linux-kernel - Re: [PATCH v8 1/9] sched/fair: fix unfairness at wakeup

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Message-ID: <CAKfTPtDnigeLQsBWU_iSPvxMbe2mpA4t1Y215WEZ4jPxjRvLGg@mail.gmail.com>
Date:   Mon, 14 Nov 2022 17:46:05 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Patrick Bellasi <patrick.bellasi@...bug.net>
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,
        qyousef@...alina.io, chris.hyser@...cle.com,
        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,
        yu.c.chen@...el.com, youssefesmat@...omium.org,
        joel@...lfernandes.org
Subject: Re: [PATCH v8 1/9] sched/fair: fix unfairness at wakeup

On Mon, 14 Nov 2022 at 17:20, Patrick Bellasi
<patrick.bellasi@...bug.net> wrote:
>
> Hi Vincent!
>
> On 10-Nov 18:50, Vincent Guittot wrote:
>
> [...]
>
> > diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> > index 1fc198be1ffd..14879d429919 100644
> > --- a/kernel/sched/sched.h
> > +++ b/kernel/sched/sched.h
> > @@ -2432,9 +2432,9 @@ extern void check_preempt_curr(struct rq *rq, struct task_struct *p, int flags);
> >  extern const_debug unsigned int sysctl_sched_nr_migrate;
> >  extern const_debug unsigned int sysctl_sched_migration_cost;
> >
> > -#ifdef CONFIG_SCHED_DEBUG
> >  extern unsigned int sysctl_sched_latency;
> >  extern unsigned int sysctl_sched_min_granularity;
> > +#ifdef CONFIG_SCHED_DEBUG
> >  extern unsigned int sysctl_sched_idle_min_granularity;
> >  extern unsigned int sysctl_sched_wakeup_granularity;
> >  extern int sysctl_resched_latency_warn_ms;
> > @@ -2448,6 +2448,34 @@ extern unsigned int sysctl_numa_balancing_scan_period_max;
> >  extern unsigned int sysctl_numa_balancing_scan_size;
> >  #endif
> >
> > +static inline unsigned long  get_sched_latency(bool idle)
>                                 ^^^^^^^^^^^^^^^^^
>
> This can be confusing since it's not always returning the sysctl_sched_latency
> value. It's also being used to tune the vruntime at wakeup time.
>
> Thus, what about renaming this to something more close to what's used for, e.g.
>    get_wakeup_latency(se)
> ?
>
> Also, in the following patches we call this always with a false parametr.
> Thus, perhaps in a following patch, we can better add something like:
>    #define max_wakeup_latency get_wakeup_latency(false)
> ?

I'm going to rename get_wakeup_latency by get_sleep_latency() as
proposed earlier.

I don't see the benefit of adding a macro of top so will keep the parameter

>
> > +{
> > +     unsigned long thresh;
> > +
> > +     if (idle)
> > +             thresh = sysctl_sched_min_granularity;
> > +     else
> > +             thresh = sysctl_sched_latency;
> > +
> > +     /*
> > +      * Halve their sleep time's effect, to allow
> > +      * for a gentler effect of sleepers:
> > +      */
> > +     if (sched_feat(GENTLE_FAIR_SLEEPERS))
> > +             thresh >>= 1;
> > +
> > +     return thresh;
> > +}
> > +
> > +static inline unsigned long  get_latency_max(void)
>                                 ^^^^^^^^^^^^^^^
>
> This is always used to cap some form of vruntime deltas in:
>  - check_preempt_tick()
>  - wakeup_latency_gran()
>  - wakeup_preempt_entity()
> It's always smaller then the max_wakeup_latency (as defined above).
>
> Thus, does not seems something like:
>    wakeup_latency_threshold()
> a better documenting naming?
>
> > +{
> > +     unsigned long thresh = get_sched_latency(false);
> > +
> > +     thresh -= sysctl_sched_min_granularity;
> > +
> > +     return thresh;
> > +}
>
> [...]
>
> Best,
> Patrick
>
> --
> #include <best/regards.h>
>
> Patrick Bellasi
>