What is the magic SysRq key?

■ Requirement : Details on magic SysRq
■ OS Environment : Linux, RHEL, Centos
■ Resolution : 

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.

Enable the magic SysRq key : 

1. CONFIG_MAGIC_SYSRQ=yes in kernel config file(which has inside /boot)
2. in /proc/sys/kernel/sysrq: echo value > /proc/sys/kernel/sysrq

value =
0 - disable sysrq completely
1 - enable all functions of sysrq
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

3. How do I use the magic SysRq key?

On x86 - You press the key combo 'ALT-SysRq-'. Note - Some
keyboards may not have a key labeled 'SysRq'. The 'SysRq' key is
also known as the 'Print Screen' key. Also some keyboards cannot
handle so many keys being pressed at the same time, so you might
have better luck with "press Alt", "press SysRq", "release SysRq",
"press ", release everything.

From command prompt :

echo command > /proc/sysrq-trigger

command =
'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 oom_kill to kill a memory hog process.

'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.

'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.)

Source : Kernel documentation.