path: root/Documentation/sysrq.txt
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authorLinus Torvalds <torvalds@ppc970.osdl.org>2005-04-16 15:20:36 -0700
committerLinus Torvalds <torvalds@ppc970.osdl.org>2005-04-16 15:20:36 -0700
commit1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 (patch)
tree0bba044c4ce775e45a88a51686b5d9f90697ea9d /Documentation/sysrq.txt
Initial git repository build. I'm not bothering with the full history, even though we have it. We can create a separate "historical" git archive of that later if we want to, and in the meantime it's about 3.2GB when imported into git - space that would just make the early git days unnecessarily complicated, when we don't have a lot of good infrastructure for it. Let it rip!
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+Linux Magic System Request Key Hacks
+Documentation for sysrq.c version 1.15
+Last update: $Date: 2001/01/28 10:15:59 $
+* What is the magic SysRq key?
+It is a 'magical' key combo you can hit which the kernel will respond to
+regardless of whatever else it is doing, unless it is completely locked up.
+* How do I enable the magic SysRq key?
+You need to say "yes" to 'Magic SysRq key (CONFIG_MAGIC_SYSRQ)' when
+configuring the kernel. When running a kernel with SysRq compiled in,
+/proc/sys/kernel/sysrq controls the functions allowed to be invoked via
+the SysRq key. By default the file contains 1 which means that every
+possible SysRq request is allowed (in older versions SysRq was disabled
+by default, and you were required to specifically enable it at run-time
+but this is not the case any more). Here is the list of possible values
+in /proc/sys/kernel/sysrq:
+ 0 - disable sysrq completely
+ 1 - enable all functions of sysrq
+ >1 - bitmask of allowed sysrq functions (see below for detailed function
+ description):
+ 2 - enable control of console logging level
+ 4 - enable control of keyboard (SAK, unraw)
+ 8 - enable debugging dumps of processes etc.
+ 16 - enable sync command
+ 32 - enable remount read-only
+ 64 - enable signalling of processes (term, kill, oom-kill)
+ 128 - allow reboot/poweroff
+ 256 - allow nicing of all RT tasks
+You can set the value in the file by the following command:
+ echo "number" >/proc/sys/kernel/sysrq
+Note that the value of /proc/sys/kernel/sysrq influences only the invocation
+via a keyboard. Invocation of any operation via /proc/sysrq-trigger is always
+* How do I use the magic SysRq key?
+On x86 - You press the key combo 'ALT-SysRq-<command key>'. Note - Some
+ keyboards may not have a key labeled 'SysRq'. The 'SysRq' key is
+ also known as the 'Print Screen' key. Also some keyboards cannot
+ handle so many keys being pressed at the same time, so you might
+ have better luck with "press Alt", "press SysRq", "release Alt",
+ "press <command key>", release everything.
+On SPARC - You press 'ALT-STOP-<command key>', I believe.
+On the serial console (PC style standard serial ports only) -
+ You send a BREAK, then within 5 seconds a command key. Sending
+ BREAK twice is interpreted as a normal BREAK.
+On PowerPC - Press 'ALT - Print Screen (or F13) - <command key>,
+ Print Screen (or F13) - <command key> may suffice.
+On other - If you know of the key combos for other architectures, please
+ let me know so I can add them to this section.
+On all - write a character to /proc/sysrq-trigger. eg:
+ echo t > /proc/sysrq-trigger
+* What are the 'command' keys?
+'r' - Turns off keyboard raw mode and sets it to XLATE.
+'k' - Secure Access Key (SAK) Kills all programs on the current virtual
+ console. NOTE: See important comments below in SAK section.
+'b' - Will immediately reboot the system without syncing or unmounting
+ your disks.
+'o' - Will shut your system off (if configured and supported).
+'s' - Will attempt to sync all mounted filesystems.
+'u' - Will attempt to remount all mounted filesystems read-only.
+'p' - Will dump the current registers and flags to your console.
+'t' - Will dump a list of current tasks and their information to your
+ console.
+'m' - Will dump current memory info to your console.
+'v' - Dumps Voyager SMP processor info to your console.
+'0'-'9' - Sets the console log level, controlling which kernel messages
+ will be printed to your console. ('0', for example would make
+ it so that only emergency messages like PANICs or OOPSes would
+ make it to your console.)
+'f' - Will call oom_kill to kill a memory hog process
+'e' - Send a SIGTERM to all processes, except for init.
+'i' - Send a SIGKILL to all processes, except for init.
+'l' - Send a SIGKILL to all processes, INCLUDING init. (Your system
+ will be non-functional after this.)
+'h' - Will display help ( actually any other key than those listed
+ above will display help. but 'h' is easy to remember :-)
+* Okay, so what can I use them for?
+Well, un'R'aw is very handy when your X server or a svgalib program crashes.
+sa'K' (Secure Access Key) is useful when you want to be sure there are no
+trojan program is running at console and which could grab your password
+when you would try to login. It will kill all programs on given console
+and thus letting you make sure that the login prompt you see is actually
+the one from init, not some trojan program.
+IMPORTANT:In its true form it is not a true SAK like the one in :IMPORTANT
+IMPORTANT:c2 compliant systems, and it should be mistook as such. :IMPORTANT
+ It seems other find it useful as (System Attention Key) which is
+useful when you want to exit a program that will not let you switch consoles.
+(For example, X or a svgalib program.)
+re'B'oot is good when you're unable to shut down. But you should also 'S'ync
+and 'U'mount first.
+'S'ync is great when your system is locked up, it allows you to sync your
+disks and will certainly lessen the chance of data loss and fscking. Note
+that the sync hasn't taken place until you see the "OK" and "Done" appear
+on the screen. (If the kernel is really in strife, you may not ever get the
+OK or Done message...)
+'U'mount is basically useful in the same ways as 'S'ync. I generally 'S'ync,
+'U'mount, then re'B'oot when my system locks. It's saved me many a fsck.
+Again, the unmount (remount read-only) hasn't taken place until you see the
+"OK" and "Done" message appear on the screen.
+The loglevel'0'-'9' is useful when your console is being flooded with
+kernel messages you do not want to see. Setting '0' will prevent all but
+the most urgent kernel messages from reaching your console. (They will
+still be logged if syslogd/klogd are alive, though.)
+t'E'rm and k'I'll are useful if you have some sort of runaway process you
+are unable to kill any other way, especially if it's spawning other
+* Sometimes SysRq seems to get 'stuck' after using it, what can I do?
+That happens to me, also. I've found that tapping shift, alt, and control
+on both sides of the keyboard, and hitting an invalid sysrq sequence again
+will fix the problem. (ie, something like alt-sysrq-z). Switching to another
+virtual console (ALT+Fn) and then back again should also help.
+* I hit SysRq, but nothing seems to happen, what's wrong?
+There are some keyboards that send different scancodes for SysRq than the
+pre-defined 0x54. So if SysRq doesn't work out of the box for a certain
+keyboard, run 'showkey -s' to find out the proper scancode sequence. Then
+use 'setkeycodes <sequence> 84' to define this sequence to the usual SysRq
+code (84 is decimal for 0x54). It's probably best to put this command in a
+boot script. Oh, and by the way, you exit 'showkey' by not typing anything
+for ten seconds.
+* I want to add SysRQ key events to a module, how does it work?
+In order to register a basic function with the table, you must first include
+the header 'include/linux/sysrq.h', this will define everything else you need.
+Next, you must create a sysrq_key_op struct, and populate it with A) the key
+handler function you will use, B) a help_msg string, that will print when SysRQ
+prints help, and C) an action_msg string, that will print right before your
+handler is called. Your handler must conform to the protoype in 'sysrq.h'.
+After the sysrq_key_op is created, you can call the macro
+register_sysrq_key(int key, struct sysrq_key_op *op_p) that is defined in
+sysrq.h, this will register the operation pointed to by 'op_p' at table
+key 'key', if that slot in the table is blank. At module unload time, you must
+call the macro unregister_sysrq_key(int key, struct sysrq_key_op *op_p), which
+will remove the key op pointed to by 'op_p' from the key 'key', if and only if
+it is currently registered in that slot. This is in case the slot has been
+overwritten since you registered it.
+The Magic SysRQ system works by registering key operations against a key op
+lookup table, which is defined in 'drivers/char/sysrq.c'. This key table has
+a number of operations registered into it at compile time, but is mutable,
+and 4 functions are exported for interface to it: __sysrq_lock_table,
+__sysrq_unlock_table, __sysrq_get_key_op, and __sysrq_put_key_op. The
+functions __sysrq_swap_key_ops and __sysrq_swap_key_ops_nolock are defined
+in the header itself, and the REGISTER and UNREGISTER macros are built from
+these. More complex (and dangerous!) manipulations of the table are possible
+using these functions, but you must be careful to always lock the table before
+you read or write from it, and to unlock it again when you are done. (And of
+course, to never ever leave an invalid pointer in the table). Null pointers in
+the table are always safe :)
+If for some reason you feel the need to call the handle_sysrq function from
+within a function called by handle_sysrq, you must be aware that you are in
+a lock (you are also in an interrupt handler, which means don't sleep!), so
+you must call __handle_sysrq_nolock instead.
+* I have more questions, who can I ask?
+You may feel free to send email to myrdraal@deathsdoor.com, and I will
+respond as soon as possible.
+ -Myrdraal
+And I'll answer any questions about the registration system you got, also
+responding as soon as possible.
+ -Crutcher
+* Credits
+Written by Mydraal <myrdraal@deathsdoor.com>
+Updated by Adam Sulmicki <adam@cfar.umd.edu>
+Updated by Jeremy M. Dolan <jmd@turbogeek.org> 2001/01/28 10:15:59
+Added to by Crutcher Dunnavant <crutcher+kernel@datastacks.com>