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From:   Vincent Guittot <vincent.guittot@linaro.org>
Date:   Tue, 20 Apr 2021 11:04:57 +0200
Message-ID: <CAKfTPtDjEMJeoZgE1an9Nh9QZPc2gJetsZHL+8QAWzqX5_znvw@mail.gmail.com>
Subject: Re: [PATCH v2] sched,fair: skip newidle_balance if a wakeup is pending
To:     Rik van Riel <riel@surriel.com>
Cc:     linux-kernel <linux-kernel@vger.kernel.org>,
        Kernel Team <kernel-team@fb.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@kernel.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Mel Gorman <mgorman@suse.de>,
        Valentin Schneider <valentin.schneider@arm.com>
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On Mon, 19 Apr 2021 at 18:51, Rik van Riel <riel@surriel.com> wrote:
>
> The try_to_wake_up function has an optimization where it can queue
> a task for wakeup on its previous CPU, if the task is still in the
> middle of going to sleep inside schedule().
>
> Once schedule() re-enables IRQs, the task will be woken up with an
> IPI, and placed back on the runqueue.
>
> If we have such a wakeup pending, there is no need to search other
> CPUs for runnable tasks. Just skip (or bail out early from) newidle
> balancing, and run the just woken up task.
>
> For a memcache like workload test, this reduces total CPU use by
> about 2%, proportionally split between user and system time,
> and p99 and p95 application response time by 2-3% on average.
> The schedstats run_delay number shows a similar improvement.
>
> Signed-off-by: Rik van Riel <riel@surriel.com>
> ---
> v2:
>  - fix !SMP build error and prev-not-CFS case by moving check into newidle_balance
>  - fix formatting of if condition
>  - audit newidle_balance return value use to make sure we get that right
>  - reset idle_stamp when breaking out of the loop due to ->ttwu_pending
>
>  kernel/sched/fair.c | 13 +++++++++++--
>  1 file changed, 11 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 69680158963f..5e26f013e182 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -10594,6 +10594,14 @@ static int newidle_balance(struct rq *this_rq, struct rq_flags *rf)
>         u64 curr_cost = 0;
>
>         update_misfit_status(NULL, this_rq);
> +
> +       /*
> +        * There is a task waiting to run. No need to search for one.
> +        * Return 0; the task will be enqueued when switching to idle.
> +        */
> +       if (this_rq->ttwu_pending)
> +               return 0;
> +
>         /*
>          * We must set idle_stamp _before_ calling idle_balance(), such that we
>          * measure the duration of idle_balance() as idle time.
> @@ -10661,7 +10669,8 @@ static int newidle_balance(struct rq *this_rq, struct rq_flags *rf)
>                  * Stop searching for tasks to pull if there are
>                  * now runnable tasks on this rq.
>                  */
> -               if (pulled_task || this_rq->nr_running > 0)
> +               if (pulled_task || this_rq->nr_running > 0 ||
> +                   this_rq->ttwu_pending)
>                         break;
>         }
>         rcu_read_unlock();
> @@ -10688,7 +10697,7 @@ static int newidle_balance(struct rq *this_rq, struct rq_flags *rf)
>         if (this_rq->nr_running != this_rq->cfs.h_nr_running)
>                 pulled_task = -1;
>
> -       if (pulled_task)
> +       if (pulled_task || this_rq->ttwu_pending)

This needs at least a comment to explain why we must clear
this_rq->idle_stamp when this_rq->ttwu_pending is set whereas it is
also done during sched_ttwu_pending()

>                 this_rq->idle_stamp = 0;
>
>         rq_repin_lock(this_rq, rf);
> --
> 2.25.4
>
>