realtime: request SCHED_RR using CAP_SYS_NICE

Try to gain SCHED_RR (round-robin) realtime scheduling privileges before
starting the server. This requires CAP_SYS_NICE on Linux systems.
We additionally register a pthread_atfork callback which resets the
scheduling class back to SCHED_OTHER (the Linux system default).

Due to CAP_SYS_NICE, setting RLIMIT_RTPRIO has no effect on the process
as documented within man 7 sched (from Linux):

  Privileged (CAP_SYS_NICE) threads ignore the RLIMIT_RTPRIO limit;
  as with older kernels, they can make arbitrary changes to
  scheduling policy and priority. See getrlimit(2) for further
  information on RLIMIT_RTPRIO

Note that this requires the sway distribution packagers to set the
CAP_SYS_NICE capability on the sway binary.

Supersedes #6992

1 files changed, 1 insertions, 0 deletions

diff --git a/sway/meson.build b/sway/meson.build
index 0ad783e3..6762ef2d 100644
--- a/sway/meson.build
+++ b/sway/meson.build
@@ -7,6 +7,7 @@ sway_sources = files(
         'ipc-server.c',
         'lock.c',
         'main.c',
+        'realtime.c',
         'server.c',
         'swaynag.c',
         'xdg_activation_v1.c',