kernelsysrq

The kernel sysrq (System Request) mechanism is a debugging and crash recovery tool built into the Linux kernel. It allows administrators to send commands directly to the kernel, even when the system is otherwise unresponsive. This is typically achieved by writing specific values to the `/proc/sysrq-trigger` file or by using dedicated hardware keys.

The SysRq functionality needs to be explicitly enabled in the kernel configuration or at runtime via `/proc/sys/kernel/sysrq`. When enabled, it provides a range of commands, each represented by a single character. For example, 'b' triggers an immediate reboot, 't' dumps a list of current tasks, and 'm' dumps kernel memory information. 'c' generates a crash dump, which can be useful for post-mortem analysis.

SysRq is a powerful tool that should be used with caution, as incorrect commands can lead to data loss or system instability. It is most often employed in scenarios where a system has frozen or crashed and standard shutdown procedures are not possible. Enabling SysRq generally involves setting the value of `/proc/sys/kernel/sysrq` to a number between 0 and 15. Each bit in this number corresponds to a specific SysRq command, with 1 enabling all commands. Disabling it is done by setting the value to 0.