Although very rare Linux might get stuck as well and won't respond to keystrokes or other user input. In the worst case there is a kernel crash, sometimes with a kernel crash dump on the console, probably caused by an unstable driver or hardware problems.
If you are lucky the system stucks but you are able to login from a remote machine by ssh. You can kill the process
kill -9 ... or reboot the system
shutdown -r now. Most of the times a jam is caused by one or more processes consuming too many resources. Symptoms are extreme high CPU usage, high load, high memory usage, a high queue of waiting processes etc. If there are no resources left the kernel might still be running but the system doesn't respond to keystrokes,
CTRL+ALT+DEL won't help and login from an other system with ssh is also impossible, as there are no resources left to start a new shell.
sysrqis the magic key if enabled on the system. By the default the magic sysrequest feature is disabled. You can enable all sysrq features by typing:
echo 1 > /proc/sys/kernel/sysrqIf you want to enable this at boottime edit
/etc/sysctl.confand replace the line
The magic sysrq key combination consists of
ALT+SYSRQ+key A good combination to safely reboot the system is typing REISUB while holding down
ALT+SYSRQ. This will stop all processes, sync the file-systems, unmount the drives and reboot.
The list of all commands:
b - Immediately reboot the system without syncing or unmounting your disks.
c - Perform a system crash by a NULL pointer dereference. A crash dump will be taken if configured.
d - Shows all locks that are held.
e - Send a SIGTERM to all processes, except for init.
f - Call oom_kill to kill a memory hog process.
g - Used by kgdb on ppc and sh platforms.
h - Display help (actually any other key than those listed
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.
l - Shows a stack backtrace for all active CPUs.
m - Dump current memory info to your console.
n - Used to make RT tasks nice-able
o - Shut your system off (if configured and supported).
p - Dump the current registers and flags to your console.
q - Dump per CPU lists of all armed hrtimers and detailed information about all clock-event devices.
r - Turns off keyboard raw mode and sets it to XLATE.
s - Attempt to sync all mounted file-systems.
t - Dump a list of current tasks and their information to your console.
u - Attempt to remount all mounted file-systems read-only.
v - Dumps Voyager SMP processor info to your console.
w - Dumps tasks that are in uninterruptable (blocked) state.
x - Used by xmon interface on ppc/powerpc platforms.
z - Dump the ftrace buffer
0-9 - Sets the console log level, controlling which kernel messages will be printed to your console.
Some security considerations:
Having the magic sysrq enabled on the system is a security risc. Anyone with physical access to the system can bring the system down. Remotely sending
break to the system will be interpreted as