1Linux Magic System Request Key Hacks
   2Documentation for sysrq.c
   4*  What is the magic SysRq key?
   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.
   9*  How do I enable the magic SysRq key?
  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
  32You can set the value in the file by the following command:
  33    echo "number" >/proc/sys/kernel/sysrq
  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
  37allowed (by a user with admin privileges).
  39*  How do I use the magic SysRq key?
  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
  45           have better luck with "press Alt", "press SysRq", "release SysRq",
  46           "press <command key>", release everything.
  48On SPARC - You press 'ALT-STOP-<command key>', I believe.
  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.
  54On PowerPC - Press 'ALT - Print Screen (or F13) - <command key>,  
  55             Print Screen (or F13) - <command key> may suffice.
  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.
  60On all -  write a character to /proc/sysrq-trigger.  e.g.:
  62                echo t > /proc/sysrq-trigger
  64*  What are the 'command' keys?
  66'b'     - Will immediately reboot the system without syncing or unmounting
  67          your disks.
  69'c'     - Will perform a kexec reboot in order to take a crashdump.
  71'd'     - Shows all locks that are held.
  73'e'     - Send a SIGTERM to all processes, except for init.
  75'f'     - Will call oom_kill to kill a memory hog process.
  77'g'     - Used by kgdb on ppc and sh platforms.
  79'h'     - Will display help (actually any other key than those listed
  80          here will display help. but 'h' is easy to remember :-)
  82'i'     - Send a SIGKILL to all processes, except for init.
  84'j'     - Forcibly "Just thaw it" - filesystems frozen by the FIFREEZE ioctl.
  86'k'     - Secure Access Key (SAK) Kills all programs on the current virtual
  87          console. NOTE: See important comments below in SAK section.
  89'l'     - Shows a stack backtrace for all active CPUs.
  91'm'     - Will dump current memory info to your console.
  93'n'     - Used to make RT tasks nice-able
  95'o'     - Will shut your system off (if configured and supported).
  97'p'     - Will dump the current registers and flags to your console.
  99'q'     - Will dump per CPU lists of all armed hrtimers (but NOT regular
 100          timer_list timers) and detailed information about all
 101          clockevent devices.
 103'r'     - Turns off keyboard raw mode and sets it to XLATE.
 105's'     - Will attempt to sync all mounted filesystems.
 107't'     - Will dump a list of current tasks and their information to your
 108          console.
 110'u'     - Will attempt to remount all mounted filesystems read-only.
 112'v'     - Dumps Voyager SMP processor info to your console.
 114'w'     - Dumps tasks that are in uninterruptable (blocked) state.
 116'x'     - Used by xmon interface on ppc/powerpc platforms.
 118'z'     - Dump the ftrace buffer
 120'0'-'9' - Sets the console log level, controlling which kernel messages
 121          will be printed to your console. ('0', for example would make
 122          it so that only emergency messages like PANICs or OOPSes would
 123          make it to your console.)
 125*  Okay, so what can I use them for?
 127Well, un'R'aw is very handy when your X server or a svgalib program crashes.
 129sa'K' (Secure Access Key) is useful when you want to be sure there is no
 130trojan program running at console which could grab your password
 131when you would try to login. It will kill all programs on given console,
 132thus letting you make sure that the login prompt you see is actually
 133the one from init, not some trojan program.
 134IMPORTANT: In its true form it is not a true SAK like the one in a :IMPORTANT
 135IMPORTANT: c2 compliant system, and it should not be mistaken as   :IMPORTANT
 136IMPORTANT: such.                                                   :IMPORTANT
 137       It seems others find it useful as (System Attention Key) which is
 138useful when you want to exit a program that will not let you switch consoles.
 139(For example, X or a svgalib program.)
 141re'B'oot is good when you're unable to shut down. But you should also 'S'ync
 142and 'U'mount first.
 144'C'rashdump can be used to manually trigger a crashdump when the system is hung.
 145The kernel needs to have been built with CONFIG_KEXEC enabled.
 147'S'ync is great when your system is locked up, it allows you to sync your
 148disks and will certainly lessen the chance of data loss and fscking. Note
 149that the sync hasn't taken place until you see the "OK" and "Done" appear
 150on the screen. (If the kernel is really in strife, you may not ever get the
 151OK or Done message...)
 153'U'mount is basically useful in the same ways as 'S'ync. I generally 'S'ync,
 154'U'mount, then re'B'oot when my system locks. It's saved me many a fsck.
 155Again, the unmount (remount read-only) hasn't taken place until you see the
 156"OK" and "Done" message appear on the screen.
 158The loglevels '0'-'9' are useful when your console is being flooded with
 159kernel messages you do not want to see. Selecting '0' will prevent all but
 160the most urgent kernel messages from reaching your console. (They will
 161still be logged if syslogd/klogd are alive, though.)
 163t'E'rm and k'I'll are useful if you have some sort of runaway process you
 164are unable to kill any other way, especially if it's spawning other
 167"'J'ust thaw it" is useful if your system becomes unresponsive due to a frozen
 168(probably root) filesystem via the FIFREEZE ioctl.
 170*  Sometimes SysRq seems to get 'stuck' after using it, what can I do?
 172That happens to me, also. I've found that tapping shift, alt, and control
 173on both sides of the keyboard, and hitting an invalid sysrq sequence again
 174will fix the problem. (i.e., something like alt-sysrq-z). Switching to another
 175virtual console (ALT+Fn) and then back again should also help.
 177*  I hit SysRq, but nothing seems to happen, what's wrong?
 179There are some keyboards that send different scancodes for SysRq than the
 180pre-defined 0x54. So if SysRq doesn't work out of the box for a certain
 181keyboard, run 'showkey -s' to find out the proper scancode sequence. Then
 182use 'setkeycodes <sequence> 84' to define this sequence to the usual SysRq
 183code (84 is decimal for 0x54). It's probably best to put this command in a
 184boot script. Oh, and by the way, you exit 'showkey' by not typing anything
 185for ten seconds.
 187*  I want to add SysRQ key events to a module, how does it work?
 189In order to register a basic function with the table, you must first include
 190the header 'include/linux/sysrq.h', this will define everything else you need.
 191Next, you must create a sysrq_key_op struct, and populate it with A) the key
 192handler function you will use, B) a help_msg string, that will print when SysRQ
 193prints help, and C) an action_msg string, that will print right before your
 194handler is called. Your handler must conform to the prototype in 'sysrq.h'.
 196After the sysrq_key_op is created, you can call the kernel function
 197register_sysrq_key(int key, struct sysrq_key_op *op_p); this will
 198register the operation pointed to by 'op_p' at table key 'key',
 199if that slot in the table is blank. At module unload time, you must call
 200the function unregister_sysrq_key(int key, struct sysrq_key_op *op_p), which
 201will remove the key op pointed to by 'op_p' from the key 'key', if and only if
 202it is currently registered in that slot. This is in case the slot has been
 203overwritten since you registered it.
 205The Magic SysRQ system works by registering key operations against a key op
 206lookup table, which is defined in 'drivers/char/sysrq.c'. This key table has
 207a number of operations registered into it at compile time, but is mutable,
 208and 2 functions are exported for interface to it:
 209        register_sysrq_key and unregister_sysrq_key.
 210Of course, never ever leave an invalid pointer in the table. I.e., when
 211your module that called register_sysrq_key() exits, it must call
 212unregister_sysrq_key() to clean up the sysrq key table entry that it used.
 213Null pointers in the table are always safe. :)
 215If for some reason you feel the need to call the handle_sysrq function from
 216within a function called by handle_sysrq, you must be aware that you are in
 217a lock (you are also in an interrupt handler, which means don't sleep!), so
 218you must call __handle_sysrq_nolock instead.
 220*  When I hit a SysRq key combination only the header appears on the console?
 222Sysrq output is subject to the same console loglevel control as all
 223other console output.  This means that if the kernel was booted 'quiet'
 224as is common on distro kernels the output may not appear on the actual
 225console, even though it will appear in the dmesg buffer, and be accessible
 226via the dmesg command and to the consumers of /proc/kmsg.  As a specific
 227exception the header line from the sysrq command is passed to all console
 228consumers as if the current loglevel was maximum.  If only the header
 229is emitted it is almost certain that the kernel loglevel is too low.
 230Should you require the output on the console channel then you will need
 231to temporarily up the console loglevel using alt-sysrq-8 or:
 233    echo 8 > /proc/sysrq-trigger
 235Remember to return the loglevel to normal after triggering the sysrq
 236command you are interested in.
 238*  I have more questions, who can I ask?
 240And I'll answer any questions about the registration system you got, also
 241responding as soon as possible.
 242 -Crutcher
 244*  Credits
 246Written by Mydraal <>
 247Updated by Adam Sulmicki <>
 248Updated by Jeremy M. Dolan <> 2001/01/28 10:15:59
 249Added to by Crutcher Dunnavant <>