linux/Documentation/sysrq.txt
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   1Linux Magic System Request Key Hacks
   2Documentation for sysrq.c
   3
   4*  What is the magic SysRq key?
   5~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   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.
   8
   9*  How do I enable the magic SysRq key?
  10~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  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
  31
  32You can set the value in the file by the following command:
  33    echo "number" >/proc/sys/kernel/sysrq
  34
  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).
  38
  39*  How do I use the magic SysRq key?
  40~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  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.
  47
  48On SPARC - You press 'ALT-STOP-<command key>', I believe.
  49
  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.
  53
  54On PowerPC - Press 'ALT - Print Screen (or F13) - <command key>,  
  55             Print Screen (or F13) - <command key> may suffice.
  56
  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.
  59
  60On all -  write a character to /proc/sysrq-trigger.  e.g.:
  61
  62                echo t > /proc/sysrq-trigger
  63
  64*  What are the 'command' keys?
  65~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  66'b'     - Will immediately reboot the system without syncing or unmounting
  67          your disks.
  68
  69'c'     - Will perform a kexec reboot in order to take a crashdump.
  70
  71'd'     - Shows all locks that are held.
  72
  73'e'     - Send a SIGTERM to all processes, except for init.
  74
  75'f'     - Will call oom_kill to kill a memory hog process.
  76
  77'g'     - Used by kgdb on ppc and sh platforms.
  78
  79'h'     - Will display help (actually any other key than those listed
  80          here will display help. but 'h' is easy to remember :-)
  81
  82'i'     - Send a SIGKILL to all processes, except for init.
  83
  84'k'     - Secure Access Key (SAK) Kills all programs on the current virtual
  85          console. NOTE: See important comments below in SAK section.
  86
  87'l'     - Shows a stack backtrace for all active CPUs.
  88
  89'm'     - Will dump current memory info to your console.
  90
  91'n'     - Used to make RT tasks nice-able
  92
  93'o'     - Will shut your system off (if configured and supported).
  94
  95'p'     - Will dump the current registers and flags to your console.
  96
  97'q'     - Will dump per CPU lists of all armed hrtimers (but NOT regular
  98          timer_list timers) and detailed information about all
  99          clockevent devices.
 100
 101'r'     - Turns off keyboard raw mode and sets it to XLATE.
 102
 103's'     - Will attempt to sync all mounted filesystems.
 104
 105't'     - Will dump a list of current tasks and their information to your
 106          console.
 107
 108'u'     - Will attempt to remount all mounted filesystems read-only.
 109
 110'v'     - Dumps Voyager SMP processor info to your console.
 111
 112'w'     - Dumps tasks that are in uninterruptable (blocked) state.
 113
 114'x'     - Used by xmon interface on ppc/powerpc platforms.
 115
 116'0'-'9' - Sets the console log level, controlling which kernel messages
 117          will be printed to your console. ('0', for example would make
 118          it so that only emergency messages like PANICs or OOPSes would
 119          make it to your console.)
 120
 121*  Okay, so what can I use them for?
 122~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 123Well, un'R'aw is very handy when your X server or a svgalib program crashes.
 124
 125sa'K' (Secure Access Key) is useful when you want to be sure there is no
 126trojan program running at console which could grab your password
 127when you would try to login. It will kill all programs on given console,
 128thus letting you make sure that the login prompt you see is actually
 129the one from init, not some trojan program.
 130IMPORTANT: In its true form it is not a true SAK like the one in a :IMPORTANT
 131IMPORTANT: c2 compliant system, and it should not be mistaken as   :IMPORTANT
 132IMPORTANT: such.                                                   :IMPORTANT
 133       It seems others find it useful as (System Attention Key) which is
 134useful when you want to exit a program that will not let you switch consoles.
 135(For example, X or a svgalib program.)
 136
 137re'B'oot is good when you're unable to shut down. But you should also 'S'ync
 138and 'U'mount first.
 139
 140'C'rashdump can be used to manually trigger a crashdump when the system is hung.
 141The kernel needs to have been built with CONFIG_KEXEC enabled.
 142
 143'S'ync is great when your system is locked up, it allows you to sync your
 144disks and will certainly lessen the chance of data loss and fscking. Note
 145that the sync hasn't taken place until you see the "OK" and "Done" appear
 146on the screen. (If the kernel is really in strife, you may not ever get the
 147OK or Done message...)
 148
 149'U'mount is basically useful in the same ways as 'S'ync. I generally 'S'ync,
 150'U'mount, then re'B'oot when my system locks. It's saved me many a fsck.
 151Again, the unmount (remount read-only) hasn't taken place until you see the
 152"OK" and "Done" message appear on the screen.
 153
 154The loglevels '0'-'9' are useful when your console is being flooded with
 155kernel messages you do not want to see. Selecting '0' will prevent all but
 156the most urgent kernel messages from reaching your console. (They will
 157still be logged if syslogd/klogd are alive, though.)
 158
 159t'E'rm and k'I'll are useful if you have some sort of runaway process you
 160are unable to kill any other way, especially if it's spawning other
 161processes.
 162
 163*  Sometimes SysRq seems to get 'stuck' after using it, what can I do?
 164~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 165That happens to me, also. I've found that tapping shift, alt, and control
 166on both sides of the keyboard, and hitting an invalid sysrq sequence again
 167will fix the problem. (i.e., something like alt-sysrq-z). Switching to another
 168virtual console (ALT+Fn) and then back again should also help.
 169
 170*  I hit SysRq, but nothing seems to happen, what's wrong?
 171~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 172There are some keyboards that send different scancodes for SysRq than the
 173pre-defined 0x54. So if SysRq doesn't work out of the box for a certain
 174keyboard, run 'showkey -s' to find out the proper scancode sequence. Then
 175use 'setkeycodes <sequence> 84' to define this sequence to the usual SysRq
 176code (84 is decimal for 0x54). It's probably best to put this command in a
 177boot script. Oh, and by the way, you exit 'showkey' by not typing anything
 178for ten seconds.
 179
 180*  I want to add SysRQ key events to a module, how does it work?
 181~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 182In order to register a basic function with the table, you must first include
 183the header 'include/linux/sysrq.h', this will define everything else you need.
 184Next, you must create a sysrq_key_op struct, and populate it with A) the key
 185handler function you will use, B) a help_msg string, that will print when SysRQ
 186prints help, and C) an action_msg string, that will print right before your
 187handler is called. Your handler must conform to the prototype in 'sysrq.h'.
 188
 189After the sysrq_key_op is created, you can call the kernel function
 190register_sysrq_key(int key, struct sysrq_key_op *op_p); this will
 191register the operation pointed to by 'op_p' at table key 'key',
 192if that slot in the table is blank. At module unload time, you must call
 193the function unregister_sysrq_key(int key, struct sysrq_key_op *op_p), which
 194will remove the key op pointed to by 'op_p' from the key 'key', if and only if
 195it is currently registered in that slot. This is in case the slot has been
 196overwritten since you registered it.
 197
 198The Magic SysRQ system works by registering key operations against a key op
 199lookup table, which is defined in 'drivers/char/sysrq.c'. This key table has
 200a number of operations registered into it at compile time, but is mutable,
 201and 2 functions are exported for interface to it:
 202        register_sysrq_key and unregister_sysrq_key.
 203Of course, never ever leave an invalid pointer in the table. I.e., when
 204your module that called register_sysrq_key() exits, it must call
 205unregister_sysrq_key() to clean up the sysrq key table entry that it used.
 206Null pointers in the table are always safe. :)
 207
 208If for some reason you feel the need to call the handle_sysrq function from
 209within a function called by handle_sysrq, you must be aware that you are in
 210a lock (you are also in an interrupt handler, which means don't sleep!), so
 211you must call __handle_sysrq_nolock instead.
 212
 213*  When I hit a SysRq key combination only the header appears on the console?
 214~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 215Sysrq output is subject to the same console loglevel control as all
 216other console output.  This means that if the kernel was booted 'quiet'
 217as is common on distro kernels the output may not appear on the actual
 218console, even though it will appear in the dmesg buffer, and be accessible
 219via the dmesg command and to the consumers of /proc/kmsg.  As a specific
 220exception the header line from the sysrq command is passed to all console
 221consumers as if the current loglevel was maximum.  If only the header
 222is emitted it is almost certain that the kernel loglevel is too low.
 223Should you require the output on the console channel then you will need
 224to temporarily up the console loglevel using alt-sysrq-8 or:
 225
 226    echo 8 > /proc/sysrq-trigger
 227
 228Remember to return the loglevel to normal after triggering the sysrq
 229command you are interested in.
 230
 231*  I have more questions, who can I ask?
 232~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 233And I'll answer any questions about the registration system you got, also
 234responding as soon as possible.
 235 -Crutcher
 236
 237*  Credits
 238~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 239Written by Mydraal <vulpyne@vulpyne.net>
 240Updated by Adam Sulmicki <adam@cfar.umd.edu>
 241Updated by Jeremy M. Dolan <jmd@turbogeek.org> 2001/01/28 10:15:59
 242Added to by Crutcher Dunnavant <crutcher+kernel@datastacks.com>
 243