8192 Mar 11 10:28 doc -rwxr-xr-x. 1 root root 7588 Dec 14 17:53 execsnoop -rwxr-xr-x. 1 root root 6373 Dec 14 17:53 ext4dist -rwxr-xr-x. 1 root root 10401 Dec 14 17:53 ext4slower ...The
doc
directory in the listing above contains documentation for each tool.
20.3. Using selected bcc-tools for performance analyses
This section describes how to use certain pre-created programs from the BPF Compiler Collection (BCC) library to efficiently and securely analyze the system performance on the per-event basis. The set of pre-created programs in the BCC library can serve as examples for creation of additional programs.
Prerequisites
- Installed bcc-tools package
- Root permissions
Using execsnoop to examine the system processes
Execute the
execsnoop
program in one terminal:# /usr/share/bcc/tools/execsnoop
In another terminal execute for example:
$ ls /usr/share/bcc/tools/doc/
The above creates a short-lived process of the
ls
command.The terminal running
execsnoop
shows the output similar to the following:PCOMM PID PPID RET ARGS ls 8382 8287 0 /usr/bin/ls --color=auto /usr/share/bcc/tools/doc/ ...
The
execsnoop
program prints a line of output for each new process, which consumes system resources. It even detects processes of programs that run very shortly, such asls
, and most monitoring tools would not register them.The
execsnoop
output displays the following fields:-
PCOMM - The parent process name. (
ls
) -
PID - The process ID. (
8382
) -
PPID - The parent process ID. (
8287
) -
RET - The return value of the
exec()
system call (0
), which loads program code into new processes. - ARGS - The location of the started program with arguments.
-
PCOMM - The parent process name. (
To see more details, examples, and options for execsnoop
, refer to the /usr/share/bcc/tools/doc/execsnoop_example.txt
file.
For more information about exec()
, see exec(3)
manual pages.
Using opensnoop to track what files a command opens
Execute the
opensnoop
program in one terminal:# /usr/share/bcc/tools/opensnoop -n uname
The above prints output for files, which are opened only by the process of the
uname
command.In another terminal execute:
$ uname
The command above opens certain files, which are captured in the next step.
The terminal running
opensnoop
shows the output similar to the following:PID COMM FD ERR PATH 8596 uname 3 0 /etc/ld.so.cache 8596 uname 3 0 /lib64/libc.so.6 8596 uname 3 0 /usr/lib/locale/locale-archive ...
The
opensnoop
program watches theopen()
system call across the whole system, and prints a line of output for each file thatuname
tried to open along the way.The
opensnoop
output displays the following fields:-
PID - The process ID. (
8596
) -
COMM - The process name. (
uname
) -
FD - The file descriptor - a value that
open()
returns to refer to the open file. (3
) - ERR - Any errors.
PATH - The location of files that
open()
tried to open.If a command tries to read a non-existent file, then the
FD
column returns-1
and theERR
column prints a value corresponding to the relevant error. As a result,opensnoop
can help you identify an application that does not behave properly.
-
PID - The process ID. (
To see more details, examples, and options for opensnoop
, refer to the /usr/share/bcc/tools/doc/opensnoop_example.txt
file.
For more information about open()
, see open(2)
manual pages.
Using biotop to examine the I/O operations on the disk
Execute the
biotop
program in one terminal:# /usr/share/bcc/tools/biotop 30
The command enables you to monitor the top processes, which perform I/O operations on the disk. The argument ensures that the command will produce a 30 second summary.
NoteWhen no argument provided, the output screen by default refreshes every 1 second.
In another terminal execute for example :
# dd if=/dev/vda of=/dev/zero
The command above reads the content from the local hard disk device and writes the output to the
/dev/zero
file. This step generates certain I/O traffic to illustratebiotop
.The terminal running
biotop
shows the output similar to the following:PID COMM D MAJ MIN DISK I/O Kbytes AVGms 9568 dd R 252 0 vda 16294 14440636.0 3.69 48 kswapd0 W 252 0 vda 1763 120696.0 1.65 7571 gnome-shell R 252 0 vda 834 83612.0 0.33 1891 gnome-shell R 252 0 vda 1379 19792.0 0.15 7515 Xorg R 252 0 vda 280 9940.0 0.28 7579 llvmpipe-1 R 252 0 vda 228 6928.0 0.19 9515 gnome-control-c R 252 0 vda 62 6444.0 0.43 8112 gnome-terminal- R 252 0 vda 67 2572.0 1.54 7807 gnome-software R 252 0 vda 31 2336.0 0.73 9578 awk R 252 0 vda 17 2228.0 0.66 7578 llvmpipe-0 R 252 0 vda 156 2204.0 0.07 9581 pgrep R 252 0 vda 58 1748.0 0.42 7531 InputThread R 252 0 vda 30 1200.0 0.48 7504 gdbus R 252 0 vda 3 1164.0 0.30 1983 llvmpipe-1 R 252 0 vda 39 724.0 0.08 1982 llvmpipe-0 R 252 0 vda 36 652.0 0.06 ...
The
biotop
output displays the following fields:-
PID - The process ID. (
9568
) -
COMM - The process name. (
dd
) -
DISK - The disk performing the read operations. (
vda
) - I/O - The number of read operations performed. (16294)
- Kbytes - The amount of Kbytes reached by the read operations. (14,440,636)
- AVGms - The average I/O time of read operations. (3.69)
-
PID - The process ID. (
To see more details, examples, and options for biotop
, refer to the /usr/share/bcc/tools/doc/biotop_example.txt
file.
For more information about dd
, see dd(1)
manual pages.
Using xfsslower to expose unexpectedly slow file system operations
Execute the
xfsslower
program in one terminal:# /usr/share/bcc/tools/xfsslower 1
The command above measures the time the XFS file system spends in performing read, write, open or sync (
fsync
) operations. The1
argument ensures that the program shows only the operations that are slower than 1 ms.NoteWhen no arguments provided,
xfsslower
by default displays operations slower than 10 ms.In another terminal execute, for example, the following:
$ vim text
The command above creates a text file in the
vim
editor to initiate certain interaction with the XFS file system.The terminal running
xfsslower
shows something similar upon saving the file from the previous step:TIME COMM PID T BYTES OFF_KB LAT(ms) FILENAME 13:07:14 b'bash' 4754 R 256 0 7.11 b'vim' 13:07:14 b'vim' 4754 R 832 0 4.03 b'libgpm.so.2.1.0' 13:07:14 b'vim' 4754 R 32 20 1.04 b'libgpm.so.2.1.0' 13:07:14 b'vim' 4754 R 1982 0 2.30 b'vimrc' 13:07:14 b'vim' 4754 R 1393 0 2.52 b'getscriptPlugin.vim' 13:07:45 b'vim' 4754 S 0 0 6.71 b'text' 13:07:45 b'pool' 2588 R 16 0 5.58 b'text' ...
Each line above represents an operation in the file system, which took more time than a certain threshold.
xfsslower
is good at exposing possible file system problems, which can take form of unexpectedly slow operations.The
xfsslower
output displays the following fields:-
COMM - The process name. (
b’bash'
) T - The operation type. (
R
)- Read
- Write
- Sync
- OFF_KB - The file offset in KB. (0)
- FILENAME - The file being read, written, or synced.
-
COMM - The process name. (
To see more details, examples, and options for xfsslower
, refer to the /usr/share/bcc/tools/doc/xfsslower_example.txt
file.
For more information about fsync
, see fsync(2)
manual pages.
Chapter 21. Enhancing security with the kernel integrity subsystem
You can increase the protection of your system by utilizing components of the kernel integrity subsystem. The following sections introduce the relevant components and provide guidance on their configuration.
21.1. The kernel integrity subsystem
The integrity subsystem is a part of the kernel which is responsible for maintaining the overall system’s data integrity. This subsystem helps to keep the state of a certain system the same from the time it was built thereby it prevents undesired modification on specific system files from users.
The kernel integrity subsystem consists of two major components:
- Integrity Measurement Architecture (IMA)
- Measures files' content whenever it is executed or opened. Users can change this behavior by applying custom policies.
- Places the measured values within the kernel’s memory space thereby it prevents any modification from the users of the system.
- Allows local and remote parties to verify the measured values.
- Extended Verification Module (EVM)
- Protects files' extended attributes (also known as xattr) that are related to the system’s security, like IMA measurements and SELinux attributes, by cryptographically hashing their corresponding values.
Both IMA and EVM also contain numerous feature extensions that bring additional functionality. For example:
- IMA-Appraisal
- Provides local validation of the current file’s content against the values previously stored in the measurement file within the kernel memory. This extension forbids any operation to be performed over a specific file in case the current and the previous measure do not match.
- EVM Digital Signatures
- Allows digital signatures to be used through cryptographic keys stored into the kernel’s keyring.
The feature extensions complement each other, but you can configure and use them independently of one another.
The kernel integrity subsystem can harness the Trusted Platform Module (TPM) to harden the system security even more. TPM is a specification by the Trusted Computing Group (TCG) for important cryptographic functions. TPMs are usually built as dedicated hardware that is attached to the platform’s motherboard and prevents software-based attacks by providing cryptographic functions from a protected and tamper-proof area of the hardware chip. Some of the TPM features are:
- Random-number generator
- Generator and secure storage for cryptographic keys
- Hashing generator
- Remote attestation
21.2. Integrity measurement architecture
Integrity Measurement Architecture (IMA) is a component of the kernel integrity subsystem. IMA aims to maintain the contents of local files. Specifically, IMA measures, stores, and appraises files' hashes before they are accessed, which prevents the reading and execution of unreliable data. Thereby, IMA enhances the security of the system.
21.3. Extended verification module
Extended Verification Module (EVM) is a component of the kernel integrity subsystem, which monitors changes in files' extended attributes (xattr). Many security-oriented technologies, including Integrity Measurement Architecture (IMA), store sensitive file information, such as content hashes, in the extended attributes. EVM creates another hash from these extended attributes and from a special key, which is loaded at boot time. The resulting hash is validated every time the extended attribute is used. For example, when IMA appraises the file.
RHEL 9 accepts the special encrypted key under the evm-key
keyring. The key was created by a master key held in the kernel keyrings.
21.4. Trusted and encrypted keys
The following section introduces trusted and encrypted keys as an important part of enhancing system security.
Trusted and encrypted keys are variable-length symmetric keys generated by the kernel that utilize the kernel keyring service. The fact that this type of keys never appear in the user space in an unencrypted form means that their integrity can be verified, which in turn means that they can be used, for example, by the extended verification module (EVM) to verify and confirm the integrity of a running system. User-level programs can only access the keys in the form of encrypted blobs.
Trusted keys need a hardware component: the Trusted Platform Module (TPM) chip, which is used to both create and encrypt (seal) the keys. The TPM seals the keys using a 2048-bit RSA key called the storage root key (SRK).
To use a TPM 1.2 specification, enable and activate it through a setting in the machine firmware or by using the tpm_setactive
command from the tpm-tools
package of utilities. Also, the TrouSers
software stack needs to be installed and the tcsd
daemon needs to be running to communicate with the TPM (dedicated hardware). The tcsd
daemon is part of the TrouSers
suite, which is available through the trousers
package. The more recent and backward incompatible TPM 2.0 uses a different software stack, where the tpm2-tools
or ibm-tss
utilities provide access to the dedicated hardware.
In addition to that, the user can seal the trusted keys with a specific set of the TPM’s platform configuration register (PCR) values. PCR contains a set of integrity-management values that reflect the firmware, boot loader, and operating system. This means that PCR-sealed keys can only be decrypted by the TPM on the same system on which they were encrypted. However, once a PCR-sealed trusted key is loaded (added to a keyring), and thus its associated PCR values are verified, it can be updated with new (or future) PCR values, so that a new kernel, for example, can be booted. A single key can also be saved as multiple blobs, each with different PCR values.
Encrypted keys do not require a TPM, as they use the kernel Advanced Encryption Standard (AES), which makes them faster than trusted keys. Encrypted keys are created using kernel-generated random numbers and encrypted by a master key when they are exported into user-space blobs. The master key is either a trusted key or a user key. If the master key is not trusted, the encrypted key is only as secure as the user key used to encrypt it.
21.5. Working with trusted keys
The following section describes how to create, export, load or update trusted keys with the keyctl
utility to improve the system security.
Prerequisites
-
For the 64-bit ARM architecture and IBM Z, the
trusted
kernel module needs to be loaded. For more information on how to load kernel modules, see Managing kernel modules. - Trusted Platform Module (TPM) needs to be enabled and active. For more information about TPM see, The kernel integrity subsystem and Trusted and encrypted keys.
Procedure
To create a trusted key using a TPM, execute:
# keyctl add trusted <name> "new <key_length> [options]" <key_ring>
Based on the syntax, construct an example command as follows:
# keyctl add trusted kmk "new 32" @u 642500861
The command creates a trusted key called
kmk
with the length of 32 bytes (256 bits) and places it in the user keyring (@u
). The keys may have a length of 32 to 128 bytes (256 to 1024 bits).
To list the current structure of the kernel keyrings:
# keyctl show Session Keyring -3 --alswrv 500 500 keyring: _ses 97833714 --alswrv 500 -1 \_ keyring: _uid.1000 642500861 --alswrv 500 500 \_ trusted: kmk
To export the key to a user-space blob, execute:
# keyctl pipe 642500861 > kmk.blob
The command uses the
pipe
subcommand and the serial number ofkmk
.To load the trusted key from the user-space blob, use the
add
subcommand with the blob as an argument:# keyctl add trusted kmk "load `cat kmk.blob`" @u 268728824
Create secure encrypted keys based on the TPM-sealed trusted key:
# keyctl add encrypted <pass:quotes[name]> "new [format] <pass:quotes[key_type]>:<pass:quotes[primary_key_name]> <pass:quotes[keylength]>" <pass:quotes[key_ring]>
Based on the syntax, generate an encrypted key using the already created trusted key:
# keyctl add encrypted encr-key "new trusted:kmk 32" @u 159771175
The command uses the TPM-sealed trusted key (
kmk
), produced in the previous step, as a primary key for generating encrypted keys.
Additional resources
-
keyctl(1)
manual page - Trusted and encrypted keys
- Kernel Key Retention Service
- The kernel integrity subsystem
21.6. Working with encrypted keys
The following section describes managing encrypted keys to improve the system security on systems where a Trusted Platform Module (TPM) is not available.
Prerequisites
-
For the 64-bit ARM architecture and IBM Z, the
encrypted-keys
kernel module needs to be loaded. For more information on how to load kernel modules, see Managing kernel modules.
Procedure
Use a random sequence of numbers to generate a user key:
# keyctl add user kmk-user "$(dd if=/dev/urandom bs=1 count=32 2>/dev/null)" @u 427069434
The command generates a user key called
kmk-user
which acts as a primary key and is used to seal the actual encrypted keys.Generate an encrypted key using the primary key from the previous step:
# keyctl add encrypted encr-key "new user:kmk-user 32" @u 1012412758
Optionally, list all keys in the specified user keyring:
# keyctl list @u 2 keys in keyring: 427069434: --alswrv 1000 1000 user: kmk-user 1012412758: --alswrv 1000 1000 encrypted: encr-key
Keep in mind that encrypted keys that are not sealed by a trusted primary key are only as secure as the user primary key (random-number key) that was used to encrypt them. Therefore, the primary user key should be loaded as securely as possible and preferably early during the boot process.
Additional resources
-
keyctl(1)
manual page - Kernel Key Retention Service
21.7. Enabling integrity measurement architecture and extended verification module
Integrity measurement architecture (IMA) and extended verification module (EVM) belong to the kernel integrity subsystem and enhance the system security in various ways. The following section describes how to enable and configure IMA and EVM to improve the security of the operating system.
Prerequisites
Verify that the
securityfs
filesystem is mounted on the/sys/kernel/security/
directory and the/sys/kernel/security/integrity/ima/
directory exists.# mount … securityfs on /sys/kernel/security type securityfs (rw,nosuid,nodev,noexec,relatime) …
Verify that the
systemd
service manager is already patched to support IMA and EVM on boot time:# dmesg | grep -i -e EVM -e IMA [ 0.000000] Command line: BOOT_IMAGE=(hd0,msdos1)/vmlinuz-5.14.0-1.el9.x86_64 root=/dev/mapper/rhel-root ro crashkernel=1G-4G:192M,4G-64G:256M,64G-:512M resume=/dev/mapper/rhel-swap rd.lvm.lv=rhel/root rd.lvm.lv=rhel/swap rhgb quiet [ 0.000000] kvm-clock: cpu 0, msr 23601001, primary cpu clock [ 0.000000] Using crashkernel=1G-4G:192M,4G-64G:256M,64G-:512M, the size chosen is a best effort estimation. [ 0.000000] Kernel command line: BOOT_IMAGE=(hd0,msdos1)/vmlinuz-5.14.0-1.el9.x86_64 root=/dev/mapper/rhel-root ro crashkernel=1G-4G:192M,4G-64G:256M,64G-:512M resume=/dev/mapper/rhel-swap rd.lvm.lv=rhel/root rd.lvm.lv=rhel/swap rhgb quiet [ 0.911527] ima: No TPM chip found, activating TPM-bypass! [ 0.911538] ima: Allocated hash algorithm: sha1 [ 0.911580] evm: Initialising EVM extended attributes: [ 0.911581] evm: security.selinux [ 0.911581] evm: security.ima [ 0.911582] evm: security.capability [ 0.911582] evm: HMAC attrs: 0x1 [ 1.715151] systemd[1]: systemd 239 running in system mode. (+PAM +AUDIT +SELINUX +IMA -APPARMOR +SMACK +SYSVINIT +UTMP +LIBCRYPTSETUP +GCRYPT +GNUTLS +ACL +XZ +LZ4 +SECCOMP +BLKID +ELFUTILS +KMOD +IDN2 -IDN +PCRE2 default-hierarchy=legacy) [ 3.824198] fbcon: qxldrmfb (fb0) is primary device [ 4.673457] PM: Image not found (code -22) [ 6.549966] systemd[1]: systemd 239 running in system mode. (+PAM +AUDIT +SELINUX +IMA -APPARMOR +SMACK +SYSVINIT +UTMP +LIBCRYPTSETUP +GCRYPT +GNUTLS +ACL +XZ +LZ4 +SECCOMP +BLKID +ELFUTILS +KMOD +IDN2 -IDN +PCRE2 default-hierarchy=legacy)
Procedure
Add the following kernel command line parameters:
# grubby --update-kernel=/boot/vmlinuz-$(uname -r) --args="ima_policy=appraise_tcb ima_appraise=fix evm=fix"
The command enables IMA and EVM in the fix mode for the current boot entry and allows users to gather and update the IMA measurements.
The
ima_policy=appraise_tcb
kernel command line parameter ensures that the kernel uses the default Trusted Computing Base (TCB) measurement policy and the appraisal step. The appraisal part forbids access to files, whose prior and current measures do not match.- Reboot to make the changes come into effect.
Optionally, verify that the parameters have been added to the kernel command line:
# cat /proc/cmdline BOOT_IMAGE=(hd0,msdos1)/vmlinuz-5.14.0-1.el9.x86_64 root=/dev/mapper/rhel-root ro crashkernel=1G-4G:192M,4G-64G:256M,64G-:512M resume=/dev/mapper/rhel-swap rd.lvm.lv=rhel/root rd.lvm.lv=rhel/swap rhgb quiet ima_policy=appraise_tcb ima_appraise=fix evm=fix
Create a kernel master key to protect the EVM key:
# keyctl add user kmk "$(dd if=/dev/urandom bs=1 count=32 2> /dev/null)" @u 748544121
The kernel master key (
kmk
) is kept entirely in the kernel space memory. The 32-byte long value of the kernel master keykmk
is generated from random bytes from the/dev/urandom
file and placed in the user (@u
) keyring. The key serial number is on the second line of the previous output.Create an encrypted EVM key based on the
kmk
key:# keyctl add encrypted evm-key "new user:kmk 64" @u 641780271
The command uses
kmk
to generate and encrypt a 64-byte long user key (namedevm-key
) and places it in the user (@u
) keyring. The key serial number is on the second line of the previous output.ImportantIt is necessary to name the user key as evm-key because that is the name the EVM subsystem is expecting and is working with.
Create a directory for exported keys.
# mkdir -p /etc/keys/
Search for the
kmk
key and export its value into a file:# keyctl pipe $(keyctl search @u user kmk) > /etc/keys/kmk
The command places the unencrypted value of the kernel master key (
kmk
) into a file of previously defined location (/etc/keys/
).Search for the
evm-key
user key and export its value into a file:# keyctl pipe $(keyctl search @u encrypted evm-key) > /etc/keys/evm-key
The command places the encrypted value of the user
evm-key
key into a file of arbitrary location. Theevm-key
has been encrypted by the kernel master key earlier.Optionally, view the newly created keys:
# keyctl show Session Keyring 974575405 --alswrv 0 0 keyring: ses 299489774 --alswrv 0 65534 \ keyring: uid.0 748544121 --alswrv 0 0 \ user: kmk 641780271 --alswrv 0 0 \_ encrypted: evm-key # ls -l /etc/keys/ total 8 -rw-r—r--. 1 root root 246 Jun 24 12:44 evm-key -rw-r—r--. 1 root root 32 Jun 24 12:43 kmk
You should be able to see a similar output.
Activate EVM:
# echo 1 > /sys/kernel/security/evm
Verification step
You can verify that EVM has been initialized by entering:
# dmesg | tail -1 […] evm: key initialized
If the system is rebooted, the keys are removed from the keyring. In such a case, you can import the already exported kmk
and evm-key
keys.
Procedure
Add the user
kmk
key (already exported to the/etc/keys/kmk
file in step 7).# keyctl add user kmk "$(cat /etc/keys/kmk)" @u 451342217 # keyctl show Session Keyring 695566911 --alswrv 0 0 keyring: ses 58982213 --alswrv 0 65534 \ keyring: uid.0 451342217 --alswrv 0 0 \ user: kmk
Import the user
evm-key
key (already exported to the/etc/keys/evm-key
file in step 8).# keyctl add encrypted evm-key "load $(cat /etc/keys/evm-key)" @u 924537557 # keyctl show Session Keyring 695566911 --alswrv 0 0 keyring: ses 58982213 --alswrv 0 65534 \ keyring: uid.0 451342217 --alswrv 0 0 \ user: kmk 924537557 --alswrv 0 0 \_ encrypted: evm-key
21.8. Collecting file hashes with integrity measurement architecture
The first level of operation of integrity measurement architecture (IMA) is the measurement phase, which allows to create file hashes and store them as extended attributes (xattrs) of those files. The following section describes how to create and inspect the files' hashes.
Prerequisites
- Enable integrity measurement architecture (IMA) and extended verification module (EVM) as described in Enabling integrity measurement architecture and extended verification module.
Verify that the
ima-evm-utils
,attr
, andkeyutils
packages are already installed:# dnf install ima-evm-utils attr keyutils Updating Subscription Management repositories. This system is registered to Red Hat Subscription Management, but is not receiving updates. You can use subscription-manager to assign subscriptions. Last metadata expiration check: 0:58:22 ago on Fri 14 Feb 2020 09:58:23 AM CET. Package ima-evm-utils-1.1-5.el8.x86_64 is already installed. Package attr-2.4.48-3.el8.x86_64 is already installed. Package keyutils-1.5.10-7.el8.x86_64 is already installed. Dependencies resolved. Nothing to do. Complete!
Procedure
Create a test file:
# echo <Test_text> > test_file
IMA and EVM ensure that the example file
test_file
is assigned hash values, which are stored as its extended attributes.Inspect extended attributes of the file:
# getfattr -m . -d test_file # file: test_file security.evm=0sAnDIy4VPA0HArpPO/EqiutnNyBql security.ima=0sAQOEDeuUnWzwwKYk+n66h/vby3eD security.selinux="unconfined_u:object_r:admin_home_t:s0"
The previous example output shows extended attributes related to SELinux and the IMA and EVM hash values. EVM actively adds a
security.evm
extended attribute and detects any offline tampering to xattrs of other files such assecurity.ima
that are directly related to content integrity of files. The value of thesecurity.evm
field is in Hash-based Message Authentication Code (HMAC-SHA1), which was generated with theevm-key
user key.
Additional resources
Chapter 22. Using Ansible roles to permanently configure kernel parameters
You can use the Kernel Settings role to configure kernel parameters on multiple clients at once. This solution:
- Provides a friendly interface with efficient input setting.
- Keeps all intended kernel parameters in one place.
After you run the Kernel Settings role from the control machine, the kernel parameters are applied to the managed systems immediately and persist across reboots.
Note that RHEL System Role delivered over RHEL channels are available to RHEL customers as an RPM package in the default AppStream repository. RHEL System Role are also available as a collection to customers with Ansible subscriptions over Ansible Automation Hub.
22.1. Introduction to the kernel settings role
RHEL System Roles is a set of roles that provide a consistent configuration interface to remotely manage multiple systems.
RHEL System Roles were introduced for automated configurations of the kernel using the Kernel Settings System Role. The rhel-system-roles
package contains this system role, and also the reference documentation.
To apply the kernel parameters on one or more systems in an automated fashion, use the Kernel Settings role with one or more of its role variables of your choice in a playbook. A playbook is a list of one or more plays that are human-readable, and are written in the YAML format.
With the Kernel Settings role you can configure:
-
The kernel parameters using the
kernel_settings_sysctl
role variable -
Various kernel subsystems, hardware devices, and device drivers using the
kernel_settings_sysfs
role variable -
The CPU affinity for the
systemd
service manager and processes it forks using thekernel_settings_systemd_cpu_affinity
role variable -
The kernel memory subsystem transparent hugepages using the
kernel_settings_transparent_hugepages
andkernel_settings_transparent_hugepages_defrag
role variables
Additional resources
-
README.md
andREADME.html
files in the/usr/share/doc/rhel-system-roles/kernel_settings/
directory - Working with playbooks
- How to build your inventory
22.2. Applying selected kernel parameters using the Kernel Settings role
Follow these steps to prepare and apply an Ansible playbook to remotely configure kernel parameters with persisting effect on multiple managed operating systems.
Prerequisites
-
You have
root
permissions. -
Entitled by your RHEL subscription, you installed the
ansible-core
andrhel-system-roles
packages on the control machine. - An inventory of managed hosts is present on the control machine and Ansible is able to connect to them.
RHEL 8.0 - 8.5 provided access to a separate Ansible repository that contains Ansible Engine 2.9 for automation based on Ansible. Ansible Engine contains command-line utilities such as ansible
, ansible-playbook
; connectors such as docker
and podman
; and the entire world of plugins and modules. For information on how to obtain and install Ansible Engine, refer to How do I Download and Install Red Hat Ansible Engine?.
RHEL 8.6 and 9.0 has introduced Ansible Core (provided as ansible-core
RPM), which contains the Ansible command-line utilities, commands, and a small set of built-in Ansible plugins. The AppStream repository provides ansible-core
, which has a limited scope of support. You can learn more by reviewing Scope of support for the ansible-core package included in the RHEL 9 AppStream.
Procedure
Optionally, review the
inventory
file for illustration purposes:# cat /home/jdoe/<ansible_project_name>/inventory [testingservers] pdoe@192.168.122.98 fdoe@192.168.122.226 [db-servers] db1.example.com db2.example.com [webservers] web1.example.com web2.example.com 192.0.2.42
The file defines the
[testingservers]
group and other groups. It allows you to run Ansible more effectively against a specific set of systems.Create a configuration file to set defaults and privilege escalation for Ansible operations.
Create a new YAML file and open it in a text editor, for example:
# vi /home/jdoe/<ansible_project_name>/ansible.cfg
Insert the following content into the file:
[defaults] inventory = ./inventory [privilege_escalation] become = true become_method = sudo become_user = root become_ask_pass = true
The
[defaults]
section specifies a path to the inventory file of managed hosts. The[privilege_escalation]
section defines that user privileges be shifted toroot
on the specified managed hosts. This is necessary for successful configuration of kernel parameters. When Ansible playbook is run, you will be prompted for user password. The user automatically switches toroot
by means ofsudo
after connecting to a managed host.
Create an Ansible playbook that uses the Kernel Settings role.
Create a new YAML file and open it in a text editor, for example:
# vi /home/jdoe/<ansible_project_name>/kernel-roles.yml
This file represents a playbook and usually contains an ordered list of tasks, also called plays, that are run against specific managed hosts selected from your
inventory
file.Insert the following content into the file:
--- - hosts: testingservers name: "Configure kernel settings" roles: - rhel-system-roles.kernel_settings vars: kernel_settings_sysctl: - name: fs.file-max value: 400000 - name: kernel.threads-max value: 65536 kernel_settings_sysfs: - name: /sys/class/net/lo/mtu value: 65000 kernel_settings_transparent_hugepages: madvise
The
name
key is optional. It associates an arbitrary string with the play as a label and identifies what the play is for. Thehosts
key in the play specifies the hosts against which the play is run. The value or values for this key can be provided as individual names of managed hosts or as groups of hosts as defined in theinventory
file.The
vars
section represents a list of variables containing selected kernel parameter names and values to which they have to be set.The
roles
key specifies what system role is going to configure the parameters and values mentioned in thevars
section.NoteYou can modify the kernel parameters and their values in the playbook to fit your needs.
Optionally, verify that the syntax in your play is correct.
# ansible-playbook --syntax-check kernel-roles.yml playbook: kernel-roles.yml
This example shows the successful verification of a playbook.
Execute your playbook.
# ansible-playbook kernel-roles.yml ... BECOME password: PLAY [Configure kernel settings] ********************************************************************************** PLAY RECAP ******************************************************************************************************** fdoe@192.168.122.226 : ok=10 changed=4 unreachable=0 failed=0 skipped=6 rescued=0 ignored=0 pdoe@192.168.122.98 : ok=10 changed=4 unreachable=0 failed=0 skipped=6 rescued=0 ignored=0
Before Ansible runs your playbook, you are going to be prompted for your password and so that a user on managed hosts can be switched to
root
, which is necessary for configuring kernel parameters.The recap section shows that the play finished successfully (
failed=0
) for all managed hosts, and that 4 kernel parameters have been applied (changed=4
).- Restart your managed hosts and check the affected kernel parameters to verify that the changes have been applied and persist across reboots.
Additional resources
- Getting started with RHEL System Roles
-
README.html
andREADME.md
files in the/usr/share/doc/rhel-system-roles/kernel_settings/
directory - Build Your Inventory
- Configuring Ansible
- Working With Playbooks
- Using Variables
- Roles