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Diffstat (limited to 'Documentation/sysrq.txt')
| -rw-r--r-- | Documentation/sysrq.txt | 85 |
1 files changed, 49 insertions, 36 deletions
diff --git a/Documentation/sysrq.txt b/Documentation/sysrq.txt index 9e592c718af..0e307c94809 100644 --- a/Documentation/sysrq.txt +++ b/Documentation/sysrq.txt @@ -11,27 +11,29 @@ regardless of whatever else it is doing, unless it is completely locked up. 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: +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 - 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 + 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). @@ -66,7 +68,8 @@ On all - write a character to /proc/sysrq-trigger. e.g.: 'b' - Will immediately reboot the system without syncing or unmounting your disks. -'c' - Will perform a kexec reboot in order to take a crashdump. +'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. @@ -74,13 +77,15 @@ On all - write a character to /proc/sysrq-trigger. e.g.: 'f' - Will call oom_kill to kill a memory hog process. -'g' - Used by kgdb on ppc and sh platforms. +'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. @@ -107,11 +112,17 @@ On all - write a character to /proc/sysrq-trigger. e.g.: 'u' - Will attempt to remount all mounted filesystems read-only. -'v' - Dumps Voyager SMP processor info to your console. +'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 @@ -120,9 +131,9 @@ On all - write a character to /proc/sysrq-trigger. e.g.: * Okay, so what can I use them for? ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ -Well, un'R'aw is very handy when your X server or a svgalib program crashes. +Well, unraw(r) 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 +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 @@ -134,20 +145,20 @@ IMPORTANT: such. :IMPORTANT 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. +reboot(b) is good when you're unable to shut down. But you should also +sync(s) and umount(u) first. -'C'rashdump can be used to manually trigger a crashdump when the system is hung. -The kernel needs to have been built with CONFIG_KEXEC enabled. +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. -'S'ync is great when your system is locked up, it allows you to sync your +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...) -'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. +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. @@ -156,10 +167,13 @@ 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 +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 @@ -169,13 +183,13 @@ 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. +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? ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ @@ -230,9 +244,8 @@ command you are interested in. * I have more questions, who can I ask? ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ -And I'll answer any questions about the registration system you got, also -responding as soon as possible. - -Crutcher +Just ask them on the linux-kernel mailing list: + linux-kernel@vger.kernel.org * Credits ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |