Add native SCHED_FIFO promotion for the Linux build without dbus#45
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HEnquist wants to merge 5 commits into
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Add native SCHED_FIFO promotion for the Linux build without dbus#45HEnquist wants to merge 5 commits into
HEnquist wants to merge 5 commits into
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The Linux build without the `dbus` feature previously fell through to the blanket fallback, where promotion was a no-op that returned Ok, so callers believed they had real-time priority when they did not. Give that build a real implementation: promote the current thread directly with pthread_setschedparam(SCHED_FIFO | SCHED_RESET_ON_FORK) at priority 10, restoring the saved policy on demotion. This needs no D-Bus daemon and works whenever the process may request real-time scheduling (root, CAP_SYS_NICE, or an RLIMIT_RTPRIO budget). The rtkit path and other platforms are untouched. - New rt_linux_native.rs with the native backend, wired into a dedicated cfg branch after the rtkit branch. - The requested priority defaults to 10 and can be overridden with the AUDIO_RT_PRIORITY environment variable (1-99). - Tests exercise the RLIMIT_RTPRIO boundary and the priority override; the fallback CI job raises RLIMIT_RTPRIO so promotion runs for real. - Docs: add a Platforms section and correct API docs that assumed D-Bus was the only Linux backend.
A library reading an environment variable it was never told about is a hidden global input that can surprise the embedding application. Replace it with an explicit, optional setter that the caller drives: set_rt_priority(Some(n)) overrides the default of 10, None restores it. Linux no-dbus only; if never called, promotion uses priority 10 as before.
- Drop the `param` field from RtPriorityThreadInfoInternal so the struct has no padding, matching the rtkit path's struct. `serialize`'s transmute no longer reads uninitialized padding bytes (which was UB). Demotion now restores the saved policy with a zeroed param, as the rtkit path already does. - rt_scheduling_available: treat a demotion failure after a successful promotion as fatal rather than ignoring it. - Fix a stale test comment that referred to the removed environment variable.
padenot
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Jul 15, 2026
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Looks good, one small question before we can merge. I'll tag a release when it's done.
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| /// Promote a thread identified by its tid to real-time priority. Promoting a thread other than the | ||
| /// caller (in particular in another process) requires the caller to be privileged. |
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How does this works in light of the comment above that says that RtPriorityThreadInfoInternal's underlying values are only meaningful from the same process?
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Great catch. I'm traveling right now without a computer, so I will fix the comment and push once I'm back home.
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Pushed the fix in d4689e1. You're right, that comment was wrong. The struct is meant to be usable across processes: the thread is identified by its system-wide tid (from gettid), which is what promote_thread_to_real_time passes to sched_setscheduler, so a suitably privileged process can promote it. Corrected the comment to match.
It claimed the thread info was only meaningful within the same process, which contradicts promote_thread_to_real_time_internal promoting a thread in another process. The thread is identified by its system-wide tid, so cross-process promotion works; correct the comment to say so.
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Give the Linux no-
dbusbuild a real implementation instead of a no-op:promote the current thread directly with
pthread_setschedparam(SCHED_FIFO).The rtkit path and other platforms are untouched. See #44 for the rationale.
rt_linux_native.rs, wired in via a dedicated cfg branch after the rtkit branchset_rt_priority()(1-99)SCHED_RESET_ON_FORKset, which anunprivileged thread cannot clear)
RLIMIT_RTPRIOboundary and the priority override; the fallback CIjob raises
RLIMIT_RTPRIOso promotion runs for realLinux backend
Edit: the priority override was originally an
AUDIO_RT_PRIORITYenvironment variable; it is nowthe optional
set_rt_priority()method instead (a library should not read env vars on its own).