Server fails to boot with Out Of Memory messages on the console

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Environment

  • Red Hat Enterprise Linux

Issue

  • Red Hat Enterprise Linux system hangs on boot where the console says "Welcome to Red Hat Enterprise Linux Server"
  • Updates or changes before the boot were minimal, if any at all, and include only a change to hugepage configurations.
  • Removed "rhgb quiet" from the command line but we still do not see any errors
  • For Red Hat Enterprise Linux 6 and under, an older kernel and single user mode hang in the same location:

  • For Red Hat Enterprise Linux 7 and above, the current kernel boots with the noted failures, but a prior installed kernel boots fine.
  • After reducing or changing DIMM sticks or changing hugepage configurations, system will not boot, and the console shows messages with Out of memory and Killed process messages similar to the following image:

Resolution

  • To avoid booting into rescue mode and to save time, one can use systemd.mask=X as a kernel command line parameter where X is the systemd service name which is responsible for making hugepages reservation. Generally, following services are responsible: systemd-sysctl.service or tuned.service. This is applicable to RHEL 7 and above.
    NOTE: This approach requires another reboot in order to bring up the masked service. Make sure to follow the rest of the steps in order to make correction in vm.nr_hugepages value before rebooting.

  • Temporarily remove any hugepage configurations, vm.nr_hugepages, from /etc/sysctl.conf or drop files under /etc/sysctl.d/* to allow the system to boot.

  • If hugepages are set via the kernel boot parameter hugepages=###, remove the parameter from the kernel parameters on boot temporarily to allow the system to boot.

  • Once the system boots, ensure the calculations for the hugepage reservation are correct and reset them via either the sysctl configuration files noted above or the kernel boot parameters.

    • Hugepage memory are each 2 MiB in size by default
    • To convert from hugepage size to megabytes, multiply the hugepage count by 2; e.g. 100 hugepages is 200 MiB in size.
    • The reservation will need to still be sized to allow regular memory to be available to the rest of the system. An application needs to be written to use hugepages and many default services on Red Hat Enterprise Linux (like SSHD and audit) do not use hugepages.

  • When using sysctl configurations to set the hugepage reservation on Red Hat Enterprise Linux 7 and above, backup and rebuild the initramfs to have the new configurations take effect.

  • In some instances, a system with hugepage reservations failed to boot because of gaps or memory holes in CPU/DIMM stick setup, and the gaps needed to be filled in with DIMM sticks.

Root Cause

  • The hugepage reservation size was set to a value larger than total RAM

    # grep vm.nr_hugepages /etc/sysctl.conf
vm.nr_hugepages = 79252380    <--- ~154,789 GiB  hugepage reservation size

# grep MemTotal /proc/meminfo
MemTotal:       137995282 kB   <--- ~132 GiB of memory installed on the system

  • For instances where the system has gaps in CPU/DIMM sticks, the DIMM banks connected to CPU were not fully filled.

Diagnostic Steps

Changing the hugepage reservation size

For Red Hat Enterprise Linux 6 and below:

  1. Add init=/bin/bash to the kernel command line on boot. This will allow you to access the system during early boot.

  2. Remount the root filesystem read-write.

    bash-4.1# mount -o remount rw /

  3. Edit the configuration.

    • If the parameter is set via sysctl configurations, edit /etc/sysctl.conf or one of the dropfiles /etc/sysctl.d/* to comment out the hugepage reservation:

      bash-4.1# vi /etc/sysctl.conf
[...]
# vm.nr_hugepages = 79252380  <--- add '#' before the vm.nr_hugepages line
[...]

    • If the parameter is set via the hugepages=### kernel boot parameters, edit /etc/grub.conf file and remove the hugepages=### parameter from thekernel` line.

    • Alternatively for either situation above, the hugepage reservation can simply be updated with the correct value rather than removed.

  4. If SELinux is enabled on the system, restore the SELinux contexts of the updated files from step 3 above. Replace <FILE> in the below command with the file that was updated in step 3:

    bash-4.1# restorecon -Rv <FILE>

  5. Reboot the system to have the changes take effect.

  6. If the configurations were removed in step 3 rather than updated, double check the calculations for the hugepage reservation and ensure enough regular memory exists for the rest of the system and workload that can not use hugepages. Once the calculations are corrected, redo the configurations removed in step 3 and reboot the system to have the hugepage reservation take effect.

For Red Hat Enterprise Linux 7 and above

  1. Temporarily boot to a prior kernel. If a prior kernel is not available, then boot the system into rescue mode.

  2. Edit the configuration.

    • If the parameter is set via sysctl configurations, edit the /etc/sysctl.conf or /etc/sysctl.d/* file with the corrected hugepage reservation.

      bash-4.1# vi /etc/sysctl.conf
[...]
vm.nr_hugepages = 5000  <--- reset reservation to 10 MiB here
[...]

    • If the parameter is set via the hugepages=### kernel parameter, set the correct hugepage parameter in the grub boot configuration file if running on Red Hat Enterprise linux 7 or correct the hugepage parameter in the grub kernelopt variables if running Red Hat Enterprise Linux 8.

  3. If the configurations were set via sysctl configuration files, rebuild the initramfs for the kernel you need to boot to.

  4. Reboot into the desired kernel.

Checking the DIMM banks

  • Once booted via one of the noted methods above, check the output of dmidecode;

    $ sed -n '/^Memory Device/,/^$/p' dmidecode  \
      | grep -e Size: -e '^[[:space:]]*Locator:' \
      | sed -e 's/\t//g' -e 's/No Module Installed/N\/A  MB/g' \
      | paste -d '\t\n' -s -  \
      | sort -n -k 6.1,2 -k 8.1,2 \
      | grep -v 'Board [2468]'

Size: 8192 MB    Locator: Board 1, DIMM 1A
Size: 8192 MB    Locator: Board 1, DIMM 2C
Size: 8192 MB    Locator: Board 1, DIMM 3B
Size: N/A  MB    Locator: Board 1, DIMM 4D  <--- hole for CPU #1
Size: N/A  MB    Locator: Board 1, DIMM 5D  <--- hole for CPU #1
Size: 8192 MB    Locator: Board 1, DIMM 6B
Size: 8192 MB    Locator: Board 1, DIMM 7C
Size: 8192 MB    Locator: Board 1, DIMM 8A

Size: 8192 MB    Locator: Board 3, DIMM 1A
Size: 8192 MB    Locator: Board 3, DIMM 2C
Size: 8192 MB    Locator: Board 3, DIMM 3B
Size: N/A  MB    Locator: Board 3, DIMM 4D   <--- hole for CPU #2
Size: N/A  MB    Locator: Board 3, DIMM 5D   <--- hole for CPU #2
Size: 8192 MB    Locator: Board 3, DIMM 6B
Size: 8192 MB    Locator: Board 3, DIMM 7C
Size: 8192 MB    Locator: Board 3, DIMM 8A

Size: 4096 MB    Locator: Board 5, DIMM 1A
Size: 4096 MB    Locator: Board 5, DIMM 2C
Size: 4096 MB    Locator: Board 5, DIMM 3B
Size: N/A  MB    Locator: Board 5, DIMM 4D   <--- hole for CPU #3
Size: N/A  MB    Locator: Board 5, DIMM 5D   <--- hole for CPU #3
Size: 4096 MB    Locator: Board 5, DIMM 6B
Size: 4096 MB    Locator: Board 5, DIMM 7C
Size: 4096 MB    Locator: Board 5, DIMM 8A

Size: 4096 MB    Locator: Board 7, DIMM 1A
Size: 4096 MB    Locator: Board 7, DIMM 2C
Size: 4096 MB    Locator: Board 7, DIMM 3B
Size: N/A  MB    Locator: Board 7, DIMM 4D   <--- hole for CPU #4
Size: N/A  MB    Locator: Board 7, DIMM 5D   <--- hole for CPU #4
Size: 4096 MB    Locator: Board 7, DIMM 6B
Size: 4096 MB    Locator: Board 7, DIMM 7C
Size: 4096 MB    Locator: Board 7, DIMM 8A

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