linux-kernel - Re: [PATCH 1/1] sched/fair: Fix the wrong sched_stat

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Message-ID: <CAKc596Jc6H5qFxrzSp_fGhqTZt-5ORLoP=E4bwG-QzkpRiL2bA@mail.gmail.com>
Date:   Wed, 30 Sep 2020 17:16:29 +0800
From:   jun qian <qianjun.kernel@...il.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     mingo@...hat.com, Vincent Guittot <vincent.guittot@...aro.org>,
        juri.lelli@...hat.com, linux-kernel@...r.kernel.org,
        dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
        mgorman@...e.de, Yafang Shao <laoar.shao@...il.com>
Subject: Re: [PATCH 1/1] sched/fair: Fix the wrong sched_stat_wait time

Peter Zijlstra <peterz@...radead.org> 于2020年9月30日周三 下午4:20写道：
>
> On Wed, Sep 30, 2020 at 10:47:12AM +0800, qianjun.kernel@...il.com wrote:
> > From: jun qian <qianjun.kernel@...il.com>
> >
> > When the sched_schedstat changes from 0 to 1, some sched se maybe
> > already in the runqueue, the se->statistics.wait_start will be 0.
> > So it will let the (rq_of(cfs_rq)) - se->statistics.wait_start)
> > wrong. We need to avoid this scenario.
>
> Is this really the only problem there? Did you do a full audit of that
> schedstat nonsense?
>

Did you mean that the sched_stat_xxx's xxx_start(sched_stat_sleep
sched_stat_iowait sched_stat_blocked
sched_stat_runtime) may be also depend the schedstat_enabled?
I have searched the codes, and found that these sched_stat_xxx's
xxx_start don't depend the schedstat_enabled
except the wait_start.

This patch is going to slove the problem that when the
schedstat_enabled is enabled, the sched_stat_wait of
the probed process will become unbelievable big probability in the fist time.

> > Signed-off-by: jun qian <qianjun.kernel@...il.com>
> > Signed-off-by: Yafang Shao <laoar.shao@...il.com>
> > ---
> >  kernel/sched/fair.c | 8 ++++++++
> >  1 file changed, 8 insertions(+)
> >
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 658aa7a..dd7c3bb 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -908,6 +908,14 @@ static void update_curr_fair(struct rq *rq)
>
> your git-diff is 'funny', it got the function ^ wrong.
>

sorry  :)

> >       if (!schedstat_enabled())
> >               return;
> >
> > +     /*
> > +      * When the sched_schedstat changes from 0 to 1, some sched se maybe
> > +      * already in the runqueue, the se->statistics.wait_start will be 0.
> > +      * So it will let the delta wrong. We need to avoid this scenario.
> > +      */
> > +     if (unlikely(!schedstat_val(se->statistics.wait_start)))
> > +             return;
> > +
> >       delta = rq_clock(rq_of(cfs_rq)) - schedstat_val(se->statistics.wait_start);
> >
> >       if (entity_is_task(se)) {
> > --
> > 1.8.3.1
> >