summaryrefslogtreecommitdiffstats
path: root/Documentation/sysrq.txt
diff options
context:
space:
mode:
Diffstat (limited to 'Documentation/sysrq.txt')
-rw-r--r--Documentation/sysrq.txt252
1 files changed, 0 insertions, 252 deletions
diff --git a/Documentation/sysrq.txt b/Documentation/sysrq.txt
deleted file mode 100644
index 642f844..0000000
--- a/Documentation/sysrq.txt
+++ /dev/null
@@ -1,252 +0,0 @@
-Linux Magic System Request Key Hacks
-Documentation for sysrq.c
-
-* 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
-allowed (by a user with admin privileges).
-
-* 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 SysRq",
- "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. e.g.:
-
- echo t > /proc/sysrq-trigger
-
-* What are the 'command' keys?
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-'b' - Will immediately reboot the system without syncing or unmounting
- your disks.
-
-'c' - Will perform a system crash by a NULL pointer dereference.
- A crashdump will be taken if configured.
-
-'d' - Shows all locks that are held.
-
-'e' - Send a SIGTERM to all processes, except for init.
-
-'f' - Will call oom_kill to kill a memory hog process.
-
-'g' - Used by kgdb (kernel debugger)
-
-'h' - Will display help (actually any other key than those listed
- here will display help. but 'h' is easy to remember :-)
-
-'i' - Send a SIGKILL to all processes, except for init.
-
-'j' - Forcibly "Just thaw it" - filesystems frozen by the FIFREEZE ioctl.
-
-'k' - Secure Access Key (SAK) Kills all programs on the current virtual
- console. NOTE: See important comments below in SAK section.
-
-'l' - Shows a stack backtrace for all active CPUs.
-
-'m' - Will dump current memory info to your console.
-
-'n' - Used to make RT tasks nice-able
-
-'o' - Will shut your system off (if configured and supported).
-
-'p' - Will dump the current registers and flags to your console.
-
-'q' - Will dump per CPU lists of all armed hrtimers (but NOT regular
- timer_list timers) and detailed information about all
- clockevent devices.
-
-'r' - Turns off keyboard raw mode and sets it to XLATE.
-
-'s' - Will attempt to sync all mounted filesystems.
-
-'t' - Will dump a list of current tasks and their information to your
- console.
-
-'u' - Will attempt to remount all mounted filesystems read-only.
-
-'v' - Forcefully restores framebuffer console
-'v' - Causes ETM buffer dump [ARM-specific]
-
-'w' - Dumps tasks that are in uninterruptable (blocked) state.
-
-'x' - Used by xmon interface on ppc/powerpc platforms.
-
-'y' - Show global CPU Registers [SPARC-64 specific]
-
-'z' - Dump the ftrace buffer
-
-'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.)
-
-* 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 is no
-trojan program running at console which could grab your password
-when you would try to login. It will kill all programs on given console,
-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 a :IMPORTANT
-IMPORTANT: c2 compliant system, and it should not be mistaken as :IMPORTANT
-IMPORTANT: such. :IMPORTANT
- It seems others 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.
-
-'C'rash can be used to manually trigger a crashdump when the system is hung.
-Note that this just triggers a crash if there is no dump mechanism available.
-
-'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 loglevels '0'-'9' are useful when your console is being flooded with
-kernel messages you do not want to see. Selecting '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
-processes.
-
-"'J'ust thaw it" is useful if your system becomes unresponsive due to a frozen
-(probably root) filesystem via the FIFREEZE ioctl.
-
-* 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. (i.e., 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 produce a different keycode for SysRq than the
-pre-defined value of 99 (see KEY_SYSRQ in include/linux/input.h), or which
-don't have a SysRq key at all. In these cases, run 'showkey -s' to find an
-appropriate scancode sequence, and use 'setkeycodes <sequence> 99' to map
-this sequence to the usual SysRq code (e.g., 'setkeycodes e05b 99'). 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 prototype in 'sysrq.h'.
-
-After the sysrq_key_op is created, you can call the kernel function
-register_sysrq_key(int key, struct sysrq_key_op *op_p); 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 function 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 2 functions are exported for interface to it:
- register_sysrq_key and unregister_sysrq_key.
-Of course, never ever leave an invalid pointer in the table. I.e., when
-your module that called register_sysrq_key() exits, it must call
-unregister_sysrq_key() to clean up the sysrq key table entry that it used.
-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.
-
-* When I hit a SysRq key combination only the header appears on the console?
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-Sysrq output is subject to the same console loglevel control as all
-other console output. This means that if the kernel was booted 'quiet'
-as is common on distro kernels the output may not appear on the actual
-console, even though it will appear in the dmesg buffer, and be accessible
-via the dmesg command and to the consumers of /proc/kmsg. As a specific
-exception the header line from the sysrq command is passed to all console
-consumers as if the current loglevel was maximum. If only the header
-is emitted it is almost certain that the kernel loglevel is too low.
-Should you require the output on the console channel then you will need
-to temporarily up the console loglevel using alt-sysrq-8 or:
-
- echo 8 > /proc/sysrq-trigger
-
-Remember to return the loglevel to normal after triggering the sysrq
-command you are interested in.
-
-* I have more questions, who can I ask?
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-Just ask them on the linux-kernel mailing list:
- linux-kernel@vger.kernel.org
-
-* Credits
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-Written by Mydraal <vulpyne@vulpyne.net>
-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>