Magic SysRq

The option

[*] Magic SysRq key

is particularly useful, because it often allows you to reduce the impact of a kernel failure and to obtain some additional information about the state of the system. It turns on some keyboard shortcuts allowing you to pass specific instructions directly to the kernel:

• Alt + SysRq + b - reboot the system immediately, without unmounting filesystems and writing the contents of disk buffers to the storage (DANGEROUS)
• Alt + SysRq + c - crashdump
• Alt + SysRq + e - send the TERM signal to all processes except for INIT
• Alt + SysRq + i - send the KILL signal to all processes except for INIT
• Alt + SysRq + k - SAK (Secure Access Key); kill all processes bound to the currently active virtual console (eg. emergency shutdown of the X server when Ctrl + Alt + Backspace does not work)
• Alt + SysRq + l - send the KILL signal to all processes including INIT
• Alt + SysRq + m - print information about the state of memory
• Alt + SysRq + o - system shutdown
• Alt + SysRq + p - print the contents of CPU registers and flags
• Alt + SysRq + r - switch the keyboard to the RAW mode (eg. when Ctrl + Alt + Backspace does not work)
• Alt + SysRq + s - synchronize filesystems and write the contents of disk buffers to the storage
• Alt + SysRq + t - print the list of all tasks
• Alt + SysRq + u - remount all filesystems read-only
• Alt + SysRq + h - print help

Which key is the SysRq? That depends on the architecture of your machine:

• x86 - Print Screen
• SPARC - STOP
• serial console - Break
• PowerPC - Print Screen (or F13)

On all architectures you can trigger the action assigned to given key (say t) by echoing the corresponding character to the file /proc/sysrq-trigger, eg.

# echo t > /proc/sysrq-trigger

What is the safest way to restart the computer?

First, you should try to press the combination of Ctrl + Alt + Backspace (X Window system) or Ctrl + Alt + Del (text console). If it does not work, you can:

• press Alt + SysRq + k to kill all processes using the current console,
• press Alt + SysRq + s, Alt + SysRq + u, Alt + SysRq + b

More information related to the ''Magic SysRq key'' mechanism can be found in the file Documentation/sysrq.txt included in the kernel sources.