sysrq is a special key combination that allows sending commands directly to the linux kernel, even if userspace is broken/frozen.
enabling it ¶
the /proc/sys/kernel/sysrq file accepts a bitmask written to it, to
control which sysrq features are enabled. a 0 disables all sysrq
features, while 1 enables all of them. see the kernel documentation
for more granular control of sysrq features
since procfs options are not persistant over reboots, it can be
enabled persistantly with the sysctl option kernel.sysrq=1 in
/etc/sysctl.d/ or by setting the sysrq_always_enabled=1 kernel
parameter if you need it before the filesystem mounts
key combinations ¶
sysrq can be used by holding alt, then hitting sysrq (print screen, you may have to press fn too), then releasing all keys except for alt, the letter(s) you want can be pressed. you can do multiple sysrq commands without needing to press sysrq again by continuing to hold alt.
| key | behavior |
| f | force an OOM kill now, even if not out of memory. mey help with a freeze |
| n | nice real-time tasks, may also help with a freeze |
| k | kill current console, can be used as a "secure attention key" |
| 0-9 | set kernel verbosity |
| v | force viewing framebuffer (show console) |
| h | print sysrq helptext to the kernel log, to see more keys not listed here |
| r | unraw the keyboard, to take back control from something like X11 |
| e | sigterm (gracefull kill) all tasks |
| i | sigkill all tasks |
| s | sync disks |
| u | unmount disks (sync first to avoid a dirty unmount) |
| b | force a reboot |
the reisub mnemonic (with a few seconds between each key) may be
used to gracefully reboot. however, realistically with modern CoW and
journaling filesystems, there is not much reason to use it over just
holding down the power button