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The Magic SysRq key
Pressing Alt + SysRq + key produces results described in the table below.
For security purposes, such as preventing casual users from using Alt+SysRq+c
to cause a system crash, the kernel.sysrq value as set by sysctl is usually
restricted by via a configuration setting. On Fedora, /usr/lib/sysctl.d/50-default.conf
contains the following lines:
# System Request functionality of the kernel (SYNC) # # Use kernel.sysrq = 1 to allow all keys. # See http://fedoraproject.org/wiki/QA/Sysrq for a list of values and keys. kernel.sysrq = 16
The value 16 enables only the Sync all mounted filesystems function; all others are disabled by default. To get access to the other functions, run:
sysctl -w kernel.sysrq=1
| Key | Action |
|---|---|
| 0 - 9 | Set the console log level (controls types of kernel messages sent to the console) |
| b | Immediately reboot the system, without unmounting or syncing filesystems |
| c | Perform a system crash. A crashdump will be taken if it is configured. |
| d | Display all currently held Locks (CONFIG_LOCKDEP kernel option is required) |
| e | Send the SIGTERM signal to all processes except init (PID 1) |
| f | Call oom_kill, which kills a process to alleviate an OOM condition |
| h | Output a terse help document to the console |
| i | Send the SIGKILL signal to all processes except init |
| j | Forcibly “Just thaw it” – filesystems frozen by the FIFREQRE ioctl. |
| k | Kill all processes on the current virtual console (can kill X and SVGALib programs) |
| l | Shows a stack backtrace for all active CPUs. |
| m | Output current memory information to the console |
| n | Reset the nice level of all high-priority and real-time tasks |
| o | Shut off the system |
| p | Output the current registers and flags to the console |
| q | Display all active high-resolution timers and clock sources. |
| r | Switch the keyboard from raw mode, used by programs such as X11 and SVGALib, to XLATE mode |
| s | Sync all mounted filesystems |
| t | Output a list of current tasks and their information to the console |
| u | Remount all mounted filesystems in read-only mode |
| v | Forcefully restore framebuffer console (ARM CPUs: cause ETM buffer dump) |
| w | Display list of blocked (D state) tasks |
| x | (Used by xmon interface on PowerPC platforms) |
| y | Show global CPU registers (SPARC-64 specific) |
| z | Dump the ftrace buffer |
| space | Print a summary of available magic SysRq keys |