1Linux Magic System Request Key Hacks
2Documentation for sysrq.c
4* What is the magic SysRq key?
6It is a 'magical' key combo you can hit which the kernel will respond to
7regardless of whatever else it is doing, unless it is completely locked up.
9* How do I enable the magic SysRq key?
11You need to say "yes" to 'Magic SysRq key (CONFIG_MAGIC_SYSRQ)' when
12configuring the kernel. When running a kernel with SysRq compiled in,
13/proc/sys/kernel/sysrq controls the functions allowed to be invoked via
14the SysRq key. By default the file contains 1 which means that every
15possible SysRq request is allowed (in older versions SysRq was disabled
16by default, and you were required to specifically enable it at run-time
17but this is not the case any more). Here is the list of possible values
18in /proc/sys/kernel/sysrq:
19   0 - disable sysrq completely
20   1 - enable all functions of sysrq
21  >1 - bitmask of allowed sysrq functions (see below for detailed function
22       description):
23          2 - enable control of console logging level
24          4 - enable control of keyboard (SAK, unraw)
25          8 - enable debugging dumps of processes etc.
26         16 - enable sync command
27         32 - enable remount read-only
28         64 - enable signalling of processes (term, kill, oom-kill)
29        128 - allow reboot/poweroff
30        256 - allow nicing of all RT tasks
32You can set the value in the file by the following command:
33    echo "number" >/proc/sys/kernel/sysrq
35Note that the value of /proc/sys/kernel/sysrq influences only the invocation
36via a keyboard. Invocation of any operation via /proc/sysrq-trigger is always
37allowed (by a user with admin privileges).
39* How do I use the magic SysRq key?
41On x86 - You press the key combo 'ALT-SysRq-<command key>'. Note - Some
42           keyboards may not have a key labeled 'SysRq'. The 'SysRq' key is
43           also known as the 'Print Screen' key. Also some keyboards cannot
44       handle so many keys being pressed at the same time, so you might
45       have better luck with "press Alt", "press SysRq", "release SysRq",
46       "press <command key>", release everything.
48On SPARC - You press 'ALT-STOP-<command key>', I believe.
50On the serial console (PC style standard serial ports only) -
51           You send a BREAK, then within 5 seconds a command key. Sending
52           BREAK twice is interpreted as a normal BREAK.
54On PowerPC - Press 'ALT - Print Screen (or F13) - <command key>,
55             Print Screen (or F13) - <command key> may suffice.
57On other - If you know of the key combos for other architectures, please
58           let me know so I can add them to this section.
60On all - write a character to /proc/sysrq-trigger. e.g.:
62        echo t > /proc/sysrq-trigger
64* What are the 'command' keys?
66'b' - Will immediately reboot the system without syncing or unmounting
67          your disks.
69'c' - Will perform a system crash by a NULL pointer dereference.
70          A crashdump will be taken if configured.
72'd' - Shows all locks that are held.
74'e' - Send a SIGTERM to all processes, except for init.
76'f' - Will call oom_kill to kill a memory hog process.
78'g' - Used by kgdb (kernel debugger)
80'h' - Will display help (actually any other key than those listed
81          here will display help. but 'h' is easy to remember :-)
83'i' - Send a SIGKILL to all processes, except for init.
85'j' - Forcibly "Just thaw it" - filesystems frozen by the FIFREEZE ioctl.
87'k' - Secure Access Key (SAK) Kills all programs on the current virtual
88          console. NOTE: See important comments below in SAK section.
90'l' - Shows a stack backtrace for all active CPUs.
92'm' - Will dump current memory info to your console.
94'n' - Used to make RT tasks nice-able
96'o' - Will shut your system off (if configured and supported).
98'p' - Will dump the current registers and flags to your console.
100'q' - Will dump per CPU lists of all armed hrtimers (but NOT regular
101          timer_list timers) and detailed information about all
102          clockevent devices.
104'r' - Turns off keyboard raw mode and sets it to XLATE.
106's' - Will attempt to sync all mounted filesystems.
108't' - Will dump a list of current tasks and their information to your
109          console.
111'u' - Will attempt to remount all mounted filesystems read-only.
113'v' - Forcefully restores framebuffer console
114'v' - Causes ETM buffer dump [ARM-specific]
116'w' - Dumps tasks that are in uninterruptable (blocked) state.
118'x' - Used by xmon interface on ppc/powerpc platforms.
120'y' - Show global CPU Registers [SPARC-64 specific]
122'z' - Dump the ftrace buffer
124'0'-'9' - Sets the console log level, controlling which kernel messages
125          will be printed to your console. ('0', for example would make
126          it so that only emergency messages like PANICs or OOPSes would
127          make it to your console.)
129* Okay, so what can I use them for?
131Well, un'R'aw is very handy when your X server or a svgalib program crashes.
133sa'K' (Secure Access Key) is useful when you want to be sure there is no
134trojan program running at console which could grab your password
135when you would try to login. It will kill all programs on given console,
136thus letting you make sure that the login prompt you see is actually
137the one from init, not some trojan program.
138IMPORTANT: In its true form it is not a true SAK like the one in a :IMPORTANT
139IMPORTANT: c2 compliant system, and it should not be mistaken as :IMPORTANT
141       It seems others find it useful as (System Attention Key) which is
142useful when you want to exit a program that will not let you switch consoles.
143(For example, X or a svgalib program.)
145re'B'oot is good when you're unable to shut down. But you should also 'S'ync
146and 'U'mount first.
148'C'rash can be used to manually trigger a crashdump when the system is hung.
149Note that this just triggers a crash if there is no dump mechanism available.
151'S'ync is great when your system is locked up, it allows you to sync your
152disks and will certainly lessen the chance of data loss and fscking. Note
153that the sync hasn't taken place until you see the "OK" and "Done" appear
154on the screen. (If the kernel is really in strife, you may not ever get the
155OK or Done message...)
157'U'mount is basically useful in the same ways as 'S'ync. I generally 'S'ync,
158'U'mount, then re'B'oot when my system locks. It's saved me many a fsck.
159Again, the unmount (remount read-only) hasn't taken place until you see the
160"OK" and "Done" message appear on the screen.
162The loglevels '0'-'9' are useful when your console is being flooded with
163kernel messages you do not want to see. Selecting '0' will prevent all but
164the most urgent kernel messages from reaching your console. (They will
165still be logged if syslogd/klogd are alive, though.)
167t'E'rm and k'I'll are useful if you have some sort of runaway process you
168are unable to kill any other way, especially if it's spawning other
171"'J'ust thaw it" is useful if your system becomes unresponsive due to a frozen
172(probably root) filesystem via the FIFREEZE ioctl.
174* Sometimes SysRq seems to get 'stuck' after using it, what can I do?
176That happens to me, also. I've found that tapping shift, alt, and control
177on both sides of the keyboard, and hitting an invalid sysrq sequence again
178will fix the problem. (i.e., something like alt-sysrq-z). Switching to another
179virtual console (ALT+Fn) and then back again should also help.
181* I hit SysRq, but nothing seems to happen, what's wrong?
183There are some keyboards that produce a different keycode for SysRq than the
184pre-defined value of 99 (see KEY_SYSRQ in include/linux/input.h), or which
185don't have a SysRq key at all. In these cases, run 'showkey -s' to find an
186appropriate scancode sequence, and use 'setkeycodes <sequence> 99' to map
187this sequence to the usual SysRq code (e.g., 'setkeycodes e05b 99'). It's
188probably best to put this command in a boot script. Oh, and by the way, you
189exit 'showkey' by not typing anything for ten seconds.
191* I want to add SysRQ key events to a module, how does it work?
193In order to register a basic function with the table, you must first include
194the header 'include/linux/sysrq.h', this will define everything else you need.
195Next, you must create a sysrq_key_op struct, and populate it with A) the key
196handler function you will use, B) a help_msg string, that will print when SysRQ
197prints help, and C) an action_msg string, that will print right before your
198handler is called. Your handler must conform to the prototype in 'sysrq.h'.
200After the sysrq_key_op is created, you can call the kernel function
201register_sysrq_key(int key, struct sysrq_key_op *op_p); this will
202register the operation pointed to by 'op_p' at table key 'key',
203if that slot in the table is blank. At module unload time, you must call
204the function unregister_sysrq_key(int key, struct sysrq_key_op *op_p), which
205will remove the key op pointed to by 'op_p' from the key 'key', if and only if
206it is currently registered in that slot. This is in case the slot has been
207overwritten since you registered it.
209The Magic SysRQ system works by registering key operations against a key op
210lookup table, which is defined in 'drivers/char/sysrq.c'. This key table has
211a number of operations registered into it at compile time, but is mutable,
212and 2 functions are exported for interface to it:
213    register_sysrq_key and unregister_sysrq_key.
214Of course, never ever leave an invalid pointer in the table. I.e., when
215your module that called register_sysrq_key() exits, it must call
216unregister_sysrq_key() to clean up the sysrq key table entry that it used.
217Null pointers in the table are always safe. :)
219If for some reason you feel the need to call the handle_sysrq function from
220within a function called by handle_sysrq, you must be aware that you are in
221a lock (you are also in an interrupt handler, which means don't sleep!), so
222you must call __handle_sysrq_nolock instead.
224* When I hit a SysRq key combination only the header appears on the console?
226Sysrq output is subject to the same console loglevel control as all
227other console output. This means that if the kernel was booted 'quiet'
228as is common on distro kernels the output may not appear on the actual
229console, even though it will appear in the dmesg buffer, and be accessible
230via the dmesg command and to the consumers of /proc/kmsg. As a specific
231exception the header line from the sysrq command is passed to all console
232consumers as if the current loglevel was maximum. If only the header
233is emitted it is almost certain that the kernel loglevel is too low.
234Should you require the output on the console channel then you will need
235to temporarily up the console loglevel using alt-sysrq-8 or:
237    echo 8 > /proc/sysrq-trigger
239Remember to return the loglevel to normal after triggering the sysrq
240command you are interested in.
242* I have more questions, who can I ask?
244And I'll answer any questions about the registration system you got, also
245responding as soon as possible.
246 -Crutcher
248* Credits
250Written by Mydraal <>
251Updated by Adam Sulmicki <>
252Updated by Jeremy M. Dolan <> 2001/01/28 10:15:59
253Added to by Crutcher Dunnavant <>

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