Magic Sys Rq Key - Configuration

Configuration

The feature is controlled both by a compile-time option in the kernel configuration, CONFIG_MAGIC_SYSRQ, and a sysctl kernel parameter, kernel.sysrq. To be able to use this functionality the CONFIG_MAGIC_SYSRQ option has to be enabled at kernel compile time.

The SysRq key can be disabled with the following command:

echo 0 > /proc/sys/kernel/sysrq

To re-enable:

echo 1 > /proc/sys/kernel/sysrq

On newer kernels (since 2.6.12), it is possible to have a more fine-grained control. On these machines, the number written to /proc/sys/kernel/sysrq can be zero, one, or a number greater than one which is a bitmask indicating which features to allow.

Possible values are:

• 0 - disable SysRq
• 1 - enable SysRq completely
• >1 - bitmask of enabled SysRq functions:
  • 2 - control of console logging level
  • 4 - control of keyboard (SAK, unraw)
  • 8 - debugging dumps of processes etc.
  • 16 - sync command
  • 32 - remount read-only
  • 64 - signalling of processes (term, kill, oom-kill)
  • 128 - reboot/poweroff
  • 256 - nicing of all RT tasks