core: don't check RLIMIT_RTPRIO

[yshui]

FreeBSD doesn't have RLIMIT_RTPRIO. So instead we skip this check and
just always try to set our priority to the lowest SCHED_RR priority
available.

Fixes #1082

Signed-off-by: Yuxuan Shui yshuiv7@gmail.com

[absolutelynothelix]

the approach taken to mitigate the mentioned issue looks good to me, i like it.

however, i have a few concerns about the scheduling stuff overall:

1. you’ve added a message suggesting to give picom the CAP_SYS_NICE capability when picom fails to set the priority. iirc, it failed for me from the very beginning but i didn’t know why and have never looked into it. now it looks like that i was missing this capability. does this work at all without it? because if it doesn’t i don’t really like the idea of giving any kind of elevated permissions to a compositor since i can’t imagine a scenario where it actually needs them;
2. i missed the exact moment when all this scheduling stuff was introduced with frame pacing so it’d be good to add a comment briefly explaining what this functions does and the most importantly why do we need it.

[yshui]

@absolutelynothelix reason for the scheduling priority is explained in: 7d96923 I could add a comment for that.

no, CAP_SYS_NICE is not mandatory, you can also raise your ulimit -r. picom needs the permission to enable realtime scheduling strategy.

[absolutelynothelix]

@yshui, ah, i thought that it’s a small part of a big mechanism (frame pacing) but it seems that it’s a standalone useful addition. the commit description makes it clear. i’d use it as a comment describing the set_rr_scheduling function and add a note about CAP_SYS_NICE and ulimit -r.

[codecov]

Codecov Report

Merging #1083 (f8cdc81) into next (b852723) will increase coverage by 0.14%.
The diff coverage is 83.33%.

@@            Coverage Diff             @@
##             next    #1083      +/-   ##
==========================================
+ Coverage   37.53%   37.67%   +0.14%     
==========================================
  Files          49       49              
  Lines       11169    11168       -1     
==========================================
+ Hits         4192     4208      +16     
+ Misses       6977     6960      -17     

Impacted Files	Coverage Δ
src/picom.c	62.09% <83.33%> (+0.63%)	⬆️

... and 2 files with indirect coverage changes