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Linus Torvalds1da177e2005-04-16 15:20:36 -07001Linux Magic System Request Key Hacks
Randy Dunlapd346cce2007-01-31 23:48:17 -08002Documentation for sysrq.c
Jesper Juhl09736bd2007-08-10 13:01:04 -07003Last update: 2007-AUG-04
Linus Torvalds1da177e2005-04-16 15:20:36 -07004
5* What is the magic SysRq key?
6~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
7It is a 'magical' key combo you can hit which the kernel will respond to
8regardless of whatever else it is doing, unless it is completely locked up.
9
10* How do I enable the magic SysRq key?
11~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
12You need to say "yes" to 'Magic SysRq key (CONFIG_MAGIC_SYSRQ)' when
13configuring the kernel. When running a kernel with SysRq compiled in,
14/proc/sys/kernel/sysrq controls the functions allowed to be invoked via
15the SysRq key. By default the file contains 1 which means that every
16possible SysRq request is allowed (in older versions SysRq was disabled
17by default, and you were required to specifically enable it at run-time
18but this is not the case any more). Here is the list of possible values
19in /proc/sys/kernel/sysrq:
20 0 - disable sysrq completely
21 1 - enable all functions of sysrq
22 >1 - bitmask of allowed sysrq functions (see below for detailed function
23 description):
24 2 - enable control of console logging level
25 4 - enable control of keyboard (SAK, unraw)
26 8 - enable debugging dumps of processes etc.
27 16 - enable sync command
28 32 - enable remount read-only
29 64 - enable signalling of processes (term, kill, oom-kill)
30 128 - allow reboot/poweroff
31 256 - allow nicing of all RT tasks
32
33You can set the value in the file by the following command:
34 echo "number" >/proc/sys/kernel/sysrq
35
36Note that the value of /proc/sys/kernel/sysrq influences only the invocation
37via a keyboard. Invocation of any operation via /proc/sysrq-trigger is always
Randy Dunlapd346cce2007-01-31 23:48:17 -080038allowed (by a user with admin privileges).
Linus Torvalds1da177e2005-04-16 15:20:36 -070039
40* How do I use the magic SysRq key?
41~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
42On x86 - You press the key combo 'ALT-SysRq-<command key>'. Note - Some
43 keyboards may not have a key labeled 'SysRq'. The 'SysRq' key is
44 also known as the 'Print Screen' key. Also some keyboards cannot
45 handle so many keys being pressed at the same time, so you might
Pavel Machekdfb00422007-10-07 00:24:37 -070046 have better luck with "press Alt", "press SysRq", "release SysRq",
Linus Torvalds1da177e2005-04-16 15:20:36 -070047 "press <command key>", release everything.
48
49On SPARC - You press 'ALT-STOP-<command key>', I believe.
50
51On the serial console (PC style standard serial ports only) -
52 You send a BREAK, then within 5 seconds a command key. Sending
53 BREAK twice is interpreted as a normal BREAK.
54
55On PowerPC - Press 'ALT - Print Screen (or F13) - <command key>,
56 Print Screen (or F13) - <command key> may suffice.
57
58On other - If you know of the key combos for other architectures, please
59 let me know so I can add them to this section.
60
Randy Dunlapd346cce2007-01-31 23:48:17 -080061On all - write a character to /proc/sysrq-trigger. e.g.:
Linus Torvalds1da177e2005-04-16 15:20:36 -070062
63 echo t > /proc/sysrq-trigger
64
65* What are the 'command' keys?
66~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Linus Torvalds1da177e2005-04-16 15:20:36 -070067'b' - Will immediately reboot the system without syncing or unmounting
68 your disks.
69
Hariprasad Nellitheertha86b1ae32005-06-25 14:58:25 -070070'c' - Will perform a kexec reboot in order to take a crashdump.
71
Randy Dunlapd346cce2007-01-31 23:48:17 -080072'd' - Shows all locks that are held.
73
Randy Dunlap78831ba2007-02-10 01:45:55 -080074'e' - Send a SIGTERM to all processes, except for init.
Linus Torvalds1da177e2005-04-16 15:20:36 -070075
Randy Dunlap78831ba2007-02-10 01:45:55 -080076'f' - Will call oom_kill to kill a memory hog process.
Linus Torvalds1da177e2005-04-16 15:20:36 -070077
Paul Mundtd29c91c2007-03-14 14:25:49 +090078'g' - Used by kgdb on ppc and sh platforms.
Linus Torvalds1da177e2005-04-16 15:20:36 -070079
Randy Dunlap78831ba2007-02-10 01:45:55 -080080'h' - Will display help (actually any other key than those listed
Jesper Juhl09736bd2007-08-10 13:01:04 -070081 here will display help. but 'h' is easy to remember :-)
Linus Torvalds1da177e2005-04-16 15:20:36 -070082
Randy Dunlap78831ba2007-02-10 01:45:55 -080083'i' - Send a SIGKILL to all processes, except for init.
84
85'k' - Secure Access Key (SAK) Kills all programs on the current virtual
86 console. NOTE: See important comments below in SAK section.
Linus Torvalds1da177e2005-04-16 15:20:36 -070087
88'm' - Will dump current memory info to your console.
89
Randy Dunlapd346cce2007-01-31 23:48:17 -080090'n' - Used to make RT tasks nice-able
91
Randy Dunlap78831ba2007-02-10 01:45:55 -080092'o' - Will shut your system off (if configured and supported).
93
94'p' - Will dump the current registers and flags to your console.
95
Johannes Weineracf11fa2007-03-22 00:11:18 -080096'q' - Will dump a list of all running timers.
97
Randy Dunlap78831ba2007-02-10 01:45:55 -080098'r' - Turns off keyboard raw mode and sets it to XLATE.
99
100's' - Will attempt to sync all mounted filesystems.
101
102't' - Will dump a list of current tasks and their information to your
103 console.
104
105'u' - Will attempt to remount all mounted filesystems read-only.
106
Linus Torvalds1da177e2005-04-16 15:20:36 -0700107'v' - Dumps Voyager SMP processor info to your console.
108
Randy Dunlapd346cce2007-01-31 23:48:17 -0800109'w' - Dumps tasks that are in uninterruptable (blocked) state.
110
111'x' - Used by xmon interface on ppc/powerpc platforms.
112
Linus Torvalds1da177e2005-04-16 15:20:36 -0700113'0'-'9' - Sets the console log level, controlling which kernel messages
114 will be printed to your console. ('0', for example would make
115 it so that only emergency messages like PANICs or OOPSes would
116 make it to your console.)
117
Linus Torvalds1da177e2005-04-16 15:20:36 -0700118* Okay, so what can I use them for?
119~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
120Well, un'R'aw is very handy when your X server or a svgalib program crashes.
121
Randy Dunlapd346cce2007-01-31 23:48:17 -0800122sa'K' (Secure Access Key) is useful when you want to be sure there is no
123trojan program running at console which could grab your password
124when you would try to login. It will kill all programs on given console,
125thus letting you make sure that the login prompt you see is actually
Linus Torvalds1da177e2005-04-16 15:20:36 -0700126the one from init, not some trojan program.
Jesper Juhl3eecd1d2006-06-25 05:48:16 -0700127IMPORTANT: In its true form it is not a true SAK like the one in a :IMPORTANT
128IMPORTANT: c2 compliant system, and it should not be mistaken as :IMPORTANT
129IMPORTANT: such. :IMPORTANT
Randy Dunlapd346cce2007-01-31 23:48:17 -0800130 It seems others find it useful as (System Attention Key) which is
Linus Torvalds1da177e2005-04-16 15:20:36 -0700131useful when you want to exit a program that will not let you switch consoles.
132(For example, X or a svgalib program.)
133
134re'B'oot is good when you're unable to shut down. But you should also 'S'ync
135and 'U'mount first.
136
Hariprasad Nellitheertha86b1ae32005-06-25 14:58:25 -0700137'C'rashdump can be used to manually trigger a crashdump when the system is hung.
138The kernel needs to have been built with CONFIG_KEXEC enabled.
139
Linus Torvalds1da177e2005-04-16 15:20:36 -0700140'S'ync is great when your system is locked up, it allows you to sync your
141disks and will certainly lessen the chance of data loss and fscking. Note
142that the sync hasn't taken place until you see the "OK" and "Done" appear
143on the screen. (If the kernel is really in strife, you may not ever get the
144OK or Done message...)
145
146'U'mount is basically useful in the same ways as 'S'ync. I generally 'S'ync,
147'U'mount, then re'B'oot when my system locks. It's saved me many a fsck.
148Again, the unmount (remount read-only) hasn't taken place until you see the
149"OK" and "Done" message appear on the screen.
150
Randy Dunlapd346cce2007-01-31 23:48:17 -0800151The loglevels '0'-'9' are useful when your console is being flooded with
152kernel messages you do not want to see. Selecting '0' will prevent all but
Linus Torvalds1da177e2005-04-16 15:20:36 -0700153the most urgent kernel messages from reaching your console. (They will
154still be logged if syslogd/klogd are alive, though.)
155
156t'E'rm and k'I'll are useful if you have some sort of runaway process you
157are unable to kill any other way, especially if it's spawning other
158processes.
159
160* Sometimes SysRq seems to get 'stuck' after using it, what can I do?
161~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
162That happens to me, also. I've found that tapping shift, alt, and control
163on both sides of the keyboard, and hitting an invalid sysrq sequence again
Randy Dunlapd346cce2007-01-31 23:48:17 -0800164will fix the problem. (i.e., something like alt-sysrq-z). Switching to another
Linus Torvalds1da177e2005-04-16 15:20:36 -0700165virtual console (ALT+Fn) and then back again should also help.
166
167* I hit SysRq, but nothing seems to happen, what's wrong?
168~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
169There are some keyboards that send different scancodes for SysRq than the
170pre-defined 0x54. So if SysRq doesn't work out of the box for a certain
171keyboard, run 'showkey -s' to find out the proper scancode sequence. Then
172use 'setkeycodes <sequence> 84' to define this sequence to the usual SysRq
173code (84 is decimal for 0x54). It's probably best to put this command in a
174boot script. Oh, and by the way, you exit 'showkey' by not typing anything
175for ten seconds.
176
177* I want to add SysRQ key events to a module, how does it work?
178~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
179In order to register a basic function with the table, you must first include
180the header 'include/linux/sysrq.h', this will define everything else you need.
181Next, you must create a sysrq_key_op struct, and populate it with A) the key
182handler function you will use, B) a help_msg string, that will print when SysRQ
183prints help, and C) an action_msg string, that will print right before your
Adrian Bunk338cec32005-09-10 00:26:54 -0700184handler is called. Your handler must conform to the prototype in 'sysrq.h'.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700185
Randy Dunlapd346cce2007-01-31 23:48:17 -0800186After the sysrq_key_op is created, you can call the kernel function
187register_sysrq_key(int key, struct sysrq_key_op *op_p); this will
188register the operation pointed to by 'op_p' at table key 'key',
189if that slot in the table is blank. At module unload time, you must call
190the function unregister_sysrq_key(int key, struct sysrq_key_op *op_p), which
Linus Torvalds1da177e2005-04-16 15:20:36 -0700191will remove the key op pointed to by 'op_p' from the key 'key', if and only if
192it is currently registered in that slot. This is in case the slot has been
193overwritten since you registered it.
194
195The Magic SysRQ system works by registering key operations against a key op
196lookup table, which is defined in 'drivers/char/sysrq.c'. This key table has
197a number of operations registered into it at compile time, but is mutable,
Randy Dunlapd346cce2007-01-31 23:48:17 -0800198and 2 functions are exported for interface to it:
199 register_sysrq_key and unregister_sysrq_key.
200Of course, never ever leave an invalid pointer in the table. I.e., when
201your module that called register_sysrq_key() exits, it must call
202unregister_sysrq_key() to clean up the sysrq key table entry that it used.
203Null pointers in the table are always safe. :)
Linus Torvalds1da177e2005-04-16 15:20:36 -0700204
205If for some reason you feel the need to call the handle_sysrq function from
206within a function called by handle_sysrq, you must be aware that you are in
207a lock (you are also in an interrupt handler, which means don't sleep!), so
208you must call __handle_sysrq_nolock instead.
209
210* I have more questions, who can I ask?
211~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Linus Torvalds1da177e2005-04-16 15:20:36 -0700212And I'll answer any questions about the registration system you got, also
213responding as soon as possible.
214 -Crutcher
215
216* Credits
217~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Adam D. Moss5e03e2c2006-01-03 13:31:01 +0100218Written by Mydraal <vulpyne@vulpyne.net>
Linus Torvalds1da177e2005-04-16 15:20:36 -0700219Updated by Adam Sulmicki <adam@cfar.umd.edu>
220Updated by Jeremy M. Dolan <jmd@turbogeek.org> 2001/01/28 10:15:59
221Added to by Crutcher Dunnavant <crutcher+kernel@datastacks.com>