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Fix real-time scheduling on Apple Silicon

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On Apple Silicon, the sysctl key hw.cpufrequency is absent because ARM
SoCs have heterogeneous core frequencies. PulseAudio's Darwin real-time
scheduling path used this to compute Mach THREAD_TIME_CONSTRAINT_POLICY
parameters in CPU cycle units, so it silently fails on all M-series Macs.

Replace the CPU-frequency approach with mach_timebase_info(), which
converts nanoseconds to Mach absolute time units regardless of
architecture. The time constraint values are chosen to match the original
intent (~6.25ms period, ~0.3ms computation, ~0.45ms constraint).
This commit is contained in:
Charles Strahan 2026-04-27 13:49:05 -05:00
parent b096704c0d
commit 8f0fea7a8a
1 changed files with 17 additions and 10 deletions
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27
src/pulse/util.c
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@ -86,6 +86,7 @@ static int _main() PA_GCC_WEAKREF(main);
#ifdef __APPLE__
#include <mach/mach_init.h>
#include <mach/mach_time.h>
#include <mach/thread_act.h>
#include <mach/thread_policy.h>
#include <sys/sysctl.h>
@ -464,22 +465,28 @@ int pa_thread_make_realtime(int rtprio) {
#if defined(OS_IS_DARWIN)
struct thread_time_constraint_policy ttcpolicy;
uint64_t freq = 0;
size_t size = sizeof(freq);
mach_timebase_info_data_t tbi;
int ret;
ret = sysctlbyname("hw.cpufrequency", &freq, &size, NULL, 0);
if (ret < 0) {
pa_log_info("Unable to read CPU frequency, acquisition of real-time scheduling failed.");
ret = mach_timebase_info(&tbi);
if (ret != KERN_SUCCESS || tbi.numer == 0) {
pa_log_info("mach_timebase_info() failed, acquisition of real-time scheduling failed.");
return -1;
}
pa_log_debug("sysctl for hw.cpufrequency: %llu", freq);
pa_log_debug("mach_timebase_info: numer=%u denom=%u", tbi.numer, tbi.denom);
/* See http://developer.apple.com/library/mac/#documentation/Darwin/Conceptual/KernelProgramming/scheduler/scheduler.html */
ttcpolicy.period = freq / 160;
ttcpolicy.computation = freq / 3300;
ttcpolicy.constraint = freq / 2200;
/* Convert desired durations from nanoseconds to Mach absolute time units.
* On Apple Silicon hw.cpufrequency is absent (heterogeneous cores), so we
* use mach_timebase_info instead, which works on both Intel and ARM.
* Values match the original intent: period ~6.25ms, computation ~0.3ms,
* constraint ~0.45ms.
* See Apple TN2169 and the Mach scheduler documentation. */
#define NS_TO_ABS(ns) ((uint32_t)((uint64_t)(ns) * tbi.denom / tbi.numer))
ttcpolicy.period = NS_TO_ABS(6250000);
ttcpolicy.computation = NS_TO_ABS(303030);
ttcpolicy.constraint = NS_TO_ABS(454545);
#undef NS_TO_ABS
ttcpolicy.preemptible = 1;
ret = thread_policy_set(mach_thread_self(),