linux-kernel - Re: Scheduling tasks on idle cpu

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Message-ID: <CAKfTPtBtzmgJNM=2ekmE0-HT+r=qLqBJ6R1cggkGCNHbb3FPdQ@mail.gmail.com>
Date:   Thu, 14 Apr 2022 09:54:20 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Qais Yousef <qais.yousef@....com>
Cc:     David Laight <David.Laight@...lab.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "juri.lelli@...hat.com" <juri.lelli@...hat.com>,
        "rostedt@...dmis.org" <rostedt@...dmis.org>,
        "bsegall@...gle.com" <bsegall@...gle.com>,
        "mgorman@...e.de" <mgorman@...e.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "parth@...ux.ibm.com" <parth@...ux.ibm.com>,
        "chris.hyser@...cle.com" <chris.hyser@...cle.com>,
        "pkondeti@...eaurora.org" <pkondeti@...eaurora.org>,
        "Valentin.Schneider@....com" <Valentin.Schneider@....com>,
        "patrick.bellasi@...bug.net" <patrick.bellasi@...bug.net>,
        "pjt@...gle.com" <pjt@...gle.com>, "pavel@....cz" <pavel@....cz>,
        "tj@...nel.org" <tj@...nel.org>,
        "qperret@...gle.com" <qperret@...gle.com>,
        "tim.c.chen@...ux.intel.com" <tim.c.chen@...ux.intel.com>,
        Wei Wang <wvw@...gle.com>
Subject: Re: Scheduling tasks on idle cpu

On Thu, 14 Apr 2022 at 01:57, Qais Yousef <qais.yousef@....com> wrote:
>
> On 04/12/22 11:07, Vincent Guittot wrote:
> > On Tue, 12 Apr 2022 at 10:39, David Laight <David.Laight@...lab.com> wrote:
> > > Yes I want the CFS scheduler to pick an idle cpu in preference
> > > to an active RT one.
> >
> > When task 34512 wakes up, scheduler checks if prev or this cpu are
> > idle which is not the case for you. Then, it compares the load of prev
> > and this_cpu and seems to select this_cpu (cpu17).
> >
> > Once cpu17 selected, it will try to find an idle cpu which shares LLC
> > but it seems that the scheduler didn't find one and finally keeps task
> > 34512 on this_cpu.
> >
> > Note that during the next tick, a load balance will be trigger if
> > this_cpu still have both RT and task 34512,
>
> David said there are idle cpus
>
>         "  There are two physical cpu with 20 cores each (with hyperthreading).
>         16, 18, 34, 36 and 38 were idle. So both 16 and 18 should be on the
>         same NUMA node. All the others are running the same RT thread code.  "
>
> Except for the possibility of them becoming idle just after the task has woken
> up, shouldn't one of them have been picked?

we don't loop on all cpus in the LLC to find an idle one but compute a
reasonable number of iteration based on the avg_idle

David can rerun is use case after disabling sched_feat(SIS_PROP)

>
> Thanks
>
> --
> Qais Yousef