[10/10] sched/timers: Explain why idle task schedules out on remote timer enqueue
Commit Message
Trying to avoid that didn't bring much value after testing, add comment
about this.
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
---
kernel/sched/core.c | 22 ++++++++++++++++++++++
1 file changed, 22 insertions(+)
Comments
On Fri, Aug 11, 2023 at 7:01 PM Frederic Weisbecker <frederic@kernel.org> wrote:
>
> Trying to avoid that didn't bring much value after testing, add comment
> about this.
>
> Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Acked-by: Rafael J. Wysocki <rafael@kernel.org>
> ---
> kernel/sched/core.c | 22 ++++++++++++++++++++++
> 1 file changed, 22 insertions(+)
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index c52c2eba7c73..e53b892167ad 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -1135,6 +1135,28 @@ 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
> --
> 2.34.1
>
@@ -1135,6 +1135,28 @@ 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