SysRq

2009-08-14T04:47:03Z

NickC: SysRq list into different order; corrected SysRq+I definition error; added mnemonics.

[[Category:Tips]]
--[[User:Drijen|Drijen]] 15:06, 22 September 2007 (EDT)

==Recovering from a System Lock==
Sometimes a process can go out to lunch, or the system will simply stop responding to user input. This could be from a process waiting for resources or a zombie, but in any case there is no reason to have to reach for that reset button!

==The Magic SysRq==
The SysRq can be used as a means to save your work, sync the file system(s) and reboot the computer in a clean manner. If you don't know where the SysRq key is, look for the "Prt Scr" (Print Screen) button on your keyboard. SysRq is generally assigned to the same keypress.

===Enabling the SysRq Function===
To enable the SysRq functionality, you must have a 2.6.15 or higher kernel, and it must be compiled directly in. Not as a Module!

To check that you have it enabled you may run this command:

:<pre>cat /boot/config |grep CONFIG_MAGIC_SYSRQ</pre>

OR

:<pre>zcat /proc/config.gz |grep SYSRQ</pre>

if you have /proc/.config support compiled in. (All base slack kernels do)

which should return:

:<pre>CONFIG_MAGIC_SYSRQ=y</pre>

If you find that you need to compile it in, the option can be found under the heading ''Kernel Hacking'' on your make <(x)(menu)config> screen.

Once you are satisfied that you have the SysRq funtion correctly compiled, ensure that it has not been disabled at boot:

:<pre>cat /proc/sys/kernel/sysrq</pre>

If the response is <tt>0</tt> :

:<pre>echo 1 > /proc/sys/kernel/sysrq</pre>

===Recovering from a Lock Up using SysRq===
If your system has gone out to lunch, you can now use the Magic SysRq key to save your work and reboot. The basic commands are as follows:

<tt>Alt+SysRq+R</tt> :Attempts to remove keyboard from Raw mode
<tt>Alt+SysRq+E</tt> :Terminate request sent to all processes (SIGTERM / kill -15)
<tt>Alt+SysRq+I</tt> :Kills all processes (SIGKILL / kill -9)
<tt>Alt+SysRq+S</tt> :Syncs all disks
<tt>Alt+SysRq+U</tt> :Force unmount and remount of all filesystems readonly
<tt>Alt+SysRq+B</tt> :Reboots (INIT 6)

and roughly in that order. You should wait several seconds between each keystroke to let the system complete your request before you issue a new one.

There are several mnemonics to help you remember the key sequence:
<tt>Raising Elephants Is So Utterly Boring</tt>
<tt>Reboot Even If System Utterly Broken</tt>
<tt>BUSIER, spelt backwards</tt>

An alternative method is to:

:<pre>Alt+SysRq+R CTRL+ALT+DEL</pre>

This command will move the keyboard out of raw mode, and attempt to issue a <tt>SHUTDOWN NOW</tt> command. However, this will not work under some configurations, or if tty1 is defunct.

==Futher Information==
Further instructions and other options can be found at:

Wikipedia article on MagicSysRq [http://en.wikipedia.org/wiki/Magic_SysRq_key]

<pre>/usr/src/<linux-version>/Documentation/sysrq.txt</pre>

SlackWiki - User contributions [en]

SysRq