Magic SysRq Key

A possible lifesaver is the Magic SysRq Key, which is enabled via the CONFIG_MAGIC_SYSRQ configure option. The SysRq (system request) key is a standard key on most keyboards. On the i386 and PPC, it is accessible via Alt-PrintScreen. When this configure option is enabled, special combinations of keys enable you to communicate with the kernel regardless of what else it is doing. This enables you to perform some useful tasks in the face of a dying system.

In addition to the configure option, there is a sysctl to toggle this feature on and off. To turn it on:

echo 1 > /proc/sys/kernel/sysrq

From the console, you can hit SysRq-h for a list of available options. SysRq-s syncs dirty buffers to disk, SysRq-u unmounts all file systems, and SysRq-b reboots the machine. Issuing these three key combinations in a row is a safer way to reboot a dying machine than simply hitting the machine reset switch.

If the machine is badly locked, it might not respond to any Magic SysRq combinations, or it might fail to complete a given command. With luck, however, these options might save your data or aid in debugging. Table 18.2 is a listing of the supported SysRq commands.

Table 18.2. Supporting SysRq Commands

Key Command	Description
SysRq-b	Reboot the machine
SysRq-e	Send a SIGTERM to all processes except init
SysRq-h	Display SysRq help on the console
SysRq-i	Send a SIGKILL to all processes except init
SysRq-k	Secure Access Key: kill all programs on this console
SysRq-l	Send a SIGKILL to all processes including init
SysRq-m	Dump memory information to console
SysRq-o	Shut down the machine
SysRq-p	Dump registers to console
SysRq-r	Turn off keyboard raw mode
SysRq-s	Sync all mounted file systems to disk
SysRq-t	Dump task information to console
SysRq-u	Unmount all mounted file systems

The file Documentation/sysrq.txt in the kernel source tree has more information. The actual implementation is in drivers/char/sysrq.c. The Magic SysRq Key is a vital tool for aiding in debugging or saving a dying system. Because it provides powerful capabilities to any user on the console, however, you should exercise caution on important machines. For your development machine, however, it is a great help.