I didn't realize that in the event of a system freeze, you can still talk to the kernel and give it some final instructions prior to a hard power cycle of the machine. This allows you to attempt to sync all mounted filesystems before a shutdown and possibly avoid file corruption. Thanks to @sec_monkey for the tip! Code: ALT+PrintScreen+s How do I use the magic SysRq keys in emergency? You need to use following key combination in order to reboot/halt/sync file system etc: ALT+SysRq+COMMAND-KEY The 'SysRq' key is also known as the 'Print Screen' key. COMMAND-KEY can be any one of the following (all keys need to hit simultaneously) : 'b' : Will immediately reboot the system without syncing or unmounting your disks. 'o' : Will shutdown your system off (if configured and supported). 's': Will attempt to sync all mounted filesystems. 'u' : Will attempt to remount all mounted filesystems read-only. 'e' : Send a SIGTERM to all processes, except for init. 'h': Show help, indeed this the one you need to remember. So whey you need to tell your Linux computer to reboot or when your X server is crashed or you don't see anything going across the screen then just press: ALT+SysRQ+s : (Press and hold down ALT, then SysRQ (Print Screen) key and press 's') -Will try to sync all mounted system ALT+SysRQ+r : (Press and hold down ALT, then SysRQ (Print Screen) key and press 'r') -Will reboot the system. If you wish to shutdown the system instead of reboot then press following key combination: ALT+SysRQ+o More commands from the man page: Code: Linux/Documentation/sysrq.txt 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. 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 29 30 You can set the value in the file by the following command: 31 echo "number" >/proc/sys/kernel/sysrq 32 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. 36 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). 40 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. 49 50 On SPARC - You press 'ALT-STOP-<command key>', I believe. 51 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. 55 56 On PowerPC - Press 'ALT - Print Screen (or F13) - <command key>, 57 Print Screen (or F13) - <command key> may suffice. 58 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. 61 62 On all - write a character to /proc/sysrq-trigger. e.g.: 63 64 echo t > /proc/sysrq-trigger 65 66 * What are the 'command' keys? 67 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 68 'b' - Will immediately reboot the system without syncing or unmounting 69 your disks. 70 71 'c' - Will perform a system crash by a NULL pointer dereference. 72 A crashdump will be taken if configured. 73 74 'd' - Shows all locks that are held. 75 76 'e' - Send a SIGTERM to all processes, except for init. 77 78 'f' - Will call oom_kill to kill a memory hog process. 79 80 'g' - Used by kgdb (kernel debugger) 81 82 'h' - Will display help (actually any other key than those listed 83 here will display help. but 'h' is easy to remember :) 84 85 'i' - Send a SIGKILL to all processes, except for init. 86 87 'j' - Forcibly "Just thaw it" - filesystems frozen by the FIFREEZE ioctl. 88 89 'k' - Secure Access Key (SAK) Kills all programs on the current virtual 90 console. NOTE: See important comments below in SAK section. 91 92 'l' - Shows a stack backtrace for all active CPUs. 93 94 'm' - Will dump current memory info to your console. 95 96 'n' - Used to make RT tasks nice-able 97 98 'o' - Will shut your system off (if configured and supported). 99 100 'p' - Will dump the current registers and flags to your console. 101 102 '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. 105 106 'r' - Turns off keyboard raw mode and sets it to XLATE. 107 108 's' - Will attempt to sync all mounted filesystems. 109 110 't' - Will dump a list of current tasks and their information to your 111 console. 112 113 'u' - Will attempt to remount all mounted filesystems read-only. 114 115 'v' - Forcefully restores framebuffer console 116 'v' - Causes ETM buffer dump [ARM-specific] 117 118 'w' - Dumps tasks that are in uninterruptable (blocked) state. 119 120 'x' - Used by xmon interface on ppc/powerpc platforms. 121 Show global PMU Registers on sparc64. 122 123 'y' - Show global CPU Registers [SPARC-64 specific] 124 125 'z' - Dump the ftrace buffer 126 127 '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.) 131 132 * 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. 135 136 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.) 147 148 reboot(b) is good when you're unable to shut down. But you should also 149 sync(s) and umount(u) first. 150 151 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. 153 154 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...) 159 160 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. 164 165 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.) 169 170 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. 173 174 "just thaw it(j)" is useful if your system becomes unresponsive due to a frozen 175 (probably root) filesystem via the FIFREEZE ioctl. 176 177 * 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. 183 184 * 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. 193 194 * 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'. 202 203 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. 211 212 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. :) 221 222 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. 226 227 * 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: 239 240 echo 8 > /proc/sysrq-trigger 241 242 Remember to return the loglevel to normal after triggering the sysrq 243 command you are interested in. 244 245 * 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 249 250 * 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> Howto Reboot or halt Linux system in emergency
