Documentation/sysrq.txt

   1 Linux Magic System Request Key Hacks
   2 Documentation for sysrq.c
   3
   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.
   8
   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. The default value in this file is set by the
  15 CONFIG_MAGIC_SYSRQ_DEFAULT_ENABLE config symbol, which itself defaults
  16 to 1. Here is the list of possible values in /proc/sys/kernel/sysrq:
  17    0 - disable sysrq completely
  18    1 - enable all functions of sysrq
  19   >1 - bitmask of allowed sysrq functions (see below for detailed function
  20        description):
  21           2 =   0x2 - enable control of console logging level
  22           4 =   0x4 - enable control of keyboard (SAK, unraw)
  23           8 =   0x8 - enable debugging dumps of processes etc.
  24          16 =  0x10 - enable sync command
  25          32 =  0x20 - enable remount read-only
  26          64 =  0x40 - enable signalling of processes (term, kill, oom-kill)
  27         128 =  0x80 - allow reboot/poweroff
  28         256 = 0x100 - allow nicing of all RT tasks
  29
  30 You can set the value in the file by the following command:
  31     echo "number" >/proc/sys/kernel/sysrq
  32
  33 The number may be written here either as decimal or as hexadecimal
  34 with the 0x prefix. CONFIG_MAGIC_SYSRQ_DEFAULT_ENABLE must always be
  35 written in hexadecimal.
  36
  37 Note that the value of /proc/sys/kernel/sysrq influences only the invocation
  38 via a keyboard. Invocation of any operation via /proc/sysrq-trigger is always
  39 allowed (by a user with admin privileges).
  40
  41 *  How do I use the magic SysRq key?
  42 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  43 On x86   - You press the key combo 'ALT-SysRq-<command key>'. Note - Some
  44            keyboards may not have a key labeled 'SysRq'. The 'SysRq' key is
  45            also known as the 'Print Screen' key. Also some keyboards cannot
  46            handle so many keys being pressed at the same time, so you might
  47            have better luck with "press Alt", "press SysRq", "release SysRq",
  48            "press <command key>", release everything.
  49
  50 On SPARC - You press 'ALT-STOP-<command key>', I believe.
  51
  52 On the serial console (PC style standard serial ports only) -
  53            You send a BREAK, then within 5 seconds a command key. Sending
  54            BREAK twice is interpreted as a normal BREAK.
  55
  56 On PowerPC - Press 'ALT - Print Screen (or F13) - <command key>,
  57              Print Screen (or F13) - <command key> may suffice.
  58
  59 On other - If you know of the key combos for other architectures, please
  60            let me know so I can add them to this section.
  61
  62 On all -  write a character to /proc/sysrq-trigger.  e.g.:
  63
  64                 echo t > /proc/sysrq-trigger
  65
  66 *  What are the 'command' keys?
  67 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  68 'b'     - Will immediately reboot the system without syncing or unmounting
  69           your disks.
  70
  71 'c'     - Will perform a system crash by a NULL pointer dereference.
  72           A crashdump will be taken if configured.
  73
  74 'd'     - Shows all locks that are held.
  75
  76 'e'     - Send a SIGTERM to all processes, except for init.
  77
  78 'f'     - Will call the oom killer to kill a memory hog process, but do not
  79           panic if nothing can be killed.
  80
  81 'g'     - Used by kgdb (kernel debugger)
  82
  83 'h'     - Will display help (actually any other key than those listed
  84           here will display help. but 'h' is easy to remember :-)
  85
  86 'i'     - Send a SIGKILL to all processes, except for init.
  87
  88 'j'     - Forcibly "Just thaw it" - filesystems frozen by the FIFREEZE ioctl.
  89
  90 'k'     - Secure Access Key (SAK) Kills all programs on the current virtual
  91           console. NOTE: See important comments below in SAK section.
  92
  93 'l'     - Shows a stack backtrace for all active CPUs.
  94
  95 'm'     - Will dump current memory info to your console.
  96
  97 'n'     - Used to make RT tasks nice-able
  98
  99 'o'     - Will shut your system off (if configured and supported).
 100
 101 'p'     - Will dump the current registers and flags to your console.
 102
 103 'q'     - Will dump per CPU lists of all armed hrtimers (but NOT regular
 104           timer_list timers) and detailed information about all
 105           clockevent devices.
 106
 107 'r'     - Turns off keyboard raw mode and sets it to XLATE.
 108
 109 's'     - Will attempt to sync all mounted filesystems.
 110
 111 't'     - Will dump a list of current tasks and their information to your
 112           console.
 113
 114 'u'     - Will attempt to remount all mounted filesystems read-only.
 115
 116 'v'     - Forcefully restores framebuffer console
 117 'v'     - Causes ETM buffer dump [ARM-specific]
 118
 119 'w'     - Dumps tasks that are in uninterruptable (blocked) state.
 120
 121 'x'     - Used by xmon interface on ppc/powerpc platforms.
 122           Show global PMU Registers on sparc64.
 123           Dump all TLB entries on MIPS.
 124
 125 'y'     - Show global CPU Registers [SPARC-64 specific]
 126
 127 'z'     - Dump the ftrace buffer
 128
 129 '0'-'9' - Sets the console log level, controlling which kernel messages
 130           will be printed to your console. ('0', for example would make
 131           it so that only emergency messages like PANICs or OOPSes would
 132           make it to your console.)
 133
 134 *  Okay, so what can I use them for?
 135 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 136 Well, unraw(r) is very handy when your X server or a svgalib program crashes.
 137
 138 sak(k) (Secure Access Key) is useful when you want to be sure there is no
 139 trojan program running at console which could grab your password
 140 when you would try to login. It will kill all programs on given console,
 141 thus letting you make sure that the login prompt you see is actually
 142 the one from init, not some trojan program.
 143 IMPORTANT: In its true form it is not a true SAK like the one in a :IMPORTANT
 144 IMPORTANT: c2 compliant system, and it should not be mistaken as   :IMPORTANT
 145 IMPORTANT: such.                                                   :IMPORTANT
 146        It seems others find it useful as (System Attention Key) which is
 147 useful when you want to exit a program that will not let you switch consoles.
 148 (For example, X or a svgalib program.)
 149
 150 reboot(b) is good when you're unable to shut down. But you should also
 151 sync(s) and umount(u) first.
 152
 153 crash(c) can be used to manually trigger a crashdump when the system is hung.
 154 Note that this just triggers a crash if there is no dump mechanism available.
 155
 156 sync(s) is great when your system is locked up, it allows you to sync your
 157 disks and will certainly lessen the chance of data loss and fscking. Note
 158 that the sync hasn't taken place until you see the "OK" and "Done" appear
 159 on the screen. (If the kernel is really in strife, you may not ever get the
 160 OK or Done message...)
 161
 162 umount(u) is basically useful in the same ways as sync(s). I generally sync(s),
 163 umount(u), then reboot(b) when my system locks. It's saved me many a fsck.
 164 Again, the unmount (remount read-only) hasn't taken place until you see the
 165 "OK" and "Done" message appear on the screen.
 166
 167 The loglevels '0'-'9' are useful when your console is being flooded with
 168 kernel messages you do not want to see. Selecting '0' will prevent all but
 169 the most urgent kernel messages from reaching your console. (They will
 170 still be logged if syslogd/klogd are alive, though.)
 171
 172 term(e) and kill(i) are useful if you have some sort of runaway process you
 173 are unable to kill any other way, especially if it's spawning other
 174 processes.
 175
 176 "just thaw it(j)" is useful if your system becomes unresponsive due to a frozen
 177 (probably root) filesystem via the FIFREEZE ioctl.
 178
 179 *  Sometimes SysRq seems to get 'stuck' after using it, what can I do?
 180 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 181 That happens to me, also. I've found that tapping shift, alt, and control
 182 on both sides of the keyboard, and hitting an invalid sysrq sequence again
 183 will fix the problem. (i.e., something like alt-sysrq-z). Switching to another
 184 virtual console (ALT+Fn) and then back again should also help.
 185
 186 *  I hit SysRq, but nothing seems to happen, what's wrong?
 187 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 188 There are some keyboards that produce a different keycode for SysRq than the
 189 pre-defined value of 99 (see KEY_SYSRQ in include/linux/input.h), or which
 190 don't have a SysRq key at all. In these cases, run 'showkey -s' to find an
 191 appropriate scancode sequence, and use 'setkeycodes <sequence> 99' to map
 192 this sequence to the usual SysRq code (e.g., 'setkeycodes e05b 99'). It's
 193 probably best to put this command in a boot script. Oh, and by the way, you
 194 exit 'showkey' by not typing anything for ten seconds.
 195
 196 *  I want to add SysRQ key events to a module, how does it work?
 197 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 198 In order to register a basic function with the table, you must first include
 199 the header 'include/linux/sysrq.h', this will define everything else you need.
 200 Next, you must create a sysrq_key_op struct, and populate it with A) the key
 201 handler function you will use, B) a help_msg string, that will print when SysRQ
 202 prints help, and C) an action_msg string, that will print right before your
 203 handler is called. Your handler must conform to the prototype in 'sysrq.h'.
 204
 205 After the sysrq_key_op is created, you can call the kernel function
 206 register_sysrq_key(int key, struct sysrq_key_op *op_p); this will
 207 register the operation pointed to by 'op_p' at table key 'key',
 208 if that slot in the table is blank. At module unload time, you must call
 209 the function unregister_sysrq_key(int key, struct sysrq_key_op *op_p), which
 210 will remove the key op pointed to by 'op_p' from the key 'key', if and only if
 211 it is currently registered in that slot. This is in case the slot has been
 212 overwritten since you registered it.
 213
 214 The Magic SysRQ system works by registering key operations against a key op
 215 lookup table, which is defined in 'drivers/char/sysrq.c'. This key table has
 216 a number of operations registered into it at compile time, but is mutable,
 217 and 2 functions are exported for interface to it:
 218         register_sysrq_key and unregister_sysrq_key.
 219 Of course, never ever leave an invalid pointer in the table. I.e., when
 220 your module that called register_sysrq_key() exits, it must call
 221 unregister_sysrq_key() to clean up the sysrq key table entry that it used.
 222 Null pointers in the table are always safe. :)
 223
 224 If for some reason you feel the need to call the handle_sysrq function from
 225 within a function called by handle_sysrq, you must be aware that you are in
 226 a lock (you are also in an interrupt handler, which means don't sleep!), so
 227 you must call __handle_sysrq_nolock instead.
 228
 229 *  When I hit a SysRq key combination only the header appears on the console?
 230 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 231 Sysrq output is subject to the same console loglevel control as all
 232 other console output.  This means that if the kernel was booted 'quiet'
 233 as is common on distro kernels the output may not appear on the actual
 234 console, even though it will appear in the dmesg buffer, and be accessible
 235 via the dmesg command and to the consumers of /proc/kmsg.  As a specific
 236 exception the header line from the sysrq command is passed to all console
 237 consumers as if the current loglevel was maximum.  If only the header
 238 is emitted it is almost certain that the kernel loglevel is too low.
 239 Should you require the output on the console channel then you will need
 240 to temporarily up the console loglevel using alt-sysrq-8 or:
 241
 242     echo 8 > /proc/sysrq-trigger
 243
 244 Remember to return the loglevel to normal after triggering the sysrq
 245 command you are interested in.
 246
 247 *  I have more questions, who can I ask?
 248 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 249 Just ask them on the linux-kernel mailing list:
 250         linux-kernel@vger.kernel.org
 251
 252 *  Credits
 253 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 254 Written by Mydraal <vulpyne@vulpyne.net>
 255 Updated by Adam Sulmicki <adam@cfar.umd.edu>
 256 Updated by Jeremy M. Dolan <jmd@turbogeek.org> 2001/01/28 10:15:59
 257 Added to by Crutcher Dunnavant <crutcher+kernel@datastacks.com>