sysrq magic keys
This is useful for trying to recover from an unresponsive system. However, by default, Ubuntu disables the key to gain raw keyboard access.\
Sometimes part of the issue is that X/Wayland display servers have crashed due to bad graphics drivers and keyboard input isn't processed anymore, whereby changing to another console, e.g.
ctrl + alt + F1 doesn't work. Since the default disables keyboard control,
alt + sysrq + r, can't help salvage the situation. Hence the suggestion to at least enable system request raw keyboard control when trying to recover a system.
Ubuntu doesn't enable control of keyboard by default
Ubuntu 16.04 LTS default value: