Every Linux host to date ran its capture/encode/send threads at nice 0:
boost_thread_priority's setpriority() needs CAP_SYS_NICE or a raised
RLIMIT_NICE, no install channel granted either, and the host binary can
never carry a file capability (a capped process's /proc/<pid>/exe is
unreadable to KWin — the 0.26.0-1 incident). A 2026-08-14 field log
showed the cost end to end: a fresh game launch's shader-compile storm
descheduled the unprioritized threads, 5 ms audio datagrams left late
enough to stutter, the client's OWD signal rose, and ABR cut a
gigabit-Ethernet session to its 5 Mbps floor at zero loss — while the
same box carried 708 Mbps cleanly once the storm passed.
The renice now falls back to RealtimeKit (MakeThreadHighPriorityWithPID,
one blocking system-bus call per boosted thread) — the same unprivileged
broker PipeWire clients use, so nothing enters the permitted set and
KWin identification is untouched. Only the nice verb, never
MakeThreadRealtime: the SCHED_RR reservations apply to rtkit-granted RR
too. zbus rides ashpd's exact backend choice (tokio, no async-io) plus
blocking-api, so the resolved graph gains no second I/O backend.
And the audio plane is boosted for the first time: the 5 ms Opus
capture->encode->send loop (critical — a stall there is directly
audible), the PipeWire capture mainloop thread (its process callbacks
run there; PipeWire's own module-rt only covers data loops we don't
use), and the pad-audio streamer (above-normal, like the session send
thread). The first two had no boost call at all; on Windows the
audio_thread boost also engages, via the SetThreadPriority arm.
enricobuehler
b21b2f6ce9