Alt + sysrq + REISUB doesn't reboot my laptop

Question

On occasion my laptop has become stuck so I've used the magic key combination of Alt + SysRq + REISUB to perform a safe reboot. However, it doesn't seem to work for me as when I press the B key to complete the magic key command it simply says resetting... and doesn't do anything.

All the other keys seem to work fine as it resets the keyboard layout, kills all tasks etc, but doesn't seem to complete with a reboot.

I was under the impression that my laptop should reboot after the magic keys have all been pressed in the order of REISUB

Answer 1

Alt+SysRq has been disabled since Ubuntu 10.10 maverick. To re-enable it you have to modify the respective kernel parameter like this:

echo 1 | sudo tee /proc/sys/kernel/sysrq 

To enable it permanently, you must edit the file /etc/sysctl.conf to include the line

kernel.sysrq=1

Answer 2

Keyboards are the issue here!

• Some just don't like the 4-keys-down at the same time and just stop being a keyboard until you release some.
• All keyboards turn into QWERTY layout when you send the SysRq signal. This makes it hard to find the right commands on keyboards with another layout. Wikipedia: SysRq commands shows a table to find the right ones for common alternative layouts.
• Some have SysRq on another key than PrtSc to hold with a function key. On my laptop this is Fn+End. Holding Alt + PrtSc only works if some system process (window manager?) translates it for me. On total crash, only the real SysRq works!
• Some only trigger SysRq key once you hold the right Alt + PrtSc (by hardware). The left Alt-key won't work in such a case, for the same reason as above. This is how my USB keyboard works.

• Tip: try out hitting Shift + Alt + SysRq to see a 'help' output on a virtual terminal. This is useful to see if it's working without doing any harm:

  SysRq : HELP : loglevel(0-9) reBoot Crash terminate-all-tasks(E) memory-full-oom-kill(F) kill-all-tasks(I) thaw-filesystems(J) saK show-backtrace-all-active-cpus(L) show-memory-usage(M) nice-all-RT-tasks(N) powerOff show-registers(P) show-all-timers(Q) unRaw Sync show-task-states(T) Unmount show-blocked-tasks(W) dump-ftrace-buffer(Z)

In all cases I've come across it didn't work, swapping the keyboard worked perfectly and showed that nothing was wrong with any kernel setting.

Answer 3

I ran into this the other day and it took a few seconds to work. I may have even done the entire REISUB sequence twice.

Also, make sure you're using the sysrq key (most laptops will require you to hold down function, so you're really holding down 4 keys at once)

Answer 4

A few years back I used to use Alt+SysRq+S,U,B,O to reboot or power off a stuck machine, because like for you, Alt+SysRq+B sometimes didn't want to work.

(Also, I need to get into the habit of using Alt+SysRq+E,I first. I don't see much point of using Alt+SysRq+R, since why does keyboard mode matter if it's going to reboot and reset it anyway?)

Answer 5

Look here: reisub-the-gentle-linux-restart then here en.wikipedia.org/wiki/System_request