* 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 on a kernel with SysRq compiled in, it
-may be DISABLED at run-time using following command:
-
- echo "0" > /proc/sys/kernel/sysrq
-
-Note that previous versions disabled sysrq by default, and you were required
-to specifically enable it at run-time. That is not the case any longer.
+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. By default the file contains 1 which means that every
+possible SysRq request is allowed (in older versions SysRq was disabled
+by default, and you were required to specifically enable it at run-time
+but this is not the case any more). 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 - enable control of console logging level
+ 4 - enable control of keyboard (SAK, unraw)
+ 8 - enable debugging dumps of processes etc.
+ 16 - enable sync command
+ 32 - enable remount read-only
+ 64 - enable signalling of processes (term, kill, oom-kill)
+ 128 - allow reboot/poweroff
+ 256 - allow nicing of all RT tasks
+
+You can set the value in the file by the following command:
+ echo "number" >/proc/sys/kernel/sysrq
+
+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.
* How do I use the magic SysRq key?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
'b' - Will immediately reboot the system without syncing or unmounting
your disks.
-'c' - Intentionally crash the system without syncing or unmounting
- your disks. This is most useful if the NETDUMP client package
- and/or the DISKDUMP package have been installed.
+'c' - Will perform a kexec reboot in order to take a crashdump.
'o' - Will shut your system off (if configured and supported).
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 SIGTERM to all processes, except for init.
'i' - Send a SIGKILL to all processes, except for init.
when you would try to login. It will kill all programs on given console
and thus letting you make sure that the login prompt you see is actually
the one from init, not some trojan program.
-IMPORTANT:In its true form it is not a true SAK like the one in :IMPORTANT
-IMPORTANT:c2 compliant systems, and it should be mistook as such. :IMPORTANT
+IMPORTANT: In its true form it is not a true SAK like the one in a :IMPORTANT
+IMPORTANT: c2 compliant system, and it should not be mistaken as :IMPORTANT
+IMPORTANT: such. :IMPORTANT
It seems other find it useful as (System Attention Key) which is
useful when you want to exit a program that will not let you switch consoles.
(For example, X or a svgalib program.)
re'B'oot is good when you're unable to shut down. But you should also 'S'ync
and 'U'mount first.
-'C'rash immediately crashes your system. This is most useful if the machine
-has been configured as a NETDUMP client because an OOPS report is generated
-and a kernel crash dump is sent to the NETDUMP server.
+'C'rashdump can be used to manually trigger a crashdump when the system is hung.
+The kernel needs to have been built with CONFIG_KEXEC enabled.
'S'ync is great when your system is locked up, it allows you to sync your
disks and will certainly lessen the chance of data loss and fscking. Note
Next, you must create a sysrq_key_op struct, and populate it with A) the key
handler function you will use, B) a help_msg string, that will print when SysRQ
prints help, and C) an action_msg string, that will print right before your
-handler is called. Your handler must conform to the protoype in 'sysrq.h'.
+handler is called. Your handler must conform to the prototype in 'sysrq.h'.
After the sysrq_key_op is created, you can call the macro
register_sysrq_key(int key, struct sysrq_key_op *op_p) that is defined in
* I have more questions, who can I ask?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-You may feel free to send email to myrdraal@deathsdoor.com, and I will
-respond as soon as possible.
- -Myrdraal
-
And I'll answer any questions about the registration system you got, also
responding as soon as possible.
-Crutcher
* Credits
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-Written by Mydraal <myrdraal@deathsdoor.com>
+Written by Mydraal <vulpyne@vulpyne.net>
Updated by Adam Sulmicki <adam@cfar.umd.edu>
Updated by Jeremy M. Dolan <jmd@turbogeek.org> 2001/01/28 10:15:59
Added to by Crutcher Dunnavant <crutcher+kernel@datastacks.com>
git://git.onelab.eu / linux-2.6.git / blobdiff