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diff --git a/Documentation/sysrq.txt b/Documentation/sysrq.txt new file mode 100644 index 000000000..0e307c948 --- /dev/null +++ b/Documentation/sysrq.txt @@ -0,0 +1,255 @@ +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. The default value in this file is set by the +CONFIG_MAGIC_SYSRQ_DEFAULT_ENABLE config symbol, which itself defaults +to 1. 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 = 0x2 - enable control of console logging level + 4 = 0x4 - enable control of keyboard (SAK, unraw) + 8 = 0x8 - enable debugging dumps of processes etc. + 16 = 0x10 - enable sync command + 32 = 0x20 - enable remount read-only + 64 = 0x40 - enable signalling of processes (term, kill, oom-kill) + 128 = 0x80 - allow reboot/poweroff + 256 = 0x100 - allow nicing of all RT tasks + +You can set the value in the file by the following command: + echo "number" >/proc/sys/kernel/sysrq + +The number may be written here either as decimal or as hexadecimal +with the 0x prefix. CONFIG_MAGIC_SYSRQ_DEFAULT_ENABLE must always be +written in hexadecimal. + +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. + Show global PMU Registers on sparc64. + +'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, unraw(r) is very handy when your X server or a svgalib program crashes. + +sak(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.) + +reboot(b) is good when you're unable to shut down. But you should also +sync(s) and umount(u) first. + +crash(c) 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. + +sync(s) 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...) + +umount(u) is basically useful in the same ways as sync(s). I generally sync(s), +umount(u), then reboot(b) 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.) + +term(e) and kill(i) 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. + +"just thaw it(j)" 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> |