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

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Message-ID: <20170223152533.GG6515@twins.programming.kicks-ass.net>
Date:   Thu, 23 Feb 2017 16:25:33 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Pavan Kondeti <pkondeti@...eaurora.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

On Thu, Feb 23, 2017 at 08:45:06PM +0530, Pavan Kondeti wrote:
> 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().

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..