Monday, March 16, 2009

Linux Kernel Magic SysRq keys in openSUSE for crash recovery

Linux Kernel offers you something that allows you to recover your
system from a crash or at the least lets you to perform a proper
shutdown using the Magic SysRq Keys. The magic SysRq key is a select
key combination in the Linux kernel which allows the user to perform
various low level commands regardless of the system’s state using the
SysRq key. It is often used to recover from freezes, or to reboot a
computer without corrupting the filesystem.

The magic Sysrq key basically has a key combination of <ALT>
+ <SysRq or Prnt Scrn> + <Command key>

The command key can be one
of the following providing a specific functionality:

  • ‘b’ - Will
    immediately reboot the system without syncing or unmounting your disks.

  • ‘c’ - Will perform
    a kexec reboot in order to take a crashdump.

  • ‘d’ - Shows all
    locks that are held.

  • ‘e’ - Send a
    SIGTERM to all processes, except for init.

  • ‘f’ - Will call
    oom_kill to kill a memory hog process.

  • ‘g’ - Used by kgdb
    on ppc and sh platforms.

  • ‘h’ - Will display
    help (actually any other key than those listed here will display help.
    but ‘h’ is easy to remember :-)

  • ‘i’ - Send a
    SIGKILL to all processes, except for init.

  • ‘k’ - Secure
    Access Key (SAK) Kills all programs on the current virtual console.
    NOTE: See important comments below in SAK section.

  • ‘m’ - Will dump
    current memory info to your console.

  • ‘n’ - Used to make
    RT tasks nice-able

  • ‘o’ - Will shut
    your system off (if configured and supported).

  • ‘p’ - Will dump
    the current registers and flags to your console.

  • ‘q’ - Will dump a
    list of all running timers.

  • ‘r’ - Turns off
    keyboard raw mode and sets it to XLATE.

  • ’s’ - Will attempt
    to sync all mounted filesystems.

  • ‘t’ - Will dump a
    list of current tasks and their information to your console.

  • ‘u’ - Will attempt
    to remount all mounted filesystems read-only.

  • ‘v’ - Dumps
    Voyager SMP processor info to your console.

  • ‘w’ - Dumps tasks
    that are in uninterruptable (blocked) state.

  • ‘x’ - Used by xmon
    interface on ppc/powerpc platforms.

  • ‘0′-’9′ - Sets the
    console log level, controlling which kernel messages will be printed to
    your console. (’0′, for example would make it so that only emergency
    messages like PANICs or OOPSes would make it to your console.)

The most useful combination is the <ALT>+<SysRq
or Prnt Scrn> + R-E-I-S-U-B

The above basically means while you Press and Hold
<ALT>+<SysRq or Prnt Scrn> Press R, E, I,
S, U, B giving sufficient time between each of these key to ensure they
perform the required job.

REISUB will:

  • Release the Keyboard of the XServer control (in
    the event your XServer crashes)

  • Try to gratiously shutdown processes (SIGTERM)

  • Forcibly shutsdown the processes (SIGKILL)

  • Sync the mounted File Systems immediatly

  • Umount the mounted File Systems

  • Reboot the system

This could avert or atleast reduce the impact of system

Magic SysRq key
is not enabled by default in openSUSE

To enable SysRq in openSUSE, click Computer and

Click System and Kernel Settings.

Click the Kernel Settings tab and place a
tick in “Enable SysRq Keys” and click Finish.

Alterntaively, click System - “/etc/sysconfig
“. Navigate down to “System - Kernel
” and set it to “Yes” and click Finish.

Another option would be to edit the

/etc/sysctl.conf file and add “kernel.sysrq =

or edit

/etc/sysconfig/sysctl and set

To temporarily enable SysRq at runtime,

opensuse11:~ # echo “1″ >

To disable

opensuse11:~ # echo “0″ >

The possible values you can
pass are:

  • 0: disable sysrq completely

  • 1: enable all functions of sysrq

  • >1 bitmask of allowed sysrq functions:

  • 2 - enable control of console logging level

  • 4 - enable control of keyboard (SAK, unraw)

  • 8 - enable debugging dumps of processes etc.

  • 16 - enable sync command

  • 32 - enable remount read-only

  • 64 - enable signalling of processes (term, kill, oom-kill)

  • 128 - allow reboot/poweroff

  • 256 - allow nicing of all RT tasks

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