Documentation/admin-guide/sysrq.rst

Source file repositories/reference/linux-study-clean/Documentation/admin-guide/sysrq.rst

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Linux Magic System Request Key Hacks
====================================

Documentation for sysrq.c

What is the magic SysRq key?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~

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.

How do I enable the magic SysRq key?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

You need to say "yes" to 'Magic SysRq key (CONFIG_MAGIC_SYSRQ)' when
configuring the kernel. When running a kernel with SysRq compiled in,
/proc/sys/kernel/sysrq controls the functions allowed to be invoked via
the SysRq key. The default value in this file is set by the
CONFIG_MAGIC_SYSRQ_DEFAULT_ENABLE config symbol, which itself defaults
to 1. Here is the list of possible values in /proc/sys/kernel/sysrq:

   -  0 - disable sysrq completely
   -  1 - enable all functions of sysrq
   - >1 - bitmask of allowed sysrq functions (see below for detailed function
     description)::

          2 =   0x2 - enable control of console logging level
          4 =   0x4 - enable control of keyboard (SAK, unraw)
          8 =   0x8 - enable debugging dumps of processes etc.
         16 =  0x10 - enable sync command
         32 =  0x20 - enable remount read-only
         64 =  0x40 - enable signalling of processes (term, kill, oom-kill)
        128 =  0x80 - allow reboot/poweroff
        256 = 0x100 - allow nicing of all RT tasks

You can set the value in the file by the following command::

    echo "number" >/proc/sys/kernel/sysrq

The number may be written here either as decimal or as hexadecimal
with the 0x prefix. CONFIG_MAGIC_SYSRQ_DEFAULT_ENABLE must always be
written in hexadecimal.

Note that the value of ``/proc/sys/kernel/sysrq`` influences only the invocation
via a keyboard. Invocation of any operation via ``/proc/sysrq-trigger`` is
always allowed (by a user with admin privileges).

How do I use the magic SysRq key?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

On x86
	You press the key combo `ALT-SysRq-<command key>`.

	.. note::
	   Some
           keyboards may not have a key labeled 'SysRq'. The 'SysRq' key is
           also known as the 'Print Screen' key. Also some keyboards cannot
	   handle so many keys being pressed at the same time, so you might
	   have better luck with press `Alt`, press `SysRq`,
	   release `SysRq`, press `<command key>`, release everything.

On SPARC
	You press `ALT-STOP-<command key>`, I believe.

On the serial console (PC style standard serial ports only)
        You send a ``BREAK``, then within 5 seconds a command key. Sending
        ``BREAK`` twice is interpreted as a normal BREAK.

On PowerPC
	Press `ALT - Print Screen` (or `F13`) - `<command key>`.

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