Linux Magic System Request Key Hacks
Documentation for sysrq.c
-Last update: 2007-JAN-06
* What is the magic SysRq key?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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",
+ 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.
* 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.
'd' - Shows all locks that are held.
-'o' - Will shut your system off (if configured and supported).
+'e' - Send a SIGTERM to all processes, except for init.
-'s' - Will attempt to sync all mounted filesystems.
+'f' - Will call oom_kill to kill a memory hog process.
-'u' - Will attempt to remount all mounted filesystems read-only.
+'g' - Used by kgdb on ppc and sh platforms.
-'p' - Will dump the current registers and flags to your console.
+'h' - Will display help (actually any other key than those listed
+ here will display help. but 'h' is easy to remember :-)
-'t' - Will dump a list of current tasks and their information to your
- console.
+'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' - Dumps Voyager SMP processor info to your console.
'w' - Dumps tasks that are in uninterruptable (blocked) state.
'x' - Used by xmon interface on ppc/powerpc platforms.
+'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.)
-'f' - Will call oom_kill to kill a memory hog process.
-
-'e' - Send a SIGTERM to all processes, except for init.
-
-'g' - Used by kgdb on ppc platforms.
-
-'i' - Send a SIGKILL to all processes, except for init.
-
-'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.
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
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?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
And I'll answer any questions about the registration system you got, also
git://ftp.safe.ca / safe / jmp / linux-2.6 / blobdiff