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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
3Last update: 2007-JAN-06
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
46 have better luck with "press Alt", "press SysRq", "release Alt",
47 "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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
67'r' - Turns off keyboard raw mode and sets it to XLATE.
68
69'k' - Secure Access Key (SAK) Kills all programs on the current virtual
70 console. NOTE: See important comments below in SAK section.
71
72'b' - Will immediately reboot the system without syncing or unmounting
73 your disks.
74
Hariprasad Nellitheertha86b1ae32005-06-25 14:58:25 -070075'c' - Will perform a kexec reboot in order to take a crashdump.
76
Randy Dunlapd346cce2007-01-31 23:48:17 -080077'd' - Shows all locks that are held.
78
Linus Torvalds1da177e2005-04-16 15:20:36 -070079'o' - Will shut your system off (if configured and supported).
80
81's' - Will attempt to sync all mounted filesystems.
82
83'u' - Will attempt to remount all mounted filesystems read-only.
84
85'p' - Will dump the current registers and flags to your console.
86
87't' - Will dump a list of current tasks and their information to your
88 console.
89
90'm' - Will dump current memory info to your console.
91
Randy Dunlapd346cce2007-01-31 23:48:17 -080092'n' - Used to make RT tasks nice-able
93
Linus Torvalds1da177e2005-04-16 15:20:36 -070094'v' - Dumps Voyager SMP processor info to your console.
95
Randy Dunlapd346cce2007-01-31 23:48:17 -080096'w' - Dumps tasks that are in uninterruptable (blocked) state.
97
98'x' - Used by xmon interface on ppc/powerpc platforms.
99
Linus Torvalds1da177e2005-04-16 15:20:36 -0700100'0'-'9' - Sets the console log level, controlling which kernel messages
101 will be printed to your console. ('0', for example would make
102 it so that only emergency messages like PANICs or OOPSes would
103 make it to your console.)
104
Randy Dunlapd346cce2007-01-31 23:48:17 -0800105'f' - Will call oom_kill to kill a memory hog process.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700106
107'e' - Send a SIGTERM to all processes, except for init.
108
Randy Dunlapd346cce2007-01-31 23:48:17 -0800109'g' - Used by kgdb on ppc platforms.
110
Linus Torvalds1da177e2005-04-16 15:20:36 -0700111'i' - Send a SIGKILL to all processes, except for init.
112
Randy Dunlapd346cce2007-01-31 23:48:17 -0800113'h' - Will display help (actually any other key than those listed
Linus Torvalds1da177e2005-04-16 15:20:36 -0700114 above will display help. but 'h' is easy to remember :-)
115
116* Okay, so what can I use them for?
117~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
118Well, un'R'aw is very handy when your X server or a svgalib program crashes.
119
Randy Dunlapd346cce2007-01-31 23:48:17 -0800120sa'K' (Secure Access Key) is useful when you want to be sure there is no
121trojan program running at console which could grab your password
122when you would try to login. It will kill all programs on given console,
123thus letting you make sure that the login prompt you see is actually
Linus Torvalds1da177e2005-04-16 15:20:36 -0700124the one from init, not some trojan program.
Jesper Juhl3eecd1d2006-06-25 05:48:16 -0700125IMPORTANT: In its true form it is not a true SAK like the one in a :IMPORTANT
126IMPORTANT: c2 compliant system, and it should not be mistaken as :IMPORTANT
127IMPORTANT: such. :IMPORTANT
Randy Dunlapd346cce2007-01-31 23:48:17 -0800128 It seems others find it useful as (System Attention Key) which is
Linus Torvalds1da177e2005-04-16 15:20:36 -0700129useful when you want to exit a program that will not let you switch consoles.
130(For example, X or a svgalib program.)
131
132re'B'oot is good when you're unable to shut down. But you should also 'S'ync
133and 'U'mount first.
134
Hariprasad Nellitheertha86b1ae32005-06-25 14:58:25 -0700135'C'rashdump can be used to manually trigger a crashdump when the system is hung.
136The kernel needs to have been built with CONFIG_KEXEC enabled.
137
Linus Torvalds1da177e2005-04-16 15:20:36 -0700138'S'ync is great when your system is locked up, it allows you to sync your
139disks and will certainly lessen the chance of data loss and fscking. Note
140that the sync hasn't taken place until you see the "OK" and "Done" appear
141on the screen. (If the kernel is really in strife, you may not ever get the
142OK or Done message...)
143
144'U'mount is basically useful in the same ways as 'S'ync. I generally 'S'ync,
145'U'mount, then re'B'oot when my system locks. It's saved me many a fsck.
146Again, the unmount (remount read-only) hasn't taken place until you see the
147"OK" and "Done" message appear on the screen.
148
Randy Dunlapd346cce2007-01-31 23:48:17 -0800149The loglevels '0'-'9' are useful when your console is being flooded with
150kernel messages you do not want to see. Selecting '0' will prevent all but
Linus Torvalds1da177e2005-04-16 15:20:36 -0700151the most urgent kernel messages from reaching your console. (They will
152still be logged if syslogd/klogd are alive, though.)
153
154t'E'rm and k'I'll are useful if you have some sort of runaway process you
155are unable to kill any other way, especially if it's spawning other
156processes.
157
158* Sometimes SysRq seems to get 'stuck' after using it, what can I do?
159~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
160That happens to me, also. I've found that tapping shift, alt, and control
161on both sides of the keyboard, and hitting an invalid sysrq sequence again
Randy Dunlapd346cce2007-01-31 23:48:17 -0800162will fix the problem. (i.e., something like alt-sysrq-z). Switching to another
Linus Torvalds1da177e2005-04-16 15:20:36 -0700163virtual console (ALT+Fn) and then back again should also help.
164
165* I hit SysRq, but nothing seems to happen, what's wrong?
166~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
167There are some keyboards that send different scancodes for SysRq than the
168pre-defined 0x54. So if SysRq doesn't work out of the box for a certain
169keyboard, run 'showkey -s' to find out the proper scancode sequence. Then
170use 'setkeycodes <sequence> 84' to define this sequence to the usual SysRq
171code (84 is decimal for 0x54). It's probably best to put this command in a
172boot script. Oh, and by the way, you exit 'showkey' by not typing anything
173for ten seconds.
174
175* I want to add SysRQ key events to a module, how does it work?
176~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
177In order to register a basic function with the table, you must first include
178the header 'include/linux/sysrq.h', this will define everything else you need.
179Next, you must create a sysrq_key_op struct, and populate it with A) the key
180handler function you will use, B) a help_msg string, that will print when SysRQ
181prints help, and C) an action_msg string, that will print right before your
Adrian Bunk338cec32005-09-10 00:26:54 -0700182handler is called. Your handler must conform to the prototype in 'sysrq.h'.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700183
Randy Dunlapd346cce2007-01-31 23:48:17 -0800184After the sysrq_key_op is created, you can call the kernel function
185register_sysrq_key(int key, struct sysrq_key_op *op_p); this will
186register the operation pointed to by 'op_p' at table key 'key',
187if that slot in the table is blank. At module unload time, you must call
188the function unregister_sysrq_key(int key, struct sysrq_key_op *op_p), which
Linus Torvalds1da177e2005-04-16 15:20:36 -0700189will remove the key op pointed to by 'op_p' from the key 'key', if and only if
190it is currently registered in that slot. This is in case the slot has been
191overwritten since you registered it.
192
193The Magic SysRQ system works by registering key operations against a key op
194lookup table, which is defined in 'drivers/char/sysrq.c'. This key table has
195a number of operations registered into it at compile time, but is mutable,
Randy Dunlapd346cce2007-01-31 23:48:17 -0800196and 2 functions are exported for interface to it:
197 register_sysrq_key and unregister_sysrq_key.
198Of course, never ever leave an invalid pointer in the table. I.e., when
199your module that called register_sysrq_key() exits, it must call
200unregister_sysrq_key() to clean up the sysrq key table entry that it used.
201Null pointers in the table are always safe. :)
Linus Torvalds1da177e2005-04-16 15:20:36 -0700202
203If for some reason you feel the need to call the handle_sysrq function from
204within a function called by handle_sysrq, you must be aware that you are in
205a lock (you are also in an interrupt handler, which means don't sleep!), so
206you must call __handle_sysrq_nolock instead.
207
208* I have more questions, who can I ask?
209~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Linus Torvalds1da177e2005-04-16 15:20:36 -0700210And I'll answer any questions about the registration system you got, also
211responding as soon as possible.
212 -Crutcher
213
214* Credits
215~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Adam D. Moss5e03e2c2006-01-03 13:31:01 +0100216Written by Mydraal <vulpyne@vulpyne.net>
Linus Torvalds1da177e2005-04-16 15:20:36 -0700217Updated by Adam Sulmicki <adam@cfar.umd.edu>
218Updated by Jeremy M. Dolan <jmd@turbogeek.org> 2001/01/28 10:15:59
219Added to by Crutcher Dunnavant <crutcher+kernel@datastacks.com>