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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
Hidetoshi Setocab8bd32009-07-29 15:04:14 -070069'c' - Will perform a system crash by a NULL pointer dereference.
70 A crashdump will be taken if configured.
Hariprasad Nellitheertha86b1ae32005-06-25 14:58:25 -070071
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
Randy Dunlap003bb8a2010-10-26 14:20:40 -070078'g' - Used by kgdb (kernel debugger)
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
Eric Sandeenc2d75432009-03-31 15:23:46 -070085'j' - Forcibly "Just thaw it" - filesystems frozen by the FIFREEZE ioctl.
86
Randy Dunlap78831ba2007-02-10 01:45:55 -080087'k' - Secure Access Key (SAK) Kills all programs on the current virtual
88 console. NOTE: See important comments below in SAK section.
Linus Torvalds1da177e2005-04-16 15:20:36 -070089
Rik van Riel5045bca2008-04-29 00:59:21 -070090'l' - Shows a stack backtrace for all active CPUs.
91
Linus Torvalds1da177e2005-04-16 15:20:36 -070092'm' - Will dump current memory info to your console.
93
Randy Dunlapd346cce2007-01-31 23:48:17 -080094'n' - Used to make RT tasks nice-able
95
Randy Dunlap78831ba2007-02-10 01:45:55 -080096'o' - Will shut your system off (if configured and supported).
97
98'p' - Will dump the current registers and flags to your console.
99
Linus Torvalds99ebcf82008-10-20 13:19:56 -0700100'q' - Will dump per CPU lists of all armed hrtimers (but NOT regular
101 timer_list timers) and detailed information about all
102 clockevent devices.
Johannes Weineracf11fa2007-03-22 00:11:18 -0800103
Randy Dunlap78831ba2007-02-10 01:45:55 -0800104'r' - Turns off keyboard raw mode and sets it to XLATE.
105
106's' - Will attempt to sync all mounted filesystems.
107
108't' - Will dump a list of current tasks and their information to your
109 console.
110
111'u' - Will attempt to remount all mounted filesystems read-only.
112
Randy Dunlap003bb8a2010-10-26 14:20:40 -0700113'v' - Forcefully restores framebuffer console
114'v' - Causes ETM buffer dump [ARM-specific]
Linus Torvalds1da177e2005-04-16 15:20:36 -0700115
Randy Dunlapd346cce2007-01-31 23:48:17 -0800116'w' - Dumps tasks that are in uninterruptable (blocked) state.
117
118'x' - Used by xmon interface on ppc/powerpc platforms.
David S. Miller916ca142012-10-16 09:34:01 -0700119 Show global PMU Registers on sparc64.
Randy Dunlapd346cce2007-01-31 23:48:17 -0800120
Randy Dunlap003bb8a2010-10-26 14:20:40 -0700121'y' - Show global CPU Registers [SPARC-64 specific]
122
Randy Dunlap3871f2f2008-12-24 16:06:57 -0800123'z' - Dump the ftrace buffer
124
Linus Torvalds1da177e2005-04-16 15:20:36 -0700125'0'-'9' - Sets the console log level, controlling which kernel messages
126 will be printed to your console. ('0', for example would make
127 it so that only emergency messages like PANICs or OOPSes would
128 make it to your console.)
129
Linus Torvalds1da177e2005-04-16 15:20:36 -0700130* Okay, so what can I use them for?
131~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
132Well, un'R'aw is very handy when your X server or a svgalib program crashes.
133
Randy Dunlapd346cce2007-01-31 23:48:17 -0800134sa'K' (Secure Access Key) is useful when you want to be sure there is no
135trojan program running at console which could grab your password
136when you would try to login. It will kill all programs on given console,
137thus letting you make sure that the login prompt you see is actually
Linus Torvalds1da177e2005-04-16 15:20:36 -0700138the one from init, not some trojan program.
Jesper Juhl3eecd1d2006-06-25 05:48:16 -0700139IMPORTANT: In its true form it is not a true SAK like the one in a :IMPORTANT
140IMPORTANT: c2 compliant system, and it should not be mistaken as :IMPORTANT
141IMPORTANT: such. :IMPORTANT
Randy Dunlapd346cce2007-01-31 23:48:17 -0800142 It seems others find it useful as (System Attention Key) which is
Linus Torvalds1da177e2005-04-16 15:20:36 -0700143useful when you want to exit a program that will not let you switch consoles.
144(For example, X or a svgalib program.)
145
146re'B'oot is good when you're unable to shut down. But you should also 'S'ync
147and 'U'mount first.
148
Hidetoshi Setocab8bd32009-07-29 15:04:14 -0700149'C'rash can be used to manually trigger a crashdump when the system is hung.
150Note that this just triggers a crash if there is no dump mechanism available.
Hariprasad Nellitheertha86b1ae32005-06-25 14:58:25 -0700151
Linus Torvalds1da177e2005-04-16 15:20:36 -0700152'S'ync is great when your system is locked up, it allows you to sync your
153disks and will certainly lessen the chance of data loss and fscking. Note
154that the sync hasn't taken place until you see the "OK" and "Done" appear
155on the screen. (If the kernel is really in strife, you may not ever get the
156OK or Done message...)
157
158'U'mount is basically useful in the same ways as 'S'ync. I generally 'S'ync,
159'U'mount, then re'B'oot when my system locks. It's saved me many a fsck.
160Again, the unmount (remount read-only) hasn't taken place until you see the
161"OK" and "Done" message appear on the screen.
162
Randy Dunlapd346cce2007-01-31 23:48:17 -0800163The loglevels '0'-'9' are useful when your console is being flooded with
164kernel messages you do not want to see. Selecting '0' will prevent all but
Linus Torvalds1da177e2005-04-16 15:20:36 -0700165the most urgent kernel messages from reaching your console. (They will
166still be logged if syslogd/klogd are alive, though.)
167
168t'E'rm and k'I'll are useful if you have some sort of runaway process you
169are unable to kill any other way, especially if it's spawning other
170processes.
171
Eric Sandeenc2d75432009-03-31 15:23:46 -0700172"'J'ust thaw it" is useful if your system becomes unresponsive due to a frozen
173(probably root) filesystem via the FIFREEZE ioctl.
174
Linus Torvalds1da177e2005-04-16 15:20:36 -0700175* Sometimes SysRq seems to get 'stuck' after using it, what can I do?
176~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
177That happens to me, also. I've found that tapping shift, alt, and control
178on both sides of the keyboard, and hitting an invalid sysrq sequence again
Randy Dunlapd346cce2007-01-31 23:48:17 -0800179will fix the problem. (i.e., something like alt-sysrq-z). Switching to another
Linus Torvalds1da177e2005-04-16 15:20:36 -0700180virtual console (ALT+Fn) and then back again should also help.
181
182* I hit SysRq, but nothing seems to happen, what's wrong?
183~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Paul Foxa2056ff2010-05-19 10:11:13 -0700184There are some keyboards that produce a different keycode for SysRq than the
185pre-defined value of 99 (see KEY_SYSRQ in include/linux/input.h), or which
186don't have a SysRq key at all. In these cases, run 'showkey -s' to find an
187appropriate scancode sequence, and use 'setkeycodes <sequence> 99' to map
188this sequence to the usual SysRq code (e.g., 'setkeycodes e05b 99'). It's
189probably best to put this command in a boot script. Oh, and by the way, you
190exit 'showkey' by not typing anything for ten seconds.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700191
192* I want to add SysRQ key events to a module, how does it work?
193~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
194In order to register a basic function with the table, you must first include
195the header 'include/linux/sysrq.h', this will define everything else you need.
196Next, you must create a sysrq_key_op struct, and populate it with A) the key
197handler function you will use, B) a help_msg string, that will print when SysRQ
198prints help, and C) an action_msg string, that will print right before your
Adrian Bunk338cec32005-09-10 00:26:54 -0700199handler is called. Your handler must conform to the prototype in 'sysrq.h'.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700200
Randy Dunlapd346cce2007-01-31 23:48:17 -0800201After the sysrq_key_op is created, you can call the kernel function
202register_sysrq_key(int key, struct sysrq_key_op *op_p); this will
203register the operation pointed to by 'op_p' at table key 'key',
204if that slot in the table is blank. At module unload time, you must call
205the function unregister_sysrq_key(int key, struct sysrq_key_op *op_p), which
Linus Torvalds1da177e2005-04-16 15:20:36 -0700206will remove the key op pointed to by 'op_p' from the key 'key', if and only if
207it is currently registered in that slot. This is in case the slot has been
208overwritten since you registered it.
209
210The Magic SysRQ system works by registering key operations against a key op
211lookup table, which is defined in 'drivers/char/sysrq.c'. This key table has
212a number of operations registered into it at compile time, but is mutable,
Randy Dunlapd346cce2007-01-31 23:48:17 -0800213and 2 functions are exported for interface to it:
214 register_sysrq_key and unregister_sysrq_key.
215Of course, never ever leave an invalid pointer in the table. I.e., when
216your module that called register_sysrq_key() exits, it must call
217unregister_sysrq_key() to clean up the sysrq key table entry that it used.
218Null pointers in the table are always safe. :)
Linus Torvalds1da177e2005-04-16 15:20:36 -0700219
220If for some reason you feel the need to call the handle_sysrq function from
221within a function called by handle_sysrq, you must be aware that you are in
222a lock (you are also in an interrupt handler, which means don't sleep!), so
223you must call __handle_sysrq_nolock instead.
224
Andy Whitcroft47c33d92009-01-15 13:50:51 -0800225* When I hit a SysRq key combination only the header appears on the console?
226~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
227Sysrq output is subject to the same console loglevel control as all
228other console output. This means that if the kernel was booted 'quiet'
229as is common on distro kernels the output may not appear on the actual
230console, even though it will appear in the dmesg buffer, and be accessible
231via the dmesg command and to the consumers of /proc/kmsg. As a specific
232exception the header line from the sysrq command is passed to all console
233consumers as if the current loglevel was maximum. If only the header
234is emitted it is almost certain that the kernel loglevel is too low.
235Should you require the output on the console channel then you will need
236to temporarily up the console loglevel using alt-sysrq-8 or:
237
238 echo 8 > /proc/sysrq-trigger
239
240Remember to return the loglevel to normal after triggering the sysrq
241command you are interested in.
242
Linus Torvalds1da177e2005-04-16 15:20:36 -0700243* I have more questions, who can I ask?
244~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Randy Dunlap21106b02012-03-30 13:37:06 -0700245Just ask them on the linux-kernel mailing list:
246 linux-kernel@vger.kernel.org
Linus Torvalds1da177e2005-04-16 15:20:36 -0700247
248* Credits
249~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Adam D. Moss5e03e2c2006-01-03 13:31:01 +0100250Written by Mydraal <vulpyne@vulpyne.net>
Linus Torvalds1da177e2005-04-16 15:20:36 -0700251Updated by Adam Sulmicki <adam@cfar.umd.edu>
252Updated by Jeremy M. Dolan <jmd@turbogeek.org> 2001/01/28 10:15:59
253Added to by Crutcher Dunnavant <crutcher+kernel@datastacks.com>