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Based on kernel version 4.2. Page generated on 2015-09-09 12:15 EST.

1	Linux Magic System Request Key Hacks
2	Documentation for sysrq.c
4	*  What is the magic SysRq key?
5	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
6	It is a 'magical' key combo you can hit which the kernel will respond to
7	regardless of whatever else it is doing, unless it is completely locked up.
9	*  How do I enable the magic SysRq key?
10	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
11	You need to say "yes" to 'Magic SysRq key (CONFIG_MAGIC_SYSRQ)' when
12	configuring the kernel. When running a kernel with SysRq compiled in,
13	/proc/sys/kernel/sysrq controls the functions allowed to be invoked via
14	the SysRq key. The default value in this file is set by the
15	CONFIG_MAGIC_SYSRQ_DEFAULT_ENABLE config symbol, which itself defaults
16	to 1. Here is the list of possible values in /proc/sys/kernel/sysrq:
17	   0 - disable sysrq completely
18	   1 - enable all functions of sysrq
19	  >1 - bitmask of allowed sysrq functions (see below for detailed function
20	       description):
21	          2 =   0x2 - enable control of console logging level
22	          4 =   0x4 - enable control of keyboard (SAK, unraw)
23	          8 =   0x8 - enable debugging dumps of processes etc.
24	         16 =  0x10 - enable sync command
25	         32 =  0x20 - enable remount read-only
26	         64 =  0x40 - enable signalling of processes (term, kill, oom-kill)
27	        128 =  0x80 - allow reboot/poweroff
28	        256 = 0x100 - allow nicing of all RT tasks
30	You can set the value in the file by the following command:
31	    echo "number" >/proc/sys/kernel/sysrq
33	The number may be written here either as decimal or as hexadecimal
34	with the 0x prefix. CONFIG_MAGIC_SYSRQ_DEFAULT_ENABLE must always be
35	written in hexadecimal.
37	Note that the value of /proc/sys/kernel/sysrq influences only the invocation
38	via a keyboard. Invocation of any operation via /proc/sysrq-trigger is always
39	allowed (by a user with admin privileges).
41	*  How do I use the magic SysRq key?
42	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
43	On x86   - You press the key combo 'ALT-SysRq-<command key>'. Note - Some
44	           keyboards may not have a key labeled 'SysRq'. The 'SysRq' key is
45	           also known as the 'Print Screen' key. Also some keyboards cannot
46		   handle so many keys being pressed at the same time, so you might
47		   have better luck with "press Alt", "press SysRq", "release SysRq",
48		   "press <command key>", release everything.
50	On SPARC - You press 'ALT-STOP-<command key>', I believe.
52	On the serial console (PC style standard serial ports only) -
53	           You send a BREAK, then within 5 seconds a command key. Sending
54	           BREAK twice is interpreted as a normal BREAK.
56	On PowerPC - Press 'ALT - Print Screen (or F13) - <command key>,  
57	             Print Screen (or F13) - <command key> may suffice.
59	On other - If you know of the key combos for other architectures, please
60	           let me know so I can add them to this section.
62	On all -  write a character to /proc/sysrq-trigger.  e.g.:
64			echo t > /proc/sysrq-trigger
66	*  What are the 'command' keys?
67	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
68	'b'     - Will immediately reboot the system without syncing or unmounting
69	          your disks.
71	'c'	- Will perform a system crash by a NULL pointer dereference.
72	          A crashdump will be taken if configured.
74	'd'	- Shows all locks that are held.
76	'e'     - Send a SIGTERM to all processes, except for init.
78	'f'	- Will call oom_kill to kill a memory hog process.
80	'g'	- Used by kgdb (kernel debugger)
82	'h'     - Will display help (actually any other key than those listed
83	          here will display help. but 'h' is easy to remember :-)
85	'i'     - Send a SIGKILL to all processes, except for init.
87	'j'     - Forcibly "Just thaw it" - filesystems frozen by the FIFREEZE ioctl.
89	'k'     - Secure Access Key (SAK) Kills all programs on the current virtual
90	          console. NOTE: See important comments below in SAK section.
92	'l'     - Shows a stack backtrace for all active CPUs.
94	'm'     - Will dump current memory info to your console.
96	'n'	- Used to make RT tasks nice-able
98	'o'     - Will shut your system off (if configured and supported).
100	'p'     - Will dump the current registers and flags to your console.
102	'q'     - Will dump per CPU lists of all armed hrtimers (but NOT regular
103	          timer_list timers) and detailed information about all
104	          clockevent devices.
106	'r'     - Turns off keyboard raw mode and sets it to XLATE.
108	's'     - Will attempt to sync all mounted filesystems.
110	't'     - Will dump a list of current tasks and their information to your
111	          console.
113	'u'     - Will attempt to remount all mounted filesystems read-only.
115	'v'	- Forcefully restores framebuffer console
116	'v'	- Causes ETM buffer dump [ARM-specific]
118	'w'	- Dumps tasks that are in uninterruptable (blocked) state.
120	'x'	- Used by xmon interface on ppc/powerpc platforms.
121	          Show global PMU Registers on sparc64.
123	'y'	- Show global CPU Registers [SPARC-64 specific]
125	'z'	- Dump the ftrace buffer
127	'0'-'9' - Sets the console log level, controlling which kernel messages
128	          will be printed to your console. ('0', for example would make
129	          it so that only emergency messages like PANICs or OOPSes would
130	          make it to your console.)
132	*  Okay, so what can I use them for?
133	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
134	Well, unraw(r) is very handy when your X server or a svgalib program crashes.
136	sak(k) (Secure Access Key) is useful when you want to be sure there is no
137	trojan program running at console which could grab your password
138	when you would try to login. It will kill all programs on given console,
139	thus letting you make sure that the login prompt you see is actually
140	the one from init, not some trojan program.
141	IMPORTANT: In its true form it is not a true SAK like the one in a :IMPORTANT
142	IMPORTANT: c2 compliant system, and it should not be mistaken as   :IMPORTANT
143	IMPORTANT: such.                                                   :IMPORTANT
144	       It seems others find it useful as (System Attention Key) which is
145	useful when you want to exit a program that will not let you switch consoles.
146	(For example, X or a svgalib program.)
148	reboot(b) is good when you're unable to shut down. But you should also
149	sync(s) and umount(u) first.
151	crash(c) can be used to manually trigger a crashdump when the system is hung.
152	Note that this just triggers a crash if there is no dump mechanism available.
154	sync(s) is great when your system is locked up, it allows you to sync your
155	disks and will certainly lessen the chance of data loss and fscking. Note
156	that the sync hasn't taken place until you see the "OK" and "Done" appear
157	on the screen. (If the kernel is really in strife, you may not ever get the
158	OK or Done message...)
160	umount(u) is basically useful in the same ways as sync(s). I generally sync(s),
161	umount(u), then reboot(b) when my system locks. It's saved me many a fsck.
162	Again, the unmount (remount read-only) hasn't taken place until you see the
163	"OK" and "Done" message appear on the screen.
165	The loglevels '0'-'9' are useful when your console is being flooded with
166	kernel messages you do not want to see. Selecting '0' will prevent all but
167	the most urgent kernel messages from reaching your console. (They will
168	still be logged if syslogd/klogd are alive, though.)
170	term(e) and kill(i) are useful if you have some sort of runaway process you
171	are unable to kill any other way, especially if it's spawning other
172	processes.
174	"just thaw it(j)" is useful if your system becomes unresponsive due to a frozen
175	(probably root) filesystem via the FIFREEZE ioctl.
177	*  Sometimes SysRq seems to get 'stuck' after using it, what can I do?
178	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
179	That happens to me, also. I've found that tapping shift, alt, and control
180	on both sides of the keyboard, and hitting an invalid sysrq sequence again
181	will fix the problem. (i.e., something like alt-sysrq-z). Switching to another
182	virtual console (ALT+Fn) and then back again should also help.
184	*  I hit SysRq, but nothing seems to happen, what's wrong?
185	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
186	There are some keyboards that produce a different keycode for SysRq than the
187	pre-defined value of 99 (see KEY_SYSRQ in include/linux/input.h), or which
188	don't have a SysRq key at all. In these cases, run 'showkey -s' to find an
189	appropriate scancode sequence, and use 'setkeycodes <sequence> 99' to map
190	this sequence to the usual SysRq code (e.g., 'setkeycodes e05b 99'). It's
191	probably best to put this command in a boot script. Oh, and by the way, you
192	exit 'showkey' by not typing anything for ten seconds.
194	*  I want to add SysRQ key events to a module, how does it work?
195	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
196	In order to register a basic function with the table, you must first include
197	the header 'include/linux/sysrq.h', this will define everything else you need.
198	Next, you must create a sysrq_key_op struct, and populate it with A) the key
199	handler function you will use, B) a help_msg string, that will print when SysRQ
200	prints help, and C) an action_msg string, that will print right before your
201	handler is called. Your handler must conform to the prototype in 'sysrq.h'.
203	After the sysrq_key_op is created, you can call the kernel function
204	register_sysrq_key(int key, struct sysrq_key_op *op_p); this will
205	register the operation pointed to by 'op_p' at table key 'key',
206	if that slot in the table is blank. At module unload time, you must call
207	the function unregister_sysrq_key(int key, struct sysrq_key_op *op_p), which
208	will remove the key op pointed to by 'op_p' from the key 'key', if and only if
209	it is currently registered in that slot. This is in case the slot has been
210	overwritten since you registered it.
212	The Magic SysRQ system works by registering key operations against a key op
213	lookup table, which is defined in 'drivers/char/sysrq.c'. This key table has
214	a number of operations registered into it at compile time, but is mutable,
215	and 2 functions are exported for interface to it:
216		register_sysrq_key and unregister_sysrq_key.
217	Of course, never ever leave an invalid pointer in the table. I.e., when
218	your module that called register_sysrq_key() exits, it must call
219	unregister_sysrq_key() to clean up the sysrq key table entry that it used.
220	Null pointers in the table are always safe. :)
222	If for some reason you feel the need to call the handle_sysrq function from
223	within a function called by handle_sysrq, you must be aware that you are in
224	a lock (you are also in an interrupt handler, which means don't sleep!), so
225	you must call __handle_sysrq_nolock instead.
227	*  When I hit a SysRq key combination only the header appears on the console?
228	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
229	Sysrq output is subject to the same console loglevel control as all
230	other console output.  This means that if the kernel was booted 'quiet'
231	as is common on distro kernels the output may not appear on the actual
232	console, even though it will appear in the dmesg buffer, and be accessible
233	via the dmesg command and to the consumers of /proc/kmsg.  As a specific
234	exception the header line from the sysrq command is passed to all console
235	consumers as if the current loglevel was maximum.  If only the header
236	is emitted it is almost certain that the kernel loglevel is too low.
237	Should you require the output on the console channel then you will need
238	to temporarily up the console loglevel using alt-sysrq-8 or:
240	    echo 8 > /proc/sysrq-trigger
242	Remember to return the loglevel to normal after triggering the sysrq
243	command you are interested in.
245	*  I have more questions, who can I ask?
246	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
247	Just ask them on the linux-kernel mailing list:
248		linux-kernel@vger.kernel.org
250	*  Credits
251	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
252	Written by Mydraal <vulpyne@vulpyne.net>
253	Updated by Adam Sulmicki <adam@cfar.umd.edu>
254	Updated by Jeremy M. Dolan <jmd@turbogeek.org> 2001/01/28 10:15:59
255	Added to by Crutcher Dunnavant <crutcher+kernel@datastacks.com>
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