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).
* 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
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'rash can be used to manually trigger a crashdump when the system is hung.
+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.
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.
-"'J'ust thaw it" is useful if your system becomes unresponsive due to a frozen
+"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?
projects / karo-tx-linux.git / blobdiff