(replying to myself as I got a message on irc from the author)
Dominique Martinet wrote on Wed, Nov 24, 2021 at 07:28:25AM +0900:
I see now that it has IOSchedulingClass=IDLE but nothing equivalent about CPU, but I don't think that'll help: if the computer is mostly idle it'll still use all the cores (and increase cpufreq automatically) for long enough to heat up and make the fans kick in.
I seem to have misremembered this part: it is single core. (doesn't change much as far as cpufreq/heat is concerned though)
Also, it had been a while but it has actually gotten much better after I added /.snapshots and similar paths to PRUNEPATHS, so this isn't as much of a problem as I remembered, but could still be a problem for large filesystems
I don't think there is a laptop friendly scheduler that says if this task is niced then keep the cpufreq at its minimum, is there? I've been manually fiddling with my cpufreqs' scaling_max_freq just low enough to not trigger the fans but it sounds like overengineering to me...
Still interested if anyone knows how to do this!