linux-kernel - Re: [PATCH] sched/fair: consider sched-idle CPU when selecting idle core

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Date:   Fri, 24 Jul 2020 14:36:44 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Jiang Biao <benbjiang@...il.com>
Cc:     Vincent Guittot <vincent.guittot@...aro.org>,
        Jiang Biao <humjb_1983@....com>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Jiang Biao <benbjiang@...cent.com>
Subject: Re: [PATCH] sched/fair: consider sched-idle CPU when selecting idle
 core


* Jiang Biao <benbjiang@...il.com> wrote:

> On Fri, 24 Jul 2020 at 18:34, Vincent Guittot
> <vincent.guittot@...aro.org> wrote:
> >
> > On Fri, 24 Jul 2020 at 10:12, Jiang Biao <benbjiang@...il.com> wrote:
> > >
> > > On Fri, 24 Jul 2020 at 15:24, Vincent Guittot
> > > <vincent.guittot@...aro.org> wrote:
> > > >
> > > > On Fri, 24 Jul 2020 at 01:39, Jiang Biao <humjb_1983@....com> wrote:
> > > > >
> > > > > From: Jiang Biao <benbjiang@...cent.com>
> > > > >
> > > > > Sched-idle CPU has been considered in select_idle_cpu and
> > > > > select_idle_smt, it also needs to be considered in select_idle_core to
> > > > > be consistent and keep the same *idle* policy.
> > > >
> > > > In the case of select_idle_core, we are looking for a core that is
> > > > fully idle but if one CPU of the core is running a sched_idle task,
> > > > the core will not be idle and we might end up having  the wakeup task
> > > > on a CPU and a sched_idle task on another CPU of the core which is not
> > > > what we want
> > > Got it. sched_idle task may interfere its sibling, which brings me
> > > another question,
> > > If there's a core with smt1 running sched_idle task and smt2 idle,
> > > selecting smt1
> > > rather than smt2 should be more helpful for wakee task, because wakee task
> > > could suppress the sched_idle task without neighbour interfering.
> >
> > But the sched_idle will then probably quickly move on the idle smt2
> >
> > > And there seems to be no consideration about that currently.
> > > Is it worth improving that?
> >
> > This will complexify and extend the duration of the search loop  and
> > as mentioned above, it will most probably be a nop at the end because
> > of sched_idle task moving on smt2
> Indeed, the complexity is not worth.
> Thanks for the explanation.

BTW., if you disagree then you could add a bit of debug 
instrumentation to measure to what extent it's a nop at the end of the 
search loop, to turn the "most probably" statement into a specific 
number.

Thanks,

	Ingo