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

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Date:   Wed, 30 Sep 2020 21:08:05 +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,
        Benjamin Segall <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5:57写道：
>
> On Wed, Sep 30, 2020 at 05:16:29PM +0800, jun qian wrote:
> > 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?
>
> Yeah, this runtime schedstat_enabled thing is fairly recent, it used to
> be an always on/off kinda thing.
>
> At the time we figured inconsistencies from dynamically
> enabling/disabling it were okay, it's just stats after all.
>
> But if you now want to 'fix' that, then a full audit might be nice.
>
> > 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.
>
> OK, so you did the audit and only found this one issue? That's good
> Changelog material :-)
>

I found another problem, when the sched_schedstat changes from 1 to 0,
the sched se
which is already in the runqueue, the statistics.wait_start already
has a value. At this moment
sched_schedstat changes from 0 to 1 again, the (rq_of(cfs_rq)) -
se->statistics.wait_start)
will not be 0 and the wait time will be bigger than the real one. So
we need to modify the  patch
to resolve the problem with this scenario.

So I really need a full audit might, ：-)

Thanks

> Thanks!