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, 2 insertions, 0 deletions

diff --git a/sway/main.c b/sway/main.c
index 78974e22..a0033c45 100644
--- a/sway/main.c
+++ b/sway/main.c
@@ -413,6 +413,8 @@ int main(int argc, char **argv) {
                 goto shutdown;
         }
 
+        set_rr_scheduling();
+
         if (!server_start(&server)) {
                 sway_terminate(EXIT_FAILURE);
                 goto shutdown;