linux-kernel - Re: [PATCH] sched,fair: skip newidle

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Date:   Mon, 19 Apr 2021 14:12:01 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Rik van Riel <riel@...riel.com>
Cc:     linux-kernel <linux-kernel@...r.kernel.org>,
        Kernel Team <kernel-team@...com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Mel Gorman <mgorman@...e.de>
Subject: Re: [PATCH] sched,fair: skip newidle_balance if a wakeup is pending

On Mon, 19 Apr 2021 at 04:18, Rik van Riel <riel@...riel.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@...riel.com>
> ---
>  kernel/sched/fair.c | 11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 69680158963f..19a92c48939f 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -7163,6 +7163,14 @@ done: __maybe_unused;
>         if (!rf)
>                 return NULL;
>
> +       /*
> +        * We have a woken up task pending here. No need to search for ones
> +        * elsewhere. This task will be enqueued the moment we unblock irqs
> +        * upon exiting the scheduler.
> +        */
> +       if (rq->ttwu_pending)
> +               return NULL;

Would it be better to put this check at the beg of newidle_balance() ?
If prev is not a cfs task, we never reach this point but instead use the path:
class->balance => balance_fair => newidle_balance

and we will not check for rq->ttwu_pending

> +
>         new_tasks = newidle_balance(rq, rf);
>
>         /*
> @@ -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();
> --
> 2.25.4
>
>