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sysrq

Reference table of Linux Magic SysRq keys for low-level system control, debugging, recovery, and emergency operations

Command Function
b Immediately reboots the system without syncing or unmounting disks.
c Performs a system crash; a crashdump will be taken if configured.
d Shows all locks that are held.
e Sends SIGTERM to all processes except init.
f Invokes the OOM killer to kill a memory-hog process (no panic if nothing can be killed).
g Used by kgdb (kernel debugger).
h Displays help (any undefined key will also show help).
i Sends SIGKILL to all processes except init.
j Forcibly "just thaw it" — unfreezes filesystems frozen by the FIFREEZE ioctl.
k Secure Access Key (SAK): kills all programs on the current virtual console.
l Shows a stack backtrace for all active CPUs.
m Dumps current memory info to the console.
n Used to make RT tasks nice-able.
o Shuts the system off (if configured and supported).
p Dumps current registers and flags to the console.
q Dumps per-CPU lists of armed hrtimers and clockevent device details.
r Turns off keyboard raw mode and sets it to XLATE.
s Attempts to sync all mounted filesystems.
t Dumps a list of current tasks and their information.
u Attempts to remount all mounted filesystems read-only.
v Forcefully restores framebuffer console.
v Dumps ETM buffer (ARM-specific).
w Dumps tasks that are in uninterruptible (blocked) state.
x Used by xmon on PPC/PowerPC; shows PMU registers on sparc64; dumps TLB entries on MIPS.
y Shows global CPU registers (SPARC64-specific).
z Dumps the ftrace buffer.
0–9 Sets the console log level (e.g. 0 = only emergencies like PANIC/OOPS).
R Replays kernel log messages on consoles.

How do I enable the magic SysRq key

  • 0 - Disable sysrq completely
  • 1 - Enable all functions of sysrq
  • >1 - Bitmask of allowed sysrq functions (see below for detailed function description):
Value 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

Search for CONFIG_MAGIC_SYSRQ i kernel config

$ rg CONFIG_MAGIC_SYSRQ /usr/src/linux/.config 
CONFIG_MAGIC_SYSRQ=y
CONFIG_MAGIC_SYSRQ_DEFAULT_ENABLE=0x1
CONFIG_MAGIC_SYSRQ_SERIAL=y
CONFIG_MAGIC_SYSRQ_SERIAL_SEQUENCE=""

Search for CONFIG_MAGIC_SYSRQ_DEFAULT_ENABL i kernel config

$ rg "CONFIG_MAGIC_SYSRQ_DEFAULT_ENABLE" /usr/src/linux/.config
CONFIG_MAGIC_SYSRQ_DEFAULT_ENABLE=0x1

You can set the value in the file by the following command

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

echo "number" >/proc/sys/kernel/sysrq

Will perform a system crash and a crashdump will be taken if configured.

echo c >/proc/sys/kernel/sysrq

How do I use the magic SysRq key

You press the key combo ALT-SysRq-<command key>

Resource(s)