In the old days, more or less all PC keyboards had a dedicated 'Print Screen/Sysrq' key and using Linux's Magic Sysrq was easy: you held down Alt, PrintScrn/Sysrq, and an appropriate key at the same time. However, this is becoming less and less common all the time, and in particular my current keyboard doesn't have a dedicated Sysrq key. Instead SysRq is overloaded on F9, and you get it by holding the keyboard's Fn key down when you push F9/PrintScrn. This presents a small difficulty in hitting Sysrq key combos, because holding that Fn key down while you hit regular keys often produces absolutely nothing. So if you just press, say, Fn + Alt + F9 + s to try to force a sync, nothing happens.
After some flailing around and unpredictable, intermittent successes, I think I have finally figured out how to use magic SysRq reliably on my keyboard and on similar keyboards without dedicated SysRq keys. The trick is this:
- press and hold Fn + Alt + your SysRq key
- release your SysRq key and Fn, while still holding Alt
- press your desired SysRq action key, such as
s.
The entire sequence has proven to be important. Releasing Fn while still holding SysRq/F9 down has had a tendency to make the kernel see an Alt+F9 sequence (which switches virtual consoles and in the process wipes away a bunch of the messages I want to keep seeing, and which obviously aborts entering magic Sysrq stuff). Releasing all three keys ends the whole magic SysRq sequence, which means my
s
does nothing.
This turns out to be the suggested approach in the kernel.org guide to magic Sysrq , although their advice is about keyboards that don't like having too many keys down at once. My keyboard specifically appears to do nothing even with just Fn + s, so I don't think it's an issue of the number of keys held down at once. And yes, I used usbmon to verify that my keyboard sends no USB events for Fn + s.
(This is perhaps trivial but I want to documented it for my own future use, because I'm sure I'm going to forget it at some point.)