linux-kernel - Re: [PATCH v7 1/9] sched/fair: fix unfairness at wakeup

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Message-ID: <CAKfTPtDTsG=PmD=CQgAyhV0YfsWKDfgyJU2BSDbc4scUwzS3CQ@mail.gmail.com>
Date:   Mon, 7 Nov 2022 09:00:12 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Chen Yu <yu.c.chen@...el.com>
Cc:     mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com,
        dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
        mgorman@...e.de, bristot@...hat.com, vschneid@...hat.com,
        linux-kernel@...r.kernel.org, parth@...ux.ibm.com,
        qais.yousef@....com, chris.hyser@...cle.com,
        patrick.bellasi@...bug.net, David.Laight@...lab.com,
        pjt@...gle.com, pavel@....cz, tj@...nel.org, qperret@...gle.com,
        tim.c.chen@...ux.intel.com, joshdon@...gle.com, timj@....org,
        kprateek.nayak@....com, youssefesmat@...omium.org,
        joel@...lfernandes.org
Subject: Re: [PATCH v7 1/9] sched/fair: fix unfairness at wakeup

On Sat, 5 Nov 2022 at 15:33, Chen Yu <yu.c.chen@...el.com> wrote:
>
> On 2022-10-28 at 11:33:55 +0200, Vincent Guittot wrote:
> [snip]
> > +static inline unsigned long  get_latency_max(void)
> > +{
> > +     unsigned long thresh = get_sched_latency(false);
> > +
> > +     thresh -= sysctl_sched_min_granularity;
> > +
> May I know why we substract sysctl_sched_min_granularity above?
> I thought thresh -= 1 would be enough to let the waking task preempt
> the current one, because:
> When a long sleeping task is enqueued on this rq, its vruntime is set
> to cfs_rq->min_vtime - latency, so
> diff = curr->vtime - cfs_rq->min_vtime + latency,
> since (curr->vtime - cfs_rq->min_vtime) >= 0, if we set the thresh to
> (latency - 1), the diff is guaranteed to be bigger than thresh and
> the waking task can preempt current task.

If the waking task failed to preempt current it could to wait up to
sysctl_sched_min_granularity before preempting it during next tick.

Vincent

>
> thanks,
> Chenyu
> > +     return thresh;
> > +}