Magic SysRq Key - Revision history

Kir: Added a link to remote console setup, added missing article

2006-06-13T11:16:10Z

Added a link to remote console setup, added missing article

Kir: Reformatted with tables, added several sections and thus TOC

2006-06-13T10:42:18Z

Reformatted with tables, added several sections and thus TOC

Vass: Initial edition of article

2006-06-13T08:57:09Z

Initial edition of article

New page

It 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.

When running a kernel with SysRq compiled in (that's right in openVZ case),
<tt>/proc/sys/kernel/sysrq</tt> 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>: 

<tt>0</tt> - disable sysrq completely 
<tt>1</tt> - enable all functions of sysrq 
><tt>1</tt> - 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>

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:

<tt>b</tt> - Will immediately reboot the system without syncing or unmounting
your disks. 
<tt>s</tt> - Will attempt to sync all mounted filesystems. 
<tt>u</tt> - Will attempt to remount all mounted filesystems read-only. 
<tt>p</tt> - Will dump the current registers and flags to your console. 
<tt>t</tt> - Will dump a list of current tasks and their information to your
console. 
<tt>m</tt> - Will dump current memory info to your console. 
<tt>0</tt>-<tt>9</tt> - 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.) 
<tt>f</tt> - Will call oom_kill to kill a memory hog process 
<tt>e</tt> - Send a SIGTERM to all processes, except for init. 
<tt>i</tt> - Send a SIGKILL to all processes, except for init. 
<tt>h</tt> - Will display help 

All ouptput will be on console or in <tt>/var/log/messages</tt>

Example of using SysRq. 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]]

@@ Line 70: / Line 70: @@
 |}
-All output will appear on the console and/or in <tt>/var/log/messages</tt> file.
+All output will appear on the console and/or in <tt>/var/log/messages</tt> file. In some cases you need to [[Remote console setup|set up a remote console]] in order to catch all the messages you need.
 == Example ==
@@ Line 77: / Line 77: @@
 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.
+and attach <tt>/var/log/messages</tt> to a request.
 [[Category: Troubleshooting]]
 [[Category: Kernel]]

@@ Line 1: / Line 1: @@
-It is a 'magical' key combo you can hit which the kernel will respond to
+'''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.
-When running a kernel with SysRq compiled in (that's right in openVZ case),
+== /proc/sys/kernel/sysrq ==
 <tt>/proc/sys/kernel/sysrq</tt> 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>
-<tt>0</tt> - disable sysrq completely<br>
+{| class="wikitable"
-<tt>1</tt> - enable all functions of sysrq<br>
+! 0
-><tt>1</tt> - bitmask of allowed sysrq functions<br>
+| disable sysrq completely
 You can set the value in the file by the following command:
@@ Line 15: / Line 24: @@
 </pre>
-To use SysRq feature you should press the key combo 'ALT-SysRq-<command key>'.
+== Available keys ==
 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
@@ Line 24: / Line 35: @@
 Here is the list of most useful command keys:
-<tt>b</tt> - Will immediately reboot the system without syncing or unmounting
+{| class="wikitable"
-your disks.<br>
+! b
-<tt>s</tt> - Will attempt to sync all mounted filesystems.<br>
+| Will immediately re'''b'''oot the system without syncing or unmounting your disks
-<tt>u</tt> - Will attempt to remount all mounted filesystems read-only.<br>
+|-
-<tt>p</tt> - Will dump the current registers and flags to your console.<br>
+! s
-<tt>t</tt> - Will dump a list of current tasks and their information to your
+| Will attempt to '''s'''ync all mounted filesystems
-console.<br>
+|-
-<tt>m</tt> - Will dump current memory info to your console.<br>
+! u
-<tt>0</tt>-<tt>9</tt> - Sets the console log level, controlling which kernel messages
+| Will attempt to remo'''u'''nt all mounted filesystems read-only
-will be printed to your console. ('0', for example would make
+|-
-it so that only emergency messages like PANICs or OOPSes would
+! p
-make it to your console.)<br>
+| Will dum'''p''' the current registers and flags to your console
-<tt>f</tt> - Will call oom_kill to kill a memory hog process<br>
+|-
-<tt>e</tt> - Send a SIGTERM to all processes, except for init.<br>
+! t
-<tt>i</tt> - Send a SIGKILL to all processes, except for init.<br>
+| Will dump a list of current '''t'''asks and their information to your console
-<tt>h</tt> - Will display help<br>
+|-
-All ouptput will be on console or in <tt>/var/log/messages</tt>
+== Example ==
-Example of using SysRq. Suppose for some strange reason your system is working slow and you want to
+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,
+<tt>Alt-SysRq-p</tt> and <tt>Alt-SysRq-t</tt> several times,
 and attach <tt>/var/log/messages</tt> to request.
 [[Category: Troubleshooting]]