Last update : 2010/11/16

Linux Kernel Documentation::sysrq.txt

1 Linux Magic System Request Key Hacks

2 Documentation for sysrq.c


4 * What is the magic SysRq key?

5 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

6 It is a 'magical' key combo you can hit which the kernel will respond to

7 regardless of whatever else it is doing, unless it is completely locked up.


9 * How do I enable the magic SysRq key?

10 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

11 You need to say "yes" to 'Magic SysRq key (CONFIG_MAGIC_SYSRQ)' when

12 configuring the kernel. When running a kernel with SysRq compiled in,

13 /proc/sys/kernel/sysrq controls the functions allowed to be invoked via

14 the SysRq key. By default the file contains 1 which means that every

15 possible SysRq request is allowed (in older versions SysRq was disabled

16 by default, and you were required to specifically enable it at run-time

17 but this is not the case any more). Here is the list of possible values

18 in /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


32 You can set the value in the file by the following command:

33 echo "number" >/proc/sys/kernel/sysrq


35 Note that the value of /proc/sys/kernel/sysrq influences only the invocation

36 via a keyboard. Invocation of any operation via /proc/sysrq-trigger is always

37 allowed (by a user with admin privileges).


39 * How do I use the magic SysRq key?

40 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

41 On 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

45 have better luck with "press Alt", "press SysRq", "release SysRq",

46 "press <command key>", release everything.


48 On SPARC - You press 'ALT-STOP-<command key>', I believe.


50 On 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.


54 On PowerPC - Press 'ALT - Print Screen (or F13) - <command key>,

55 Print Screen (or F13) - <command key> may suffice.


57 On other - If you know of the key combos for other architectures, please

58 let me know so I can add them to this section.


60 On all - write a character to /proc/sysrq-trigger. e.g.:


62 echo t > /proc/sysrq-trigger


64 * What are the 'command' keys?

65 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

66 'b' - Will immediately reboot the system without syncing or unmounting

67 your disks.


69 'c' - Will perform a system crash by a NULL pointer dereference.

70 A crashdump will be taken if configured.


72 'd' - Shows all locks that are held.


74 'e' - Send a SIGTERM to all processes, except for init.


76 'f' - Will call oom_kill to kill a memory hog process.


78 'g' - Used by kgdb on ppc and sh platforms.


80 'h' - Will display help (actually any other key than those listed

81 here will display help. but 'h' is easy to remember :-)


83 'i' - Send a SIGKILL to all processes, except for init.


85 'j' - Forcibly "Just thaw it" - filesystems frozen by the FIFREEZE ioctl.


87 'k' - Secure Access Key (SAK) Kills all programs on the current virtual

88 console. NOTE: See important comments below in SAK section.


90 'l' - Shows a stack backtrace for all active CPUs.


92 'm' - Will dump current memory info to your console.


94 'n' - Used to make RT tasks nice-able


96 'o' - Will shut your system off (if configured and supported).


98 'p' - Will dump the current registers and flags to your console.


100 '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.


104 'r' - Turns off keyboard raw mode and sets it to XLATE.


106 's' - Will attempt to sync all mounted filesystems.


108 't' - Will dump a list of current tasks and their information to your

109 console.


111 'u' - Will attempt to remount all mounted filesystems read-only.


113 'v' - Dumps Voyager SMP processor info to your console.


115 'w' - Dumps tasks that are in uninterruptable (blocked) state.


117 'x' - Used by xmon interface on ppc/powerpc platforms.


119 'z' - Dump the ftrace buffer


121 '0'-'9' - Sets the console log level, controlling which kernel messages

122 will be printed to your console. ('0', for example would make

123 it so that only emergency messages like PANICs or OOPSes would

124 make it to your console.)


126 * Okay, so what can I use them for?

127 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

128 Well, un'R'aw is very handy when your X server or a svgalib program crashes.


130 sa'K' (Secure Access Key) is useful when you want to be sure there is no

131 trojan program running at console which could grab your password

132 when you would try to login. It will kill all programs on given console,

133 thus letting you make sure that the login prompt you see is actually

134 the one from init, not some trojan program.

135 IMPORTANT: In its true form it is not a true SAK like the one in a :IMPORTANT

136 IMPORTANT: c2 compliant system, and it should not be mistaken as :IMPORTANT


138 It seems others find it useful as (System Attention Key) which is

139 useful when you want to exit a program that will not let you switch consoles.

140 (For example, X or a svgalib program.)


142 re'B'oot is good when you're unable to shut down. But you should also 'S'ync

143 and 'U'mount first.


145 'C'rash can be used to manually trigger a crashdump when the system is hung.

146 Note that this just triggers a crash if there is no dump mechanism available.


148 'S'ync is great when your system is locked up, it allows you to sync your

149 disks and will certainly lessen the chance of data loss and fscking. Note

150 that the sync hasn't taken place until you see the "OK" and "Done" appear

151 on the screen. (If the kernel is really in strife, you may not ever get the

152 OK or Done message...)


154 'U'mount is basically useful in the same ways as 'S'ync. I generally 'S'ync,

155 'U'mount, then re'B'oot when my system locks. It's saved me many a fsck.

156 Again, the unmount (remount read-only) hasn't taken place until you see the

157 "OK" and "Done" message appear on the screen.


159 The loglevels '0'-'9' are useful when your console is being flooded with

160 kernel messages you do not want to see. Selecting '0' will prevent all but

161 the most urgent kernel messages from reaching your console. (They will

162 still be logged if syslogd/klogd are alive, though.)


164 t'E'rm and k'I'll are useful if you have some sort of runaway process you

165 are unable to kill any other way, especially if it's spawning other

166 processes.


168 "'J'ust thaw it" is useful if your system becomes unresponsive due to a frozen

169 (probably root) filesystem via the FIFREEZE ioctl.


171 * Sometimes SysRq seems to get 'stuck' after using it, what can I do?

172 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

173 That happens to me, also. I've found that tapping shift, alt, and control

174 on both sides of the keyboard, and hitting an invalid sysrq sequence again

175 will fix the problem. (i.e., something like alt-sysrq-z). Switching to another

176 virtual console (ALT+Fn) and then back again should also help.


178 * I hit SysRq, but nothing seems to happen, what's wrong?

179 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

180 There are some keyboards that produce a different keycode for SysRq than the

181 pre-defined value of 99 (see KEY_SYSRQ in include/linux/input.h), or which

182 don't have a SysRq key at all. In these cases, run 'showkey -s' to find an

183 appropriate scancode sequence, and use 'setkeycodes <sequence> 99' to map

184 this sequence to the usual SysRq code (e.g., 'setkeycodes e05b 99'). It's

185 probably best to put this command in a boot script. Oh, and by the way, you

186 exit 'showkey' by not typing anything for ten seconds.


188 * I want to add SysRQ key events to a module, how does it work?

189 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

190 In order to register a basic function with the table, you must first include

191 the header 'include/linux/sysrq.h', this will define everything else you need.

192 Next, you must create a sysrq_key_op struct, and populate it with A) the key

193 handler function you will use, B) a help_msg string, that will print when SysRQ

194 prints help, and C) an action_msg string, that will print right before your

195 handler is called. Your handler must conform to the prototype in 'sysrq.h'.


197 After the sysrq_key_op is created, you can call the kernel function

198 register_sysrq_key(int key, struct sysrq_key_op *op_p); this will

199 register the operation pointed to by 'op_p' at table key 'key',

200 if that slot in the table is blank. At module unload time, you must call

201 the function unregister_sysrq_key(int key, struct sysrq_key_op *op_p), which

202 will remove the key op pointed to by 'op_p' from the key 'key', if and only if

203 it is currently registered in that slot. This is in case the slot has been

204 overwritten since you registered it.


206 The Magic SysRQ system works by registering key operations against a key op

207 lookup table, which is defined in 'drivers/char/sysrq.c'. This key table has

208 a number of operations registered into it at compile time, but is mutable,

209 and 2 functions are exported for interface to it:

210 register_sysrq_key and unregister_sysrq_key.

211 Of course, never ever leave an invalid pointer in the table. I.e., when

212 your module that called register_sysrq_key() exits, it must call

213 unregister_sysrq_key() to clean up the sysrq key table entry that it used.

214 Null pointers in the table are always safe. :)


216 If for some reason you feel the need to call the handle_sysrq function from

217 within a function called by handle_sysrq, you must be aware that you are in

218 a lock (you are also in an interrupt handler, which means don't sleep!), so

219 you must call __handle_sysrq_nolock instead.


221 * When I hit a SysRq key combination only the header appears on the console?

222 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

223 Sysrq output is subject to the same console loglevel control as all

224 other console output. This means that if the kernel was booted 'quiet'

225 as is common on distro kernels the output may not appear on the actual

226 console, even though it will appear in the dmesg buffer, and be accessible

227 via the dmesg command and to the consumers of /proc/kmsg. As a specific

228 exception the header line from the sysrq command is passed to all console

229 consumers as if the current loglevel was maximum. If only the header

230 is emitted it is almost certain that the kernel loglevel is too low.

231 Should you require the output on the console channel then you will need

232 to temporarily up the console loglevel using alt-sysrq-8 or:


234 echo 8 > /proc/sysrq-trigger


236 Remember to return the loglevel to normal after triggering the sysrq

237 command you are interested in.


239 * I have more questions, who can I ask?

240 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

241 And I'll answer any questions about the registration system you got, also

242 responding as soon as possible.

243 -Crutcher


245 * Credits

246 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

247 Written by Mydraal <>

248 Updated by Adam Sulmicki <>

249 Updated by Jeremy M. Dolan <> 2001/01/28 10:15:59

250 Added to by Crutcher Dunnavant <>