From: eLinux.org
MagicSysRq
Note: This article is currently only a draft and is a part of a series of articles I'm going to publish the next few months - if you want to contribute to it, please feel free to. However it would be nice if you could coordinate your efforts with me
This article is still work in progress and still needs a lot of effort.
Contents
Introduction
MagicSysRq is a special key combination which lets the user perform certain low level tasks of the kernel and is especially useful when the whole system seems to be hung. MagicSysRq e.g. makes it possible to, more or less, cleanly shutdown a system, which doesn't even respond to CTRL+ALT+DEL any more.
The MagicSysRq functions are invoked by pressing ALT+SysRq+Function Key where function key describes one of the following.
TODO:Insert table here
Example
The probably most used combination of MagicSysRq functions is the one RSEIUB, which can be easyly remebered by the mnemonic trick ``Raising Elephants is so utterly boring*\footnote{``You can still find several references the* mnemonic Raising Skinny Elephants is utterly boring on the internet - however it is probably better to do the sync after the tasks exited/were killed} - although we're dealing with penguins here ;) According to the table above this performs, in order: R Put Keyboard in Raw Mode E terminate all tasks I kill all tasks S Sync U remount all filesystems read-only B Reboot
Troubleshooting
You can easily and safely check whether MagicSysRq works on your System by pressing Alt+SysRq+H - on a console you should now see a short help text of the MagicSysRq Feature or at least print this to `dmesg`.
If you don't see the help text, check whether CONFIG\_MAGIC\_SYSRQ=y is set in your Kernel Config. If it is set, please check the output of 'cat /proc/sys/kernel/sysrq' which should report 1 if MagicSysRq is enabled. To enable it just do a simple echo 1 > /proc/sys/kernel/sysrq
Important Files
/proc/sys/kernel/sysrq - enable/disable MagicSysRq at runtime /proc/sysrq-trigger - trigger MagicSysRq from the console (e.g. useful over ssh)
TODO Content:
- Explanation of remaining keys
- using it over serial line -> BREAK+key, in kermit ctrl-\ b KEY
- Interaction with KDB
- Interaction with kexec / crashkernel
