Documentation for sysrq.c
What is the magic SysRq key?
It is a 'magical' key combo you can hit which the kernel will respond to
regardless of whatever else it is doing, unless it is completely locked up.
How do I enable the magic SysRq key?
You need to say "yes" to 'Magic SysRq key (CONFIG_MAGIC_SYSRQ)' when configuring the kernel. When running a kernel with SysRq compiled in, /proc/sys/kernel/sysrq controls the functions allowed to be invoked via the SysRq key. The default value in this file is set by the CONFIG_MAGIC_SYSRQ_DEFAULT_ENABLE config symbol, which itself defaults to 1. Here is the list of possible values in /proc/sys/kernel/sysrq:
-
0 - disable sysrq completely
-
1 - enable all functions of sysrq
-
1 - bitmask of allowed sysrq functions (see below for detailed function description)::
2 = 0x2 - enable control of console logging level 4 = 0x4 - enable control of keyboard (SAK, unraw) 8 = 0x8 - enable debugging dumps of processes etc. 16 = 0x10 - enable sync command 32 = 0x20 - enable remount read-only 64 = 0x40 - enable signalling of processes (term, kill, oom-kill)128 = 0x80 - allow reboot/poweroff 256 = 0x100 - allow nicing of all RT tasks
You can set the value in the file by the following command::
echo "number" >/proc/sys/kernel/sysrq
The number may be written here either as decimal or as hexadecimal with the 0x prefix. CONFIG_MAGIC_SYSRQ_DEFAULT_ENABLE must always be written in hexadecimal.
Note that the value of /proc/sys/kernel/sysrq influences only the invocation
via a keyboard. Invocation of any operation via /proc/sysrq-trigger is
always allowed (by a user with admin privileges).
What are the 'command' keys?
=========== ===================================================================
Command Function
=========== ===================================================================
``b`` Will immediately reboot the system without syncing or unmounting
your disks.
``c`` Will perform a system crash by a NULL pointer dereference.
A crashdump will be taken if configured.
``d`` Shows all locks that are held.
``e`` Send a SIGTERM to all processes, except for init.
``f`` Will call the oom killer to kill a memory hog process, but do not
panic if nothing can be killed.
``g`` Used by kgdb (kernel debugger)
``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.
``j`` Forcibly "Just thaw it" - filesystems frozen by the FIFREEZE ioctl.
``k`` Secure Access Key (SAK) Kills all programs on the current virtual
console. NOTE: See important comments below in SAK section.
``l`` Shows a stack backtrace for all active CPUs.
``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 per CPU lists of all armed hrtimers (but NOT regular
timer_list timers) and detailed information about all
clockevent devices.
``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`` Forcefully restores framebuffer console
``v`` Causes ETM buffer dump [ARM-specific]
``w`` Dumps tasks that are in uninterruptable (blocked) state.
``x`` Used by xmon interface on ppc/powerpc platforms.
Show global PMU Registers on sparc64.
Dump all TLB entries on MIPS.
``y`` Show global CPU Registers [SPARC-64 specific]
``z`` Dump the ftrace buffer
``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.)
=========== ===================================================================
A common use of the magic SysRq key is to perform a safe reboot of a Linux computer which has otherwise locked up (abbr. REISUB).
This can prevent a fsck being required on reboot and gives some programs a chance to save emergency backups of unsaved work.
The QWERTY (or AZERTY) mnemonics: "Raising Elephants Is So Utterly Boring", "Reboot Even If System Utterly Broken" or simply the word "BUSIER" read backwards, are often used to remember the following SysRq-keys sequence:
unRaw (take control of keyboard back from X),
tErminate (send SIGTERM to all processes, allowing them to terminate gracefully),
kIll (send SIGKILL to all processes, forcing them to terminate immediately),
Sync (flush data to disk),
Unmount (remount all filesystems read-only),
reBoot.