There are several low level shortcuts that are implemented in the kernel which can be used for debugging and
recovering from an unresponsive system. Whenever possible, it is recommended that you use these shortcuts
instead of doing a hard shutdown (holding down the power button to completely power off the system).

A common idiom to remember this is "Reboot Even If System Utterly Broken" (also referred to as "REISUB").
Alternatively, think of it as "BUSIER" backwards.

Keyboard shorcut / description:

Alt+SysRq+r = Unraw / Take control of keyboard back from X.
Alt+SysRq+i = Kill / Send SIGKILL to all processes, forcing them to terminate immediately.
Alt+SysRq+u = Unmount / Unmount and remount all filesystems read-only.
Alt+SysRq+e = Terminate / Send SIGTERM to all processes, allowing them to terminate gracefully.
Alt+SysRq+s = Sync / Flush data to disk.
Alt+SysRq+b = Reboot / Reboot.

** Note: Depending on the computer or keyboard, replace Alt+SysyRq with Alt+PrtScr or in notebooks with Alt+FN+SysRq or Alt+FN+PrtScr, followd by a shorcut.

Complete the REISUB sequence in order to close the system the at least sub-normally and then reboot.

Tips:

If you are using a display manager and after Alt+SysRq+e you are presented with the login screen (or
full desktop if autologin is enabled), it is most likely caused by Restart=always directive in the
relevant service file. If necessary, edit the unit, however this should not prevent the "REISUB" sequence
from working.

If all the above combinations work except Alt+SysRq+b , try using the contralateral Alt key.
On laptops that use Fn key to differentiate SysRq from PrtScrn, it may not actually be necessary to
use the Fn key (i.e., Alt+PrtSc+letter could work).

You may need to press Ctrl along with Alt. So for example, full key shortcut would be
Ctrl+Alt+SysRq+b.

References:
Web: https://wiki.archlinux.org/index.php/Keyboard_shortcuts
File: Keyboard shortcuts - Reboot Linux from keyboard.pdf
Web: https://en.wikipedia.org/wiki/Magic_SysRq_key

Note: If the sequence fails the system is frozen. In this case you can still use CTRL+ALT+DEL or press the power button.

Note: You can assign actions to keys. Look key binding in the guide of Li-Speak, in the section "Setup the keyboard keys:" (Link)



References:
Topic: Kill programs and system users processes
Guide: Manage processes
