A few times a month, I find that I can no longer change the brightness with the usual keys. Also, my laptop won't suspend when I close the lid, and the power icon won't update when I unplug or plug in my power cord. Based on these symptoms, I think this is an issue with powerdevil. Fortunately, there's an easy workaround:

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systemctl suspend

After this, my laptop suspends, I close and re-open the lid, unlock, and everything is back to normal.

Unfortunately for actually fixing this bug, it's difficult to reproduce and I haven't been able to find any relevant error messages or diagnostics. Here's what I've tried checking so far:

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systemctl status upower.service
systemctl status org.kde.powerdevil.backlighthelper.service
acpi -V
systemd-inhibit --list
journalctl --boot --lines=all | grep powerdevil
grep powerdevil ~/.xsession-errors


I'd really appreciate any tips on debugging powerdevil specifically. I've tried running strace and gdb on the powerdevil process when it's stuck that way, and unfortunately I haven't learned anything useful from it. The backtrace from gdb looks normal, and the strace output is almost-non-existent. I first noticed this on version 5.12.7, but it's also present on version 5.12.8 and it's been bugging me since early 2019. (I'm running KUbuntu 18.04 in case that matters.)

P.S. Full bug report is here:

https://bugs.launchpad.net/ubuntu/+source/powerdevil/+bug/1928997