Magic SysRq Tutorial - What, Kernel Configuation, Use, Commands

What is SysRq?

SysRq is the magical key comibnation when you hit, kernel will respond regardless of whatever else it is doing, unless it is completely locked up.

Kernel Configuration to Enable SysRq?

During Linux Kernel Compilation: CONFIG_MAGIC_SYSRQ = 'y'
While running Linux:

• Kernel Command Line Parameter: "sysrq_always_enabled"
• echo "number" > /proc/sys/kernel/sysrq
• 0 - Disable sysrq completely
• 1 - Enable all functions of sysrq
• > 1 - Bitmask of allowed sysrq functions

How to use SysRq?

If X86: Press Key Combo: Alt + SysRq + <Command Key>. If you don't find 'SysRq' on your keyboard , use 'PrtScr', then the key combo becomes: Alt + PrtScr + <Command Key>

On All other machines ( This will be useful when there is no keyboard attached and you have a serial console) : echo <command key> > /proc/sysrq-trigger

E.g. echo b > /proc/sysrq-trigger to reboot the system

echo g > /proc/sysrq-trigger to get in debugger mode used by kgdb

echo p > /proc/sysrq-trigger to dump the current registers and flags to your console

Will it work under all conditions?

It won't work in the following conditions:

• Kernel Panic
• Hardware Failure preventing the kernel to run properly

Use cases

1. When the X Server is Frozen

Press the "Alt + SysRq + r" combo to take the keyboard from the X Server and then perform "CTRL + ALT+ F1" to get to the terminal

2. When the system is Frozen: "Alt + SysRq + REISUB" keys

List of commands supported:

Command	Function
b	Will immediately reboot the system without syncing or unmounting your disks.
c	Will perform a system crash by a NULL pointer dereference. A crashdump will be taken if configured.
d	Shows all locks that are held.
e	Send a SIGTERM to all processes, except for init.
f	Will call the oom killer to kill a memory hog process, but do not panic if nothing can be killed.
g	Used by kgdb (kernel debugger)
h	Will display help (actually any other key than those listed here will display help. but h is easy to remember :-)
i	Send a SIGKILL to all processes, except for init.
j	Forcibly “Just thaw it” - filesystems frozen by the FIFREEZE ioctl.
k	Secure Access Key (SAK) Kills all programs on the current virtual console. NOTE: See important comments below in SAK section.
l	Shows a stack backtrace for all active CPUs.
m	Will dump current memory info to your console.
n	Used to make RT tasks nice-able
o	Will shut your system off (if configured and supported).
p	Will dump the current registers and flags to your console.
q	Will dump per CPU lists of all armed hrtimers (but NOT regular timer_list timers) and detailed information about all clockevent devices.
r	Turns off keyboard raw mode and sets it to XLATE.
s	Will attempt to sync all mounted filesystems.
t	Will dump a list of current tasks and their information to your console.
u	Will attempt to remount all mounted filesystems read-only.
v	Forcefully restores framebuffer console
v	Causes ETM buffer dump [ARM-specific]
w	Dumps tasks that are in uninterruptable (blocked) state.
x	Used by xmon interface on ppc/powerpc platforms. Show global PMU Registers on sparc64. Dump all TLB entries on MIPS.
y	Show global CPU Registers [SPARC-64 specific]
z	Dump the ftrace buffer
0-9	Sets the console log level, controlling which kernel messages will be printed to your console. (0, for example would make it so that only emergency messages like PANICs or OOPSes would make it to your console.)