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Chapter 2. Using SystemTap

This chapter instructs users how to install SystemTap, and provides an introduction on how to run SystemTap scripts.

2.1. Installation and Setup

To deploy SystemTap, you need to install the SystemTap packages along with the corresponding set of -devel, -debuginfo and -debuginfo-common packages for your kernel. If your system has multiple kernels installed, and you wish to use SystemTap on more than one kernel kernel, you will need to install the -devel and -debuginfo packages for each of those kernel versions.
These procedures will be discussed in detail in the following sections.

Important

Many users confuse -debuginfo with -debug. Remember that the deployment of SystemTap requires the installation of the -debuginfo package of the kernel, not the -debug version of the kernel.

2.1.1. Installing SystemTap

To deploy Systemtap, you will need to to install the following RPMs:
  • systemtap
  • systemtap-runtime
Assuming that yum is installed in the system, these two rpms can be installed with yum install systemtap systemtap-runtime. Note that before you can use SystemTap, you will still need to install the required kernel information RPMs.

2.1.2. Installing Required Kernel Information RPMs

SystemTap needs information about the kernel in order to place instrumentation in it (i.e. probe it). This information also allows SystemTap to generate the code for the instrumentation. This information is contained in the matching -devel, -debuginfo, and -debuginfo-common packages for your kernel. The necessary -devel and -debuginfo packages for the ordinary "vanilla" kernel are as follows:
  • kernel-debuginfo
  • kernel-debuginfo-common
  • kernel-devel
Likewise, the necessary packages for the PAE kernel would be kernel-PAE-debuginfo, kernel-PAE-debuginfo-common, and kernel-PAE-devel.
To determine what kernel your system is currently using, use: 
uname -r
For example, if you wish to use SystemTap on kernel version 2.6.18-53.el5 on an i686 machine, then you would need to download and install the following RPMs:
  • kernel-debuginfo-2.6.18-53.1.13.el5.i686.rpm
  • kernel-debuginfo-common-2.6.18-53.1.13.el5.i686.rpm
  • kernel-devel-2.6.18-53.1.13.el5.i686.rpm

Important

The version, variant, and architecture of the -devel, -debuginfo and -debuginfo-common packages must match the kernel you wish to probe with SystemTap exactly.
The easiest way to install the required kernel information packages is through yum install and debuginfo-install. debuginfo-install is included with later versions of the yum-utils package (for example, version 1.1.10), and also requires an appropriate yum repository from which to download and install -debuginfo/-debuginfo-common packages.
Most required kernel packages can be found at ftp://ftp.redhat.com/pub/redhat/linux/enterprise/; navigate there until you find the appropriate Debuginfo directory for your system. Configure yum accordingly by adding a new "debug" yum repository file under /etc/yum.repos.d containing the following lines: 
[rhel-debuginfo]
name=Red Hat Enterprise Linux $releasever - $basearch - Debug
baseurl=ftp://ftp.redhat.com/pub/redhat/linux/enterprise/$releasever/en/os/$basearch/Debuginfo/
enabled=1
After configuring yum with the appropriate repository, you can now install the required -devel, -debuginfo, and -debuginfo-common packages for your kernel. To install the corresponding packages for a specific kernel, run the following commands:
  • yum install kernelname-devel-version
  • debuginfo-install kernelname-version
Replace kernelname with the appropriate kernel variant name (for example, kernel-PAE), and version with the target kernel's version. For example, to install the required kernel information packages for the kernel-PAE--2.6.18-53.1.13.el5 kernel, run:
  • yum install kernel-PAE-devel-2.6.18-53.1.13.el5
  • debuginfo-install kernel-PAE-2.6.18-53.1.13.el5
If you do not have yum and yum-utils installed (and you are unable to install them), you will have to manually download and install the required kernel information packages. To generate the URL from which to download the required packages, use the following script:
rheldebugurl.sh

#! /bin/bash
pkg="redhat-release"
releasever=`rpm -q --qf "%{version}" $pkg`
base=`uname -m`
echo "ftp://ftp.redhat.com/pub/redhat/linux/\
enterprise/$releasever/en/os/$base/Debuginfo"

Once you have manually downloaded the required packages to the machine, install the RPMs by running rpm --force -ivh package_names.

2.1.3. Initial Testing

If you are currently using the kernel you wish to probe with SystemTap, you can immediately test whether the deployment was successful. If not, you will need to reboot and load the appropriate kernel.
To start the test, run the command stap -v -e 'probe vfs.read {printf("read performed\n"); exit()}'. This command simply instructs SystemTap to print read performed then exit properly once a virtual file system read is detected. If the SystemTap deployment was successful, you should get output similar to the following: 
Pass 1: parsed user script and 45 library script(s) in 340usr/0sys/358real ms.
Pass 2: analyzed script: 1 probe(s), 1 function(s), 0 embed(s), 0 global(s) in 290usr/260sys/568real ms.
Pass 3: translated to C into "/tmp/stapiArgLX/stap_e5886fa50499994e6a87aacdc43cd392_399.c" in 490usr/430sys/938real ms.
Pass 4: compiled C into "stap_e5886fa50499994e6a87aacdc43cd392_399.ko" in 3310usr/430sys/3714real ms.
Pass 5: starting run.
read performed
Pass 5: run completed in 10usr/40sys/73real ms.
The last three lines of the output (i.e. beginning with Pass 5 indicate that SystemTap was able to successfully create the instrumentation to probe the kernel, run the instrumentation, detect the event being probed (in this case, a virtual file system read), and execute a valid handler (print text then close it with no errors).