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. By default the file contains 1 which means that every possible SysRq request is allowed (in older versions SysRq was disabled by default, and you were required to specifically enable it at run-time but this is not the case any more). 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 - 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
You can set the value in the file by the following command:
echo "number" >/proc/sys/kernel/sysrq
How do I use the magic SysRq key?
You press the key combo 'ALT-SysRq-<command key>'.
You press 'ALT-STOP-<command key>'
Serial Console (PC style standard serial ports only )
You send a BREAK, then within 5 seconds a command key.
Press 'ALT - Print Screen (or F13) - <command key>
Write a character to /proc/sysrq-trigger: echo t > /proc/sysrq-trigger
What are the 'command' keys?
'b' - Will immediately reboot the system without syncing or unmounting
'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.
'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 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
'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.)
Okay, so what can I use them for?
un'R'aw is very handy when your X server or a svgalib program crashes.
sa'K' (Secure Access Key) is useful when you want to be sure there is no trojan program running at console which could grab your password when you would try to login. It will kill all programs on given console, thus letting you make sure that the login prompt you see is actually the one from init, not some trojan program. IMPORTANT: In its true form it is not a true SAK like the one in a :IMPORTANT IMPORTANT: c2 compliant system, and it should not be mistaken as :IMPORTANT IMPORTANT: such. :IMPORTANT
It seems others find it useful as (System Attention Key) which is
useful when you want to exit a program that will not let you switch consoles. (For example, X or a svgalib program.)
re'B'oot is good when you're unable to shut down. But you should also 'S'ync and 'U'mount first.
'C'rashdump can be used to manually trigger a crashdump when the system is hung. The kernel needs to have been built with CONFIG_KEXEC enabled.
'S'ync is great when your system is locked up, it allows you to sync your disks and will certainly lessen the chance of data loss and fscking. Note that the sync hasn't taken place until you see the "OK" and "Done" appear on the screen. (If the kernel is really in strife, you may not ever get the OK or Done message...)
'U'mount is basically useful in the same ways as 'S'ync. I generally 'S'ync, 'U'mount, then re'B'oot when my system locks. It's saved me many a fsck. Again, the unmount (remount read-only) hasn't taken place until you see the "OK" and "Done" message appear on the screen.
The loglevels '0'-'9' are useful when your console is being flooded with kernel messages you do not want to see. Selecting '0' will prevent all but the most urgent kernel messages from reaching your console. (They will still be logged if syslogd/klogd are alive, though.)
t'E'rm and k'I'll are useful if you have some sort of runaway process you are unable to kill any other way, especially if it's spawning other processes.