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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
Linus Torvalds1da177e2005-04-16 15:20:36 -07003
4* What is the magic SysRq key?
5~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
6It is a 'magical' key combo you can hit which the kernel will respond to
7regardless of whatever else it is doing, unless it is completely locked up.
8
9* How do I enable the magic SysRq key?
10~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
11You need to say "yes" to 'Magic SysRq key (CONFIG_MAGIC_SYSRQ)' when
12configuring the kernel. When running a kernel with SysRq compiled in,
13/proc/sys/kernel/sysrq controls the functions allowed to be invoked via
14the SysRq key. By default the file contains 1 which means that every
15possible SysRq request is allowed (in older versions SysRq was disabled
16by default, and you were required to specifically enable it at run-time
17but this is not the case any more). Here is the list of possible values
18in /proc/sys/kernel/sysrq:
19 0 - disable sysrq completely
20 1 - enable all functions of sysrq
21 >1 - bitmask of allowed sysrq functions (see below for detailed function
22 description):
23 2 - enable control of console logging level
24 4 - enable control of keyboard (SAK, unraw)
25 8 - enable debugging dumps of processes etc.
26 16 - enable sync command
27 32 - enable remount read-only
28 64 - enable signalling of processes (term, kill, oom-kill)
29 128 - allow reboot/poweroff
30 256 - allow nicing of all RT tasks
31
32You can set the value in the file by the following command:
33 echo "number" >/proc/sys/kernel/sysrq
34
35Note that the value of /proc/sys/kernel/sysrq influences only the invocation
36via a keyboard. Invocation of any operation via /proc/sysrq-trigger is always
Randy Dunlapd346cce2007-01-31 23:48:17 -080037allowed (by a user with admin privileges).
Linus Torvalds1da177e2005-04-16 15:20:36 -070038
39* How do I use the magic SysRq key?
40~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
41On x86 - You press the key combo 'ALT-SysRq-<command key>'. Note - Some
42 keyboards may not have a key labeled 'SysRq'. The 'SysRq' key is
43 also known as the 'Print Screen' key. Also some keyboards cannot
44 handle so many keys being pressed at the same time, so you might
Pavel Machekdfb00422007-10-07 00:24:37 -070045 have better luck with "press Alt", "press SysRq", "release SysRq",
Linus Torvalds1da177e2005-04-16 15:20:36 -070046 "press <command key>", release everything.
47
48On SPARC - You press 'ALT-STOP-<command key>', I believe.
49
50On the serial console (PC style standard serial ports only) -
51 You send a BREAK, then within 5 seconds a command key. Sending
52 BREAK twice is interpreted as a normal BREAK.
53
54On PowerPC - Press 'ALT - Print Screen (or F13) - <command key>,
55 Print Screen (or F13) - <command key> may suffice.
56
57On other - If you know of the key combos for other architectures, please
58 let me know so I can add them to this section.
59
Randy Dunlapd346cce2007-01-31 23:48:17 -080060On all - write a character to /proc/sysrq-trigger. e.g.:
Linus Torvalds1da177e2005-04-16 15:20:36 -070061
62 echo t > /proc/sysrq-trigger
63
64* What are the 'command' keys?
65~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Linus Torvalds1da177e2005-04-16 15:20:36 -070066'b' - Will immediately reboot the system without syncing or unmounting
67 your disks.
68
Hariprasad Nellitheertha86b1ae32005-06-25 14:58:25 -070069'c' - Will perform a kexec reboot in order to take a crashdump.
70
Randy Dunlapd346cce2007-01-31 23:48:17 -080071'd' - Shows all locks that are held.
72
Randy Dunlap78831ba2007-02-10 01:45:55 -080073'e' - Send a SIGTERM to all processes, except for init.
Linus Torvalds1da177e2005-04-16 15:20:36 -070074
Randy Dunlap78831ba2007-02-10 01:45:55 -080075'f' - Will call oom_kill to kill a memory hog process.
Linus Torvalds1da177e2005-04-16 15:20:36 -070076
Paul Mundtd29c91c2007-03-14 14:25:49 +090077'g' - Used by kgdb on ppc and sh platforms.
Linus Torvalds1da177e2005-04-16 15:20:36 -070078
Randy Dunlap78831ba2007-02-10 01:45:55 -080079'h' - Will display help (actually any other key than those listed
Jesper Juhl09736bd2007-08-10 13:01:04 -070080 here will display help. but 'h' is easy to remember :-)
Linus Torvalds1da177e2005-04-16 15:20:36 -070081
Randy Dunlap78831ba2007-02-10 01:45:55 -080082'i' - Send a SIGKILL to all processes, except for init.
83
84'k' - Secure Access Key (SAK) Kills all programs on the current virtual
85 console. NOTE: See important comments below in SAK section.
Linus Torvalds1da177e2005-04-16 15:20:36 -070086
Rik van Riel5045bca2008-04-29 00:59:21 -070087'l' - Shows a stack backtrace for all active CPUs.
88
Linus Torvalds1da177e2005-04-16 15:20:36 -070089'm' - Will dump current memory info to your console.
90
Randy Dunlapd346cce2007-01-31 23:48:17 -080091'n' - Used to make RT tasks nice-able
92
Randy Dunlap78831ba2007-02-10 01:45:55 -080093'o' - Will shut your system off (if configured and supported).
94
95'p' - Will dump the current registers and flags to your console.
96
Linus Torvalds99ebcf82008-10-20 13:19:56 -070097'q' - Will dump per CPU lists of all armed hrtimers (but NOT regular
98 timer_list timers) and detailed information about all
99 clockevent devices.
Johannes Weineracf11fa2007-03-22 00:11:18 -0800100
Randy Dunlap78831ba2007-02-10 01:45:55 -0800101'r' - Turns off keyboard raw mode and sets it to XLATE.
102
103's' - Will attempt to sync all mounted filesystems.
104
105't' - Will dump a list of current tasks and their information to your
106 console.
107
108'u' - Will attempt to remount all mounted filesystems read-only.
109
Linus Torvalds1da177e2005-04-16 15:20:36 -0700110'v' - Dumps Voyager SMP processor info to your console.
111
Randy Dunlapd346cce2007-01-31 23:48:17 -0800112'w' - Dumps tasks that are in uninterruptable (blocked) state.
113
114'x' - Used by xmon interface on ppc/powerpc platforms.
115
Linus Torvalds1da177e2005-04-16 15:20:36 -0700116'0'-'9' - Sets the console log level, controlling which kernel messages
117 will be printed to your console. ('0', for example would make
118 it so that only emergency messages like PANICs or OOPSes would
119 make it to your console.)
120
Linus Torvalds1da177e2005-04-16 15:20:36 -0700121* Okay, so what can I use them for?
122~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
123Well, un'R'aw is very handy when your X server or a svgalib program crashes.
124
Randy Dunlapd346cce2007-01-31 23:48:17 -0800125sa'K' (Secure Access Key) is useful when you want to be sure there is no
126trojan program running at console which could grab your password
127when you would try to login. It will kill all programs on given console,
128thus letting you make sure that the login prompt you see is actually
Linus Torvalds1da177e2005-04-16 15:20:36 -0700129the one from init, not some trojan program.
Jesper Juhl3eecd1d2006-06-25 05:48:16 -0700130IMPORTANT: In its true form it is not a true SAK like the one in a :IMPORTANT
131IMPORTANT: c2 compliant system, and it should not be mistaken as :IMPORTANT
132IMPORTANT: such. :IMPORTANT
Randy Dunlapd346cce2007-01-31 23:48:17 -0800133 It seems others find it useful as (System Attention Key) which is
Linus Torvalds1da177e2005-04-16 15:20:36 -0700134useful when you want to exit a program that will not let you switch consoles.
135(For example, X or a svgalib program.)
136
137re'B'oot is good when you're unable to shut down. But you should also 'S'ync
138and 'U'mount first.
139
Hariprasad Nellitheertha86b1ae32005-06-25 14:58:25 -0700140'C'rashdump can be used to manually trigger a crashdump when the system is hung.
141The kernel needs to have been built with CONFIG_KEXEC enabled.
142
Linus Torvalds1da177e2005-04-16 15:20:36 -0700143'S'ync is great when your system is locked up, it allows you to sync your
144disks and will certainly lessen the chance of data loss and fscking. Note
145that the sync hasn't taken place until you see the "OK" and "Done" appear
146on the screen. (If the kernel is really in strife, you may not ever get the
147OK or Done message...)
148
149'U'mount is basically useful in the same ways as 'S'ync. I generally 'S'ync,
150'U'mount, then re'B'oot when my system locks. It's saved me many a fsck.
151Again, the unmount (remount read-only) hasn't taken place until you see the
152"OK" and "Done" message appear on the screen.
153
Randy Dunlapd346cce2007-01-31 23:48:17 -0800154The loglevels '0'-'9' are useful when your console is being flooded with
155kernel messages you do not want to see. Selecting '0' will prevent all but
Linus Torvalds1da177e2005-04-16 15:20:36 -0700156the most urgent kernel messages from reaching your console. (They will
157still be logged if syslogd/klogd are alive, though.)
158
159t'E'rm and k'I'll are useful if you have some sort of runaway process you
160are unable to kill any other way, especially if it's spawning other
161processes.
162
163* Sometimes SysRq seems to get 'stuck' after using it, what can I do?
164~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
165That happens to me, also. I've found that tapping shift, alt, and control
166on both sides of the keyboard, and hitting an invalid sysrq sequence again
Randy Dunlapd346cce2007-01-31 23:48:17 -0800167will fix the problem. (i.e., something like alt-sysrq-z). Switching to another
Linus Torvalds1da177e2005-04-16 15:20:36 -0700168virtual console (ALT+Fn) and then back again should also help.
169
170* I hit SysRq, but nothing seems to happen, what's wrong?
171~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
172There are some keyboards that send different scancodes for SysRq than the
173pre-defined 0x54. So if SysRq doesn't work out of the box for a certain
174keyboard, run 'showkey -s' to find out the proper scancode sequence. Then
175use 'setkeycodes <sequence> 84' to define this sequence to the usual SysRq
176code (84 is decimal for 0x54). It's probably best to put this command in a
177boot script. Oh, and by the way, you exit 'showkey' by not typing anything
178for ten seconds.
179
180* I want to add SysRQ key events to a module, how does it work?
181~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
182In order to register a basic function with the table, you must first include
183the header 'include/linux/sysrq.h', this will define everything else you need.
184Next, you must create a sysrq_key_op struct, and populate it with A) the key
185handler function you will use, B) a help_msg string, that will print when SysRQ
186prints help, and C) an action_msg string, that will print right before your
Adrian Bunk338cec32005-09-10 00:26:54 -0700187handler is called. Your handler must conform to the prototype in 'sysrq.h'.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700188
Randy Dunlapd346cce2007-01-31 23:48:17 -0800189After the sysrq_key_op is created, you can call the kernel function
190register_sysrq_key(int key, struct sysrq_key_op *op_p); this will
191register the operation pointed to by 'op_p' at table key 'key',
192if that slot in the table is blank. At module unload time, you must call
193the function unregister_sysrq_key(int key, struct sysrq_key_op *op_p), which
Linus Torvalds1da177e2005-04-16 15:20:36 -0700194will remove the key op pointed to by 'op_p' from the key 'key', if and only if
195it is currently registered in that slot. This is in case the slot has been
196overwritten since you registered it.
197
198The Magic SysRQ system works by registering key operations against a key op
199lookup table, which is defined in 'drivers/char/sysrq.c'. This key table has
200a number of operations registered into it at compile time, but is mutable,
Randy Dunlapd346cce2007-01-31 23:48:17 -0800201and 2 functions are exported for interface to it:
202 register_sysrq_key and unregister_sysrq_key.
203Of course, never ever leave an invalid pointer in the table. I.e., when
204your module that called register_sysrq_key() exits, it must call
205unregister_sysrq_key() to clean up the sysrq key table entry that it used.
206Null pointers in the table are always safe. :)
Linus Torvalds1da177e2005-04-16 15:20:36 -0700207
208If for some reason you feel the need to call the handle_sysrq function from
209within a function called by handle_sysrq, you must be aware that you are in
210a lock (you are also in an interrupt handler, which means don't sleep!), so
211you must call __handle_sysrq_nolock instead.
212
Andy Whitcroft47c33d92009-01-15 13:50:51 -0800213* When I hit a SysRq key combination only the header appears on the console?
214~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
215Sysrq output is subject to the same console loglevel control as all
216other console output. This means that if the kernel was booted 'quiet'
217as is common on distro kernels the output may not appear on the actual
218console, even though it will appear in the dmesg buffer, and be accessible
219via the dmesg command and to the consumers of /proc/kmsg. As a specific
220exception the header line from the sysrq command is passed to all console
221consumers as if the current loglevel was maximum. If only the header
222is emitted it is almost certain that the kernel loglevel is too low.
223Should you require the output on the console channel then you will need
224to temporarily up the console loglevel using alt-sysrq-8 or:
225
226 echo 8 > /proc/sysrq-trigger
227
228Remember to return the loglevel to normal after triggering the sysrq
229command you are interested in.
230
Linus Torvalds1da177e2005-04-16 15:20:36 -0700231* I have more questions, who can I ask?
232~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Linus Torvalds1da177e2005-04-16 15:20:36 -0700233And I'll answer any questions about the registration system you got, also
234responding as soon as possible.
235 -Crutcher
236
237* Credits
238~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Adam D. Moss5e03e2c2006-01-03 13:31:01 +0100239Written by Mydraal <vulpyne@vulpyne.net>
Linus Torvalds1da177e2005-04-16 15:20:36 -0700240Updated by Adam Sulmicki <adam@cfar.umd.edu>
241Updated by Jeremy M. Dolan <jmd@turbogeek.org> 2001/01/28 10:15:59
242Added to by Crutcher Dunnavant <crutcher+kernel@datastacks.com>