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Based on kernel version 3.9. Page generated on 2013-05-02 23:15 EST.

1	Linux Magic System Request Key Hacks
2	Documentation for sysrq.c
3	
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.
8	
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. By default the file contains 1 which means that every
15	possible SysRq request is allowed (in older versions SysRq was disabled
16	by default, and you were required to specifically enable it at run-time
17	but this is not the case any more). Here is the list of possible values
18	in /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
31	
32	You can set the value in the file by the following command:
33	    echo "number" >/proc/sys/kernel/sysrq
34	
35	Note that the value of /proc/sys/kernel/sysrq influences only the invocation
36	via a keyboard. Invocation of any operation via /proc/sysrq-trigger is always
37	allowed (by a user with admin privileges).
38	
39	*  How do I use the magic SysRq key?
40	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
41	On 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.
47	
48	On SPARC - You press 'ALT-STOP-<command key>', I believe.
49	
50	On 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.
53	
54	On PowerPC - Press 'ALT - Print Screen (or F13) - <command key>,  
55	             Print Screen (or F13) - <command key> may suffice.
56	
57	On other - If you know of the key combos for other architectures, please
58	           let me know so I can add them to this section.
59	
60	On all -  write a character to /proc/sysrq-trigger.  e.g.:
61	
62			echo t > /proc/sysrq-trigger
63	
64	*  What are the 'command' keys?
65	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
66	'b'     - Will immediately reboot the system without syncing or unmounting
67	          your disks.
68	
69	'c'	- Will perform a system crash by a NULL pointer dereference.
70	          A crashdump will be taken if configured.
71	
72	'd'	- Shows all locks that are held.
73	
74	'e'     - Send a SIGTERM to all processes, except for init.
75	
76	'f'	- Will call oom_kill to kill a memory hog process.
77	
78	'g'	- Used by kgdb (kernel debugger)
79	
80	'h'     - Will display help (actually any other key than those listed
81	          here will display help. but 'h' is easy to remember :-)
82	
83	'i'     - Send a SIGKILL to all processes, except for init.
84	
85	'j'     - Forcibly "Just thaw it" - filesystems frozen by the FIFREEZE ioctl.
86	
87	'k'     - Secure Access Key (SAK) Kills all programs on the current virtual
88	          console. NOTE: See important comments below in SAK section.
89	
90	'l'     - Shows a stack backtrace for all active CPUs.
91	
92	'm'     - Will dump current memory info to your console.
93	
94	'n'	- Used to make RT tasks nice-able
95	
96	'o'     - Will shut your system off (if configured and supported).
97	
98	'p'     - Will dump the current registers and flags to your console.
99	
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.
103	
104	'r'     - Turns off keyboard raw mode and sets it to XLATE.
105	
106	's'     - Will attempt to sync all mounted filesystems.
107	
108	't'     - Will dump a list of current tasks and their information to your
109	          console.
110	
111	'u'     - Will attempt to remount all mounted filesystems read-only.
112	
113	'v'	- Forcefully restores framebuffer console
114	'v'	- Causes ETM buffer dump [ARM-specific]
115	
116	'w'	- Dumps tasks that are in uninterruptable (blocked) state.
117	
118	'x'	- Used by xmon interface on ppc/powerpc platforms.
119	          Show global PMU Registers on sparc64.
120	
121	'y'	- Show global CPU Registers [SPARC-64 specific]
122	
123	'z'	- Dump the ftrace buffer
124	
125	'0'-'9' - Sets the console log level, controlling which kernel messages
126	          will be printed to your console. ('0', for example would make
127	          it so that only emergency messages like PANICs or OOPSes would
128	          make it to your console.)
129	
130	*  Okay, so what can I use them for?
131	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
132	Well, un'R'aw is very handy when your X server or a svgalib program crashes.
133	
134	sa'K' (Secure Access Key) is useful when you want to be sure there is no
135	trojan program running at console which could grab your password
136	when you would try to login. It will kill all programs on given console,
137	thus letting you make sure that the login prompt you see is actually
138	the one from init, not some trojan program.
139	IMPORTANT: In its true form it is not a true SAK like the one in a :IMPORTANT
140	IMPORTANT: c2 compliant system, and it should not be mistaken as   :IMPORTANT
141	IMPORTANT: such.                                                   :IMPORTANT
142	       It seems others find it useful as (System Attention Key) which is
143	useful when you want to exit a program that will not let you switch consoles.
144	(For example, X or a svgalib program.)
145	
146	re'B'oot is good when you're unable to shut down. But you should also 'S'ync
147	and 'U'mount first.
148	
149	'C'rash can be used to manually trigger a crashdump when the system is hung.
150	Note that this just triggers a crash if there is no dump mechanism available.
151	
152	'S'ync is great when your system is locked up, it allows you to sync your
153	disks and will certainly lessen the chance of data loss and fscking. Note
154	that the sync hasn't taken place until you see the "OK" and "Done" appear
155	on the screen. (If the kernel is really in strife, you may not ever get the
156	OK or Done message...)
157	
158	'U'mount is basically useful in the same ways as 'S'ync. I generally 'S'ync,
159	'U'mount, then re'B'oot when my system locks. It's saved me many a fsck.
160	Again, the unmount (remount read-only) hasn't taken place until you see the
161	"OK" and "Done" message appear on the screen.
162	
163	The loglevels '0'-'9' are useful when your console is being flooded with
164	kernel messages you do not want to see. Selecting '0' will prevent all but
165	the most urgent kernel messages from reaching your console. (They will
166	still be logged if syslogd/klogd are alive, though.)
167	
168	t'E'rm and k'I'll are useful if you have some sort of runaway process you
169	are unable to kill any other way, especially if it's spawning other
170	processes.
171	
172	"'J'ust thaw it" is useful if your system becomes unresponsive due to a frozen
173	(probably root) filesystem via the FIFREEZE ioctl.
174	
175	*  Sometimes SysRq seems to get 'stuck' after using it, what can I do?
176	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
177	That happens to me, also. I've found that tapping shift, alt, and control
178	on both sides of the keyboard, and hitting an invalid sysrq sequence again
179	will fix the problem. (i.e., something like alt-sysrq-z). Switching to another
180	virtual console (ALT+Fn) and then back again should also help.
181	
182	*  I hit SysRq, but nothing seems to happen, what's wrong?
183	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
184	There are some keyboards that produce a different keycode for SysRq than the
185	pre-defined value of 99 (see KEY_SYSRQ in include/linux/input.h), or which
186	don't have a SysRq key at all. In these cases, run 'showkey -s' to find an
187	appropriate scancode sequence, and use 'setkeycodes <sequence> 99' to map
188	this sequence to the usual SysRq code (e.g., 'setkeycodes e05b 99'). It's
189	probably best to put this command in a boot script. Oh, and by the way, you
190	exit 'showkey' by not typing anything for ten seconds.
191	
192	*  I want to add SysRQ key events to a module, how does it work?
193	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
194	In order to register a basic function with the table, you must first include
195	the header 'include/linux/sysrq.h', this will define everything else you need.
196	Next, you must create a sysrq_key_op struct, and populate it with A) the key
197	handler function you will use, B) a help_msg string, that will print when SysRQ
198	prints help, and C) an action_msg string, that will print right before your
199	handler is called. Your handler must conform to the prototype in 'sysrq.h'.
200	
201	After the sysrq_key_op is created, you can call the kernel function
202	register_sysrq_key(int key, struct sysrq_key_op *op_p); this will
203	register the operation pointed to by 'op_p' at table key 'key',
204	if that slot in the table is blank. At module unload time, you must call
205	the function unregister_sysrq_key(int key, struct sysrq_key_op *op_p), which
206	will remove the key op pointed to by 'op_p' from the key 'key', if and only if
207	it is currently registered in that slot. This is in case the slot has been
208	overwritten since you registered it.
209	
210	The Magic SysRQ system works by registering key operations against a key op
211	lookup table, which is defined in 'drivers/char/sysrq.c'. This key table has
212	a number of operations registered into it at compile time, but is mutable,
213	and 2 functions are exported for interface to it:
214		register_sysrq_key and unregister_sysrq_key.
215	Of course, never ever leave an invalid pointer in the table. I.e., when
216	your module that called register_sysrq_key() exits, it must call
217	unregister_sysrq_key() to clean up the sysrq key table entry that it used.
218	Null pointers in the table are always safe. :)
219	
220	If for some reason you feel the need to call the handle_sysrq function from
221	within a function called by handle_sysrq, you must be aware that you are in
222	a lock (you are also in an interrupt handler, which means don't sleep!), so
223	you must call __handle_sysrq_nolock instead.
224	
225	*  When I hit a SysRq key combination only the header appears on the console?
226	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
227	Sysrq output is subject to the same console loglevel control as all
228	other console output.  This means that if the kernel was booted 'quiet'
229	as is common on distro kernels the output may not appear on the actual
230	console, even though it will appear in the dmesg buffer, and be accessible
231	via the dmesg command and to the consumers of /proc/kmsg.  As a specific
232	exception the header line from the sysrq command is passed to all console
233	consumers as if the current loglevel was maximum.  If only the header
234	is emitted it is almost certain that the kernel loglevel is too low.
235	Should you require the output on the console channel then you will need
236	to temporarily up the console loglevel using alt-sysrq-8 or:
237	
238	    echo 8 > /proc/sysrq-trigger
239	
240	Remember to return the loglevel to normal after triggering the sysrq
241	command you are interested in.
242	
243	*  I have more questions, who can I ask?
244	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
245	Just ask them on the linux-kernel mailing list:
246		linux-kernel@vger.kernel.org
247	
248	*  Credits
249	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
250	Written by Mydraal <vulpyne@vulpyne.net>
251	Updated by Adam Sulmicki <adam@cfar.umd.edu>
252	Updated by Jeremy M. Dolan <jmd@turbogeek.org> 2001/01/28 10:15:59
253	Added to by Crutcher Dunnavant <crutcher+kernel@datastacks.com>
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