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

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Message-ID: <CAEU1=PnoHKiBQyk7ArRbci4BFokhyr-9aazSHTMm9YbW8PHrQQ@mail.gmail.com>
Date:   Thu, 23 Feb 2017 22:59:15 +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

On Thu, Feb 23, 2017 at 10:07 PM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Thu, Feb 23, 2017 at 04:25:33PM +0100, Peter Zijlstra wrote:
>>
>> Ah, I read your question wrong. Yes I think you're right, we now loose
>> the pull when the last RT task goes away.
>>
>> Hmm.. how to fix that nicely..
>
> Something like so perhaps? This would make a pull happen when the last
> RT task on this CPU goes away.
>
> Steve?
>
> ---
> diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
> index 9f3e40226dec..283d591078b0 100644
> --- a/kernel/sched/rt.c
> +++ b/kernel/sched/rt.c
> @@ -1336,6 +1336,9 @@ static void dequeue_task_rt(struct rq *rq, struct task_struct *p, int flags)
>         dequeue_rt_entity(rt_se, flags);
>
>         dequeue_pushable_task(rq, p);
> +
> +       if (!rq->rt.rt_nr_running)
> +               queue_pull_task(rq);
>  }
>
>  /*

The next balance_callback() is not called until the context switch is
completed. So we potentially pick a lower class task before the pull
happens. Would it be wrong to call pull_rt_task() directly instead of
queuing the callback.


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