1Linux Magic System Request Key Hacks
   2Documentation for sysrq.c
   3Last update: 2007-AUG-04
   5*  What is the magic SysRq key?
   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.
  10*  How do I enable the magic SysRq key?
  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
  33You can set the value in the file by the following command:
  34    echo "number" >/proc/sys/kernel/sysrq
  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
  38allowed (by a user with admin privileges).
  40*  How do I use the magic SysRq key?
  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 SysRq",
  47           "press <command key>", release everything.
  49On SPARC - You press 'ALT-STOP-<command key>', I believe.
  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.
  55On PowerPC - Press 'ALT - Print Screen (or F13) - <command key>,  
  56             Print Screen (or F13) - <command key> may suffice.
  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.
  61On all -  write a character to /proc/sysrq-trigger.  e.g.:
  63                echo t > /proc/sysrq-trigger
  65*  What are the 'command' keys?
  67'b'     - Will immediately reboot the system without syncing or unmounting
  68          your disks.
  70'c'     - Will perform a kexec reboot in order to take a crashdump.
  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'k'     - Secure Access Key (SAK) Kills all programs on the current virtual
  86          console. NOTE: See important comments below in SAK section.
  88'l'     - Shows a stack backtrace for all active CPUs.
  90'm'     - Will dump current memory info to your console.
  92'n'     - Used to make RT tasks nice-able
  94'o'     - Will shut your system off (if configured and supported).
  96'p'     - Will dump the current registers and flags to your console.
  98'q'     - Will dump per CPU lists of all armed hrtimers (but NOT regular
  99          timer_list timers) and detailed information about all
 100          clockevent devices.
 102'r'     - Turns off keyboard raw mode and sets it to XLATE.
 104's'     - Will attempt to sync all mounted filesystems.
 106't'     - Will dump a list of current tasks and their information to your
 107          console.
 109'u'     - Will attempt to remount all mounted filesystems read-only.
 111'v'     - Dumps Voyager SMP processor info to your console.
 113'w'     - Dumps tasks that are in uninterruptable (blocked) state.
 115'x'     - Used by xmon interface on ppc/powerpc platforms.
 117'0'-'9' - Sets the console log level, controlling which kernel messages
 118          will be printed to your console. ('0', for example would make
 119          it so that only emergency messages like PANICs or OOPSes would
 120          make it to your console.)
 122*  Okay, so what can I use them for?
 124Well, un'R'aw is very handy when your X server or a svgalib program crashes.
 126sa'K' (Secure Access Key) is useful when you want to be sure there is no
 127trojan program running at console which could grab your password
 128when you would try to login. It will kill all programs on given console,
 129thus letting you make sure that the login prompt you see is actually
 130the one from init, not some trojan program.
 131IMPORTANT: In its true form it is not a true SAK like the one in a :IMPORTANT
 132IMPORTANT: c2 compliant system, and it should not be mistaken as   :IMPORTANT
 133IMPORTANT: such.                                                   :IMPORTANT
 134       It seems others find it useful as (System Attention Key) which is
 135useful when you want to exit a program that will not let you switch consoles.
 136(For example, X or a svgalib program.)
 138re'B'oot is good when you're unable to shut down. But you should also 'S'ync
 139and 'U'mount first.
 141'C'rashdump can be used to manually trigger a crashdump when the system is hung.
 142The kernel needs to have been built with CONFIG_KEXEC enabled.
 144'S'ync is great when your system is locked up, it allows you to sync your
 145disks and will certainly lessen the chance of data loss and fscking. Note
 146that the sync hasn't taken place until you see the "OK" and "Done" appear
 147on the screen. (If the kernel is really in strife, you may not ever get the
 148OK or Done message...)
 150'U'mount is basically useful in the same ways as 'S'ync. I generally 'S'ync,
 151'U'mount, then re'B'oot when my system locks. It's saved me many a fsck.
 152Again, the unmount (remount read-only) hasn't taken place until you see the
 153"OK" and "Done" message appear on the screen.
 155The loglevels '0'-'9' are useful when your console is being flooded with
 156kernel messages you do not want to see. Selecting '0' will prevent all but
 157the most urgent kernel messages from reaching your console. (They will
 158still be logged if syslogd/klogd are alive, though.)
 160t'E'rm and k'I'll are useful if you have some sort of runaway process you
 161are unable to kill any other way, especially if it's spawning other
 164*  Sometimes SysRq seems to get 'stuck' after using it, what can I do?
 166That happens to me, also. I've found that tapping shift, alt, and control
 167on both sides of the keyboard, and hitting an invalid sysrq sequence again
 168will fix the problem. (i.e., something like alt-sysrq-z). Switching to another
 169virtual console (ALT+Fn) and then back again should also help.
 171*  I hit SysRq, but nothing seems to happen, what's wrong?
 173There are some keyboards that send different scancodes for SysRq than the
 174pre-defined 0x54. So if SysRq doesn't work out of the box for a certain
 175keyboard, run 'showkey -s' to find out the proper scancode sequence. Then
 176use 'setkeycodes <sequence> 84' to define this sequence to the usual SysRq
 177code (84 is decimal for 0x54). It's probably best to put this command in a
 178boot script. Oh, and by the way, you exit 'showkey' by not typing anything
 179for ten seconds.
 181*  I want to add SysRQ key events to a module, how does it work?
 183In order to register a basic function with the table, you must first include
 184the header 'include/linux/sysrq.h', this will define everything else you need.
 185Next, you must create a sysrq_key_op struct, and populate it with A) the key
 186handler function you will use, B) a help_msg string, that will print when SysRQ
 187prints help, and C) an action_msg string, that will print right before your
 188handler is called. Your handler must conform to the prototype in 'sysrq.h'.
 190After the sysrq_key_op is created, you can call the kernel function
 191register_sysrq_key(int key, struct sysrq_key_op *op_p); this will
 192register the operation pointed to by 'op_p' at table key 'key',
 193if that slot in the table is blank. At module unload time, you must call
 194the function unregister_sysrq_key(int key, struct sysrq_key_op *op_p), which
 195will remove the key op pointed to by 'op_p' from the key 'key', if and only if
 196it is currently registered in that slot. This is in case the slot has been
 197overwritten since you registered it.
 199The Magic SysRQ system works by registering key operations against a key op
 200lookup table, which is defined in 'drivers/char/sysrq.c'. This key table has
 201a number of operations registered into it at compile time, but is mutable,
 202and 2 functions are exported for interface to it:
 203        register_sysrq_key and unregister_sysrq_key.
 204Of course, never ever leave an invalid pointer in the table. I.e., when
 205your module that called register_sysrq_key() exits, it must call
 206unregister_sysrq_key() to clean up the sysrq key table entry that it used.
 207Null pointers in the table are always safe. :)
 209If for some reason you feel the need to call the handle_sysrq function from
 210within a function called by handle_sysrq, you must be aware that you are in
 211a lock (you are also in an interrupt handler, which means don't sleep!), so
 212you must call __handle_sysrq_nolock instead.
 214*  I have more questions, who can I ask?
 216And I'll answer any questions about the registration system you got, also
 217responding as soon as possible.
 218 -Crutcher
 220*  Credits
 222Written by Mydraal <>
 223Updated by Adam Sulmicki <>
 224Updated by Jeremy M. Dolan <> 2001/01/28 10:15:59
 225Added to by Crutcher Dunnavant <>
 226 kindly hosted by Redpill Linpro AS, provider of Linux consulting and operations services since 1995.