[tip: sched/core] sched/timers: Explain why idle task schedules out on remote timer enqueue

[tip-bot2 for Frederic Weisbecker]

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