Contents of /alx-src/tags/kernel26-2.6.12-alx-r9/Documentation/sysrq.txt
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Tag kernel26-2.6.12-alx-r9
1 | Linux Magic System Request Key Hacks |
2 | Documentation for sysrq.c version 1.15 |
3 | Last update: $Date: 2001/01/28 10:15:59 $ |
4 | |
5 | * What is the magic SysRq key? |
6 | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
7 | It is a 'magical' key combo you can hit which the kernel will respond to |
8 | regardless of whatever else it is doing, unless it is completely locked up. |
9 | |
10 | * How do I enable the magic SysRq key? |
11 | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
12 | You need to say "yes" to 'Magic SysRq key (CONFIG_MAGIC_SYSRQ)' when |
13 | configuring the kernel. When running a kernel with SysRq compiled in, |
14 | /proc/sys/kernel/sysrq controls the functions allowed to be invoked via |
15 | the SysRq key. By default the file contains 1 which means that every |
16 | possible SysRq request is allowed (in older versions SysRq was disabled |
17 | by default, and you were required to specifically enable it at run-time |
18 | but this is not the case any more). Here is the list of possible values |
19 | in /proc/sys/kernel/sysrq: |
20 | 0 - disable sysrq completely |
21 | 1 - enable all functions of sysrq |
22 | >1 - bitmask of allowed sysrq functions (see below for detailed function |
23 | description): |
24 | 2 - enable control of console logging level |
25 | 4 - enable control of keyboard (SAK, unraw) |
26 | 8 - enable debugging dumps of processes etc. |
27 | 16 - enable sync command |
28 | 32 - enable remount read-only |
29 | 64 - enable signalling of processes (term, kill, oom-kill) |
30 | 128 - allow reboot/poweroff |
31 | 256 - allow nicing of all RT tasks |
32 | |
33 | You can set the value in the file by the following command: |
34 | echo "number" >/proc/sys/kernel/sysrq |
35 | |
36 | Note that the value of /proc/sys/kernel/sysrq influences only the invocation |
37 | via a keyboard. Invocation of any operation via /proc/sysrq-trigger is always |
38 | allowed. |
39 | |
40 | * How do I use the magic SysRq key? |
41 | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
42 | On x86 - You press the key combo 'ALT-SysRq-<command key>'. Note - Some |
43 | keyboards may not have a key labeled 'SysRq'. The 'SysRq' key is |
44 | also known as the 'Print Screen' key. Also some keyboards cannot |
45 | handle so many keys being pressed at the same time, so you might |
46 | have better luck with "press Alt", "press SysRq", "release Alt", |
47 | "press <command key>", release everything. |
48 | |
49 | On SPARC - You press 'ALT-STOP-<command key>', I believe. |
50 | |
51 | On the serial console (PC style standard serial ports only) - |
52 | You send a BREAK, then within 5 seconds a command key. Sending |
53 | BREAK twice is interpreted as a normal BREAK. |
54 | |
55 | On PowerPC - Press 'ALT - Print Screen (or F13) - <command key>, |
56 | Print Screen (or F13) - <command key> may suffice. |
57 | |
58 | On other - If you know of the key combos for other architectures, please |
59 | let me know so I can add them to this section. |
60 | |
61 | On all - write a character to /proc/sysrq-trigger. eg: |
62 | |
63 | echo t > /proc/sysrq-trigger |
64 | |
65 | * What are the 'command' keys? |
66 | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
67 | 'r' - Turns off keyboard raw mode and sets it to XLATE. |
68 | |
69 | 'k' - Secure Access Key (SAK) Kills all programs on the current virtual |
70 | console. NOTE: See important comments below in SAK section. |
71 | |
72 | 'b' - Will immediately reboot the system without syncing or unmounting |
73 | your disks. |
74 | |
75 | 'o' - Will shut your system off (if configured and supported). |
76 | |
77 | 's' - Will attempt to sync all mounted filesystems. |
78 | |
79 | 'u' - Will attempt to remount all mounted filesystems read-only. |
80 | |
81 | 'p' - Will dump the current registers and flags to your console. |
82 | |
83 | 't' - Will dump a list of current tasks and their information to your |
84 | console. |
85 | |
86 | 'm' - Will dump current memory info to your console. |
87 | |
88 | 'v' - Dumps Voyager SMP processor info to your console. |
89 | |
90 | '0'-'9' - Sets the console log level, controlling which kernel messages |
91 | will be printed to your console. ('0', for example would make |
92 | it so that only emergency messages like PANICs or OOPSes would |
93 | make it to your console.) |
94 | |
95 | 'f' - Will call oom_kill to kill a memory hog process |
96 | |
97 | 'e' - Send a SIGTERM to all processes, except for init. |
98 | |
99 | 'i' - Send a SIGKILL to all processes, except for init. |
100 | |
101 | 'l' - Send a SIGKILL to all processes, INCLUDING init. (Your system |
102 | will be non-functional after this.) |
103 | |
104 | 'h' - Will display help ( actually any other key than those listed |
105 | above will display help. but 'h' is easy to remember :-) |
106 | |
107 | * Okay, so what can I use them for? |
108 | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
109 | Well, un'R'aw is very handy when your X server or a svgalib program crashes. |
110 | |
111 | sa'K' (Secure Access Key) is useful when you want to be sure there are no |
112 | trojan program is running at console and which could grab your password |
113 | when you would try to login. It will kill all programs on given console |
114 | and thus letting you make sure that the login prompt you see is actually |
115 | the one from init, not some trojan program. |
116 | IMPORTANT:In its true form it is not a true SAK like the one in :IMPORTANT |
117 | IMPORTANT:c2 compliant systems, and it should be mistook as such. :IMPORTANT |
118 | It seems other find it useful as (System Attention Key) which is |
119 | useful when you want to exit a program that will not let you switch consoles. |
120 | (For example, X or a svgalib program.) |
121 | |
122 | re'B'oot is good when you're unable to shut down. But you should also 'S'ync |
123 | and 'U'mount first. |
124 | |
125 | 'S'ync is great when your system is locked up, it allows you to sync your |
126 | disks and will certainly lessen the chance of data loss and fscking. Note |
127 | that the sync hasn't taken place until you see the "OK" and "Done" appear |
128 | on the screen. (If the kernel is really in strife, you may not ever get the |
129 | OK or Done message...) |
130 | |
131 | 'U'mount is basically useful in the same ways as 'S'ync. I generally 'S'ync, |
132 | 'U'mount, then re'B'oot when my system locks. It's saved me many a fsck. |
133 | Again, the unmount (remount read-only) hasn't taken place until you see the |
134 | "OK" and "Done" message appear on the screen. |
135 | |
136 | The loglevel'0'-'9' is useful when your console is being flooded with |
137 | kernel messages you do not want to see. Setting '0' will prevent all but |
138 | the most urgent kernel messages from reaching your console. (They will |
139 | still be logged if syslogd/klogd are alive, though.) |
140 | |
141 | t'E'rm and k'I'll are useful if you have some sort of runaway process you |
142 | are unable to kill any other way, especially if it's spawning other |
143 | processes. |
144 | |
145 | * Sometimes SysRq seems to get 'stuck' after using it, what can I do? |
146 | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
147 | That happens to me, also. I've found that tapping shift, alt, and control |
148 | on both sides of the keyboard, and hitting an invalid sysrq sequence again |
149 | will fix the problem. (ie, something like alt-sysrq-z). Switching to another |
150 | virtual console (ALT+Fn) and then back again should also help. |
151 | |
152 | * I hit SysRq, but nothing seems to happen, what's wrong? |
153 | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
154 | There are some keyboards that send different scancodes for SysRq than the |
155 | pre-defined 0x54. So if SysRq doesn't work out of the box for a certain |
156 | keyboard, run 'showkey -s' to find out the proper scancode sequence. Then |
157 | use 'setkeycodes <sequence> 84' to define this sequence to the usual SysRq |
158 | code (84 is decimal for 0x54). It's probably best to put this command in a |
159 | boot script. Oh, and by the way, you exit 'showkey' by not typing anything |
160 | for ten seconds. |
161 | |
162 | * I want to add SysRQ key events to a module, how does it work? |
163 | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
164 | In order to register a basic function with the table, you must first include |
165 | the header 'include/linux/sysrq.h', this will define everything else you need. |
166 | Next, you must create a sysrq_key_op struct, and populate it with A) the key |
167 | handler function you will use, B) a help_msg string, that will print when SysRQ |
168 | prints help, and C) an action_msg string, that will print right before your |
169 | handler is called. Your handler must conform to the protoype in 'sysrq.h'. |
170 | |
171 | After the sysrq_key_op is created, you can call the macro |
172 | register_sysrq_key(int key, struct sysrq_key_op *op_p) that is defined in |
173 | sysrq.h, this will register the operation pointed to by 'op_p' at table |
174 | key 'key', if that slot in the table is blank. At module unload time, you must |
175 | call the macro unregister_sysrq_key(int key, struct sysrq_key_op *op_p), which |
176 | will remove the key op pointed to by 'op_p' from the key 'key', if and only if |
177 | it is currently registered in that slot. This is in case the slot has been |
178 | overwritten since you registered it. |
179 | |
180 | The Magic SysRQ system works by registering key operations against a key op |
181 | lookup table, which is defined in 'drivers/char/sysrq.c'. This key table has |
182 | a number of operations registered into it at compile time, but is mutable, |
183 | and 4 functions are exported for interface to it: __sysrq_lock_table, |
184 | __sysrq_unlock_table, __sysrq_get_key_op, and __sysrq_put_key_op. The |
185 | functions __sysrq_swap_key_ops and __sysrq_swap_key_ops_nolock are defined |
186 | in the header itself, and the REGISTER and UNREGISTER macros are built from |
187 | these. More complex (and dangerous!) manipulations of the table are possible |
188 | using these functions, but you must be careful to always lock the table before |
189 | you read or write from it, and to unlock it again when you are done. (And of |
190 | course, to never ever leave an invalid pointer in the table). Null pointers in |
191 | the table are always safe :) |
192 | |
193 | If for some reason you feel the need to call the handle_sysrq function from |
194 | within a function called by handle_sysrq, you must be aware that you are in |
195 | a lock (you are also in an interrupt handler, which means don't sleep!), so |
196 | you must call __handle_sysrq_nolock instead. |
197 | |
198 | * I have more questions, who can I ask? |
199 | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
200 | You may feel free to send email to myrdraal@deathsdoor.com, and I will |
201 | respond as soon as possible. |
202 | -Myrdraal |
203 | |
204 | And I'll answer any questions about the registration system you got, also |
205 | responding as soon as possible. |
206 | -Crutcher |
207 | |
208 | * Credits |
209 | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
210 | Written by Mydraal <myrdraal@deathsdoor.com> |
211 | Updated by Adam Sulmicki <adam@cfar.umd.edu> |
212 | Updated by Jeremy M. Dolan <jmd@turbogeek.org> 2001/01/28 10:15:59 |
213 | Added to by Crutcher Dunnavant <crutcher+kernel@datastacks.com> |