linux-kernel - Re: [PATCH] sched: Optimize pick_next_task for idle_sched

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Message-ID: <CAEU1=P=zZuYb-0W861Ek8wrqvZGfx1aVXC9_x4T8cyJptoPZWA@mail.gmail.com>
Date:   Thu, 23 Feb 2017 16:04:22 +0530
From:   Pavan Kondeti <pkondeti@...eaurora.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] sched: Optimize pick_next_task for idle_sched_class too

Hi Peter,

> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 49ce1cb..51ca21e 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -3321,15 +3321,14 @@ static inline void schedule_debug(struct task_struct *prev)
>  static inline struct task_struct *
>  pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
>  {
> -       const struct sched_class *class = &fair_sched_class;
> +       const struct sched_class *class;
>         struct task_struct *p;
>
>         /*
>          * Optimization: we know that if all tasks are in
>          * the fair class we can call that function directly:
>          */
> -       if (likely(prev->sched_class == class &&
> -                  rq->nr_running == rq->cfs.h_nr_running)) {
> +       if (likely(rq->nr_running == rq->cfs.h_nr_running)) {
>                 p = fair_sched_class.pick_next_task(rq, prev, rf);
>                 if (unlikely(p == RETRY_TASK))
>                         goto again;

Would this delay pulling RT tasks from other CPUs? Lets say this CPU
has 2 fair tasks and 1 RT task. The RT task is sleeping now. Earlier,
we attempt to pull RT tasks from other CPUs in pick_next_task_rt(),
which is not done anymore.

Thanks,
Pavan
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