The sysreq keys should always work if the OS is locked up.

This used to be the case, but the default configuration of systemd actually disables them and requires manual activation.

But yeah, knowing what each does and waiting for sigterm, flush, and umount to complete is crucial to avoid borking your system. From the kernel documentation:

sync(s) is handy before yanking removable medium or after using a rescue shell that provides no graceful shutdown – it will ensure your data is safely written to the disk. Note that the sync hasn’t taken place until you see the “OK” and “Done” appear on the screen.

umount(u) can be used to mark filesystems as properly unmounted. From the running system’s point of view, they will be remounted read-only. The remount isn’t complete until you see the “OK” and “Done” message appear on the screen.

A little correction:

Ctrl + Alt + F7 should take you to a different TTY session where you can login and give a reboot command. (actually any F# key should work)

Virtual consoles 1, 7, and sometimes 2 are usually “special” in the sense that they host a graphic session or kernel logs or something else that is not a login prompt, while 8 through 12 are not usually allocated.

Definitely try pressing Ctrl-Alt-F3 if your session hangs (i.e. mouse cursor is unresponsive) before going nuclear with SysRq. Then you can look at logs and resource utilization to try and figure out what’s going wrong rather than murdering your CPU mid-thought.

Solid troubleshooting advice though! Num lock/caps lock are the definitive way to tell if your CPU is halted or not - but only reliable over a wired USB or PS/2 connection ;)