Sep 282008
 

The 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 crashes.

Magic SysRq key is not enabled by default in openSUSE.

To enable SysRq in openSUSE, click Computer and YaST.

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 editor“.  Navigate down to “System – Kernel – ENABLE_SYSRQ” and set it to “Yes” and click Finish.

Another option would be to edit the

/etc/sysctl.conf file and add “kernel.sysrq = 1″

or edit

/etc/sysconfig/sysctl and set ENABLE_SYSRQ=”yes”

To temporarily enable SysRq at runtime,

opensuse11:~ # echo “1” > /proc/sys/kernel/sysrq

To disable

opensuse11:~ # echo “0” > /proc/sys/kernel/sysrq

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