linux-kernel - Re: [RFC 5/6] sched/fair: Take into account latency nice at wakeup

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Date:   Wed, 4 May 2022 14:39:10 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Chen Yu <yu.chen.surf@...il.com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        "Peter Zijlstra (Intel)" <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 Mailing List <linux-kernel@...r.kernel.org>,
        parth@...ux.ibm.com, qais.yousef@....com, chris.hyser@...cle.com,
        pkondeti@...eaurora.org,
        Valentin Schneider <valentin.schneider@....com>,
        patrick.bellasi@...bug.net, David.Laight@...lab.com,
        Paul Turner <pjt@...gle.com>, Pavel Machek <pavel@....cz>,
        tj@...nel.org, dhaval.giani@...cle.com, qperret@...gle.com,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Chen Yu <yu.c.chen@...el.com>
Subject: Re: [RFC 5/6] sched/fair: Take into account latency nice at wakeup

On Wed, 4 May 2022 at 13:15, Chen Yu <yu.chen.surf@...il.com> wrote:
>
> On Sat, Mar 12, 2022 at 7:11 AM Vincent Guittot
> <vincent.guittot@...aro.org> wrote:
> >
> > Take into account the nice latency priority of a thread when deciding to
> > preempt the current running thread. We don't want to provide more CPU
> > bandwidth to a thread but reorder the scheduling to run latency sensitive
> > task first whenever possible.
> >
> ---------->8-------------------
> >  #endif /* CONFIG_SMP */
> >
> > +static long wakeup_latency_gran(int latency_weight)
> > +{
> > +       long thresh = sysctl_sched_latency;
> If I understood correctly, this is to consider the latency weight and
> 'shrink/expand'
> current task's time slice thus to facilitate preemption. And may I
> know why don't we use
> __sched_period() but to use sysctl_sched_latency directly? Is it
> possible the rq has
> more than 8(sched_nr_latency) tasks thus the period is longer than
> sysctl_sched_latency?

Main reason is to be aligned with place_entity which also uses
sysctl_sched_latency to cap entity's vruntime to be higher than
min_vruntime-sysctl_sched_latency

>
> Thanks,
> Chenyu
> > +
> > +       if (!latency_weight)
> > +               return 0;
> > +
> > +       if (sched_feat(GENTLE_FAIR_SLEEPERS))
> > +               thresh >>= 1;
> > +
> > +       /*
> > +        * Clamp the delta to stay in the scheduler period range
> > +        * [-sysctl_sched_latency:sysctl_sched_latency]
> > +        */
> > +       latency_weight = clamp_t(long, latency_weight,
> > +                               -1 * NICE_LATENCY_WEIGHT_MAX,
> > +                               NICE_LATENCY_WEIGHT_MAX);
> > +
> > +       return (thresh * latency_weight) >> NICE_LATENCY_SHIFT;
> > +}
> > +