linux-kernel - Re: [RFC PATCH] sched: fix the nonsense shares when load of cfs

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Date:   Wed, 4 Mar 2020 09:47:34 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     王贇 <yun.wang@...ux.alibaba.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        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>,
        "open list:SCHEDULER" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH] sched: fix the nonsense shares when load of cfs_rq is
 too, small

On Wed, 4 Mar 2020 at 02:19, 王贇 <yun.wang@...ux.alibaba.com> wrote:
>
>
>
> On 2020/3/4 上午3:52, Peter Zijlstra wrote:
> [snip]
> >> The reason is because we have group B with shares as 2, which make
> >> the group A 'cfs_rq->load.weight' very small.
> >>
> >> And in calc_group_shares() we calculate shares as:
> >>
> >>   load = max(scale_load_down(cfs_rq->load.weight), cfs_rq->avg.load_avg);
> >>   shares = (tg_shares * load) / tg_weight;
> >>
> >> Since the 'cfs_rq->load.weight' is too small, the load become 0
> >> in here, although 'tg_shares' is 102400, shares of the se which
> >> stand for group A on root cfs_rq become 2.
> >
> > Argh, because A->cfs_rq.load.weight is B->se.load.weight which is
> > B->shares/nr_cpus.
>
> Yeah, that's exactly why it happens, even the share 2 scale up to 2048,
> on 96 CPUs platform, each CPU get only 21 in equal case.
>
> >
> >> While the se of D on root cfs_rq is far more bigger than 2, so it
> >> wins the battle.
> >>
> >> This patch add a check on the zero load and make it as MIN_SHARES
> >> to fix the nonsense shares, after applied the group C wins as
> >> expected.
> >>
> >> Signed-off-by: Michael Wang <yun.wang@...ux.alibaba.com>
> >> ---
> >>  kernel/sched/fair.c | 2 ++
> >>  1 file changed, 2 insertions(+)
> >>
> >> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> >> index 84594f8aeaf8..53d705f75fa4 100644
> >> --- a/kernel/sched/fair.c
> >> +++ b/kernel/sched/fair.c
> >> @@ -3182,6 +3182,8 @@ static long calc_group_shares(struct cfs_rq *cfs_rq)
> >>      tg_shares = READ_ONCE(tg->shares);
> >>
> >>      load = max(scale_load_down(cfs_rq->load.weight), cfs_rq->avg.load_avg);
> >> +    if (!load && cfs_rq->load.weight)
> >> +            load = MIN_SHARES;
> >>
> >>      tg_weight = atomic_long_read(&tg->load_avg);
> >
> > Yeah, I suppose that'll do. Hurmph, wants a comment though.
> >
> > But that has me looking at other users of scale_load_down(), and doesn't
> > at least update_tg_cfs_load() suffer the same problem?
>
> Good point :-) I'm not sure but is scale_load_down() supposed to scale small
> value into 0? If not, maybe we should fix the helper to make sure it at
> least return some real load? like:
>
> # define scale_load_down(w) ((w + (1 << SCHED_FIXEDPOINT_SHIFT)) >> SCHED_FIXEDPOINT_SHIFT)

you will add +1 of nice prio for each device

should we use instead
# define scale_load_down(w) ((w >> SCHED_FIXEDPOINT_SHIFT) ? (w >>
SCHED_FIXEDPOINT_SHIFT) : MIN_SHARES)

Regards,
Vincent

>
> Regards,
> Michael Wang
>
> >