The following commit has been merged into the sched/core branch of tip: Commit-ID: 194600008d5c43b5a4ba98c4b81633397e34ffad Gitweb: https://git.kernel.org/tip/194600008d5c43b5a4ba98c4b81633397e34ffad Author: Frederic Weisbecker <frederic@xxxxxxxxxx> AuthorDate: Tue, 14 Nov 2023 14:38:40 -05:00 Committer: Peter Zijlstra <peterz@xxxxxxxxxxxxx> CommitterDate: Wed, 15 Nov 2023 09:57:52 +01:00 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@xxxxxxxxxx> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx> Acked-by: Rafael J. Wysocki <rafael@xxxxxxxxxx> Link: https://lkml.kernel.org/r/20231114193840.4041-3-frederic@xxxxxxxxxx --- kernel/sched/core.c | 22 ++++++++++++++++++++++ 1 file changed, 22 insertions(+) diff --git a/kernel/sched/core.c b/kernel/sched/core.c index f5f4495..2de77a6 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
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