diff options
| author | Frederic Weisbecker <frederic@kernel.org> | 2023-11-14 14:38:40 -0500 |
|---|---|---|
| committer | Peter Zijlstra <peterz@infradead.org> | 2023-11-15 09:57:52 +0100 |
| commit | 194600008d5c43b5a4ba98c4b81633397e34ffad (patch) | |
| tree | 4421046692e3eac2559f69bf9df605e1bc9db072 /kernel/sched/core.c | |
| parent | dd5403869a40595eb953f12e8cd2bb57bb88bb67 (diff) | |
sched/timers: Explain why idle task schedules out on remote timer enqueue
Trying to avoid that didn't bring much value after testing, add comment
about this.
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Rafael J. Wysocki <rafael@kernel.org>
Link: https://lkml.kernel.org/r/20231114193840.4041-3-frederic@kernel.org
Diffstat (limited to 'kernel/sched/core.c')
| -rw-r--r-- | kernel/sched/core.c | 22 |
1 files changed, 22 insertions, 0 deletions
diff --git a/kernel/sched/core.c b/kernel/sched/core.c index f5f4495d1768..2de77a6d5ef8 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -1131,6 +1131,28 @@ static void wake_up_idle_cpu(int cpu) if (cpu == smp_processor_id()) return; + /* + * Set TIF_NEED_RESCHED and send an IPI if in the non-polling + * part of the idle loop. This forces an exit from the idle loop + * and a round trip to schedule(). Now this could be optimized + * because a simple new idle loop iteration is enough to + * re-evaluate the next tick. Provided some re-ordering of tick + * nohz functions that would need to follow TIF_NR_POLLING + * clearing: + * + * - On most archs, a simple fetch_or on ti::flags with a + * "0" value would be enough to know if an IPI needs to be sent. + * + * - x86 needs to perform a last need_resched() check between + * monitor and mwait which doesn't take timers into account. + * There a dedicated TIF_TIMER flag would be required to + * fetch_or here and be checked along with TIF_NEED_RESCHED + * before mwait(). + * + * However, remote timer enqueue is not such a frequent event + * and testing of the above solutions didn't appear to report + * much benefits. + */ if (set_nr_and_not_polling(rq->idle)) smp_send_reschedule(cpu); else |