linux-kernel - Re: [PATCH] sched/fair: add protection for delta of wait time

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Date:   Tue, 19 Jan 2021 00:05:40 +0800
From:   Jiang Biao <benbjiang@...il.com>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <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>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Jiang Biao <benbjiang@...cent.com>
Subject: Re: [PATCH] sched/fair: add protection for delta of wait time

Hi, Vincent

On Mon, 18 Jan 2021 at 23:32, Vincent Guittot
<vincent.guittot@...aro.org> wrote:
>
> On Mon, 18 Jan 2021 at 15:11, Jiang Biao <benbjiang@...il.com> wrote:
> >
> > Hi, Vincent
> >
> > On Mon, 18 Jan 2021 at 15:56, Vincent Guittot
> > <vincent.guittot@...aro.org> wrote:
> > >
> > > On Sun, 17 Jan 2021 at 13:31, Jiang Biao <benbjiang@...il.com> wrote:
> > > >
> > > > From: Jiang Biao <benbjiang@...cent.com>
> > > >
> > > > delta in update_stats_wait_end() might be negative, which would
> > > > make following statistics go wrong.
> > >
> > > Could you describe the use case that generates a negative delta ?
> > >
> > > rq_clock is always increasing so this should not lead to a negative
> > > value even if update_stats_wait_end/start are not called in the right
> > > order,
> > Yes, indeed.
> >
> > > This situation could happen after a migration if we forgot to call
> > > update_stats_wait_start
> > The migration case was what I worried about, but no regular use case
> > comes into my mind. :)
>
> IIUC, you haven't faced the problem and it's only based on studying the code.
Not yet. :).  Just found there are protections for
sleep_time/block_time, but no protection
for wait_time.

Think more later, the sleep_time/block_time do need to be protected
for the migration case,
because update_stats_enqueue_sleeper could be called right after
migration with src cpu's
sleep_start/block_start. But wait_time is not the case.

The following case might be too extreme to happen. :)

Thanks a lot for your patience.

Regards,
Jiang

>
> > As an extreme case, would it be a problem if we disable/re-enable
> > sched_schedstats during migration?
> >
> > static inline void
> > update_stats_wait_start(struct cfs_rq *cfs_rq, struct sched_entity *se)
> > {
> >         u64 wait_start, prev_wait_start;
> >
> >         if (!schedstat_enabled()) // disable during migration
> >                 return; // return here, and skip updating wait_start
> > ...
> > }
> >
> > static inline void
> > update_stats_wait_end(struct cfs_rq *cfs_rq, struct sched_entity *se)
> > {
> >         struct task_struct *p;
> >         u64 delta;
> >
> >         if (!schedstat_enabled())  // re-enable again
> >                 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;
> >          //stale wait_start which might be bigger than rq_clock would
> > be used. -)
> >         delta = rq_clock(rq_of(cfs_rq)) -
> > schedstat_val(se->statistics.wait_start);
> > ...
> >
> > Thanks a lot.
> > Regards,
> > Jiang