Commit 19460000 authored by Frederic Weisbecker's avatar Frederic Weisbecker Committed by Peter Zijlstra
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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: default avatarFrederic Weisbecker <frederic@kernel.org>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: default avatarRafael J. Wysocki <rafael@kernel.org>
Link: https://lkml.kernel.org/r/20231114193840.4041-3-frederic@kernel.org
parent dd540386

kernel/sched/core.c

+22 −0
Original line number Diff line number Diff line
@@ -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