Editing Magic SysRq Key

'''Alt-SysRq-*''' is a "magical" key combo you can hit which the kernel will respond to
regardless of whatever else it is doing, unless it is completely locked up.

== /proc/sys/kernel/sysrq ==

When running a kernel with SysRq compiled in (as in all binary kernels released by OpenVZ),
<tt>/proc/sys/kernel/sysrq</tt> file controls the functions allowed to be invoked via the SysRq key.
Here is the list of possible values in <tt>/proc/sys/kernel/sysrq</tt>:<br>

{| class="wikitable"
! 0
| disable sysrq completely
|-
! 1
| enable all functions of sysrq
|-
! >1
| bitmask of allowed sysrq functions
|}

You can set the value in the file by the following command:
<pre>
echo "number" >/proc/sys/kernel/sysrq
</pre>

== Available keys ==

To use SysRq feature you should press the key combo 'Alt-SysRq-<command key>'.
The 'SysRq' key is also known as the 'Print Screen' key. Other way, if you don't have
physical access to the node, only ssh, you can do
<pre>
echo "command key" > /proc/sysrq-trigger
</pre>

Here is the list of most useful command keys:

{| class="wikitable"
! b
| Will immediately re'''b'''oot the system without syncing or unmounting your disks
|-
! s 
| Will attempt to '''s'''ync all mounted filesystems
|-
! u
| Will attempt to remo'''u'''nt all mounted filesystems read-only
|-
! p
| Will dum'''p''' the current registers and flags to your console
|-
! t
| Will dump a list of current '''t'''asks and their information to your console
|-
! m
| Will dump current '''m'''emory info to your console
|-
! 0...9
| Sets the console log level, controlling which kernel messages will be printed to your console. ('''0''', for example would make it so that only emergency messages like PANICs or OOPSes would make it to your console)
|-
! f
| Will call oom_kill to kill a memory hog process
|-
! e
| Send a SIGT'''E'''RM to all processes, except for init
|-
! i
| Send a SIGK'''I'''LL to all processes, except for init
|-
! h
| Will display '''h'''elp
|}

All output will appear on the console and/or in <tt>/var/log/messages</tt> file.

== Example ==

Suppose for some strange reason your system is working slow and you want to
ask developers what happens, then don't forget to press
<tt>Alt-SysRq-p</tt> and <tt>Alt-SysRq-t</tt> several times,
and attach <tt>/var/log/messages</tt> to request.

[[Category: Troubleshooting]]
[[Category: Kernel]]