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

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Date:   Thu, 23 Feb 2017 20:45:06 +0530
From:   Pavan Kondeti <pkondeti@...eaurora.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Pavan Kondeti <pkondeti@...eaurora.org>,
        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,

On Thu, Feb 23, 2017 at 7:24 PM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Thu, Feb 23, 2017 at 04:04:22PM +0530, Pavan Kondeti wrote:
>> 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.
>
> It should not; the two places of interrests are when we leave the RT
> class to run anything lower (fair,idle), at which point we'll pull,
> or when an RT tasks wakes up, at which point it'll push.

Can you kindly show me where we are pulling when a RT task goes to
sleep? Apart from class/prio change, I see pull happening only from
pick_next_task_rt().

Thanks,
Pavan

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