SysRq and Cumulus Linux

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Issue

/var/log/syslog may show SysRq output that is unexpected. The output may look something like this:

[ 8638.455521] SysRq : HELP : loglevel(0-9) reBoot Crash terminate-all-tasks(E) memory-full-oom-kill(F) kill-all-tasks(I) thaw-filesystems(J) show-backtrace-all-active-cpus(L) show-memory-usage(M) nice-all-RT-tasks(N) powerOff show-registers(P) show-all-timers(Q) Sync show-task-states(T) Unmount show-blocked-tasks(W) dump-ftrace-buffer(Z)

Environment

  • Cumulus Linux, all versions

Solution

You can disable the SysRq facility temporarily by running the following command:

cumulus@switch:~$ sudo bash
cumulus@switch:~$ echo 0 > /proc/sys/kernel/sysrq

The above command does not persist if you reboot the switch. To make this configuration persistent, create a sysrq.conf file in /etc/sysctl.d that contains the following information:

kernel.sysrq = 0

Causes

The SysRq facility is a method of interacting with the kernel directly, and is normally used when the switch/device is hung or not working correctly. There are several triggers to reach SysRq and it is possible that one of them may have been invoked inadvertently. These triggers include:

  • Pressing the SysRq key while connected to the console
  • Pressing the Break key while connected to the console
  • Issuing a Send Break from a telnet session into the switch
  • Issuing echo ? > /proc/sysrq-trigger

Note: Different terminal emulators may use different key sequences to generate a Break signal. On a directly attached laptop, a Break may be sent based on an application-specific hotkey.

Since a Break key presents an extended space (low signal without start/stop bits), it can also be simulated by connecting a bad/incorrect cable or malfunctioning device to the console.

Note: Verify that your serial console port connection is correctly connected, as there have been cases where a Break was seen when a live Ethernet cable was inserted into the console port.

If the correct key sequence is evaluated after the SysRq trigger, it can also cause the switch to power off or reboot. Care should be taken while connecting cables and devices to the Cumulus Linux switch console.

You can find more information on the Linux SysRq facility at https://www.kernel.org/doc/Documentation/sysrq.txt.

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