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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.
| 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 |
