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 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 (kernel debugger)
  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'     - Forcefully restores framebuffer console
 114'v'     - Causes ETM buffer dump [ARM-specific]
 116'w'     - Dumps tasks that are in uninterruptable (blocked) state.
 118'x'     - Used by xmon interface on ppc/powerpc platforms.
 119          Show global PMU Registers on sparc64.
 121'y'     - Show global CPU Registers [SPARC-64 specific]
 123'z'     - Dump the ftrace buffer
 125'0'-'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.)
 130*  Okay, so what can I use them for?
 132Well, un'R'aw is very handy when your X server or a svgalib program crashes.
 134sa'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
 138the one from init, not some trojan program.
 139IMPORTANT: 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
 142       It seems others find it useful as (System Attention Key) which is
 143useful when you want to exit a program that will not let you switch consoles.
 144(For example, X or a svgalib program.)
 146re'B'oot is good when you're unable to shut down. But you should also 'S'ync
 147and 'U'mount first.
 149'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.
 152'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...)
 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.
 163The 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
 165the most urgent kernel messages from reaching your console. (They will
 166still be logged if syslogd/klogd are alive, though.)
 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
 172"'J'ust thaw it" is useful if your system becomes unresponsive due to a frozen
 173(probably root) filesystem via the FIFREEZE ioctl.
 175*  Sometimes SysRq seems to get 'stuck' after using it, what can I do?
 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
 179will fix the problem. (i.e., something like alt-sysrq-z). Switching to another
 180virtual console (ALT+Fn) and then back again should also help.
 182*  I hit SysRq, but nothing seems to happen, what's wrong?
 184There 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.
 192*  I want to add SysRQ key events to a module, how does it work?
 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
 199handler is called. Your handler must conform to the prototype in 'sysrq.h'.
 201After 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
 206will 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.
 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,
 213and 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. :)
 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.
 225*  When I hit a SysRq key combination only the header appears on the console?
 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:
 238    echo 8 > /proc/sysrq-trigger
 240Remember to return the loglevel to normal after triggering the sysrq
 241command you are interested in.
 243*  I have more questions, who can I ask?
 245Just ask them on the linux-kernel mailing list:
 248*  Credits
 250Written by Mydraal <>
 251Updated by Adam Sulmicki <>
 252Updated by Jeremy M. Dolan <> 2001/01/28 10:15:59
 253Added to by Crutcher Dunnavant <>