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

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Date:   Thu, 5 Mar 2020 08:53:48 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     王贇 <yun.wang@...ux.alibaba.com>
Cc:     Ben Segall <bsegall@...gle.com>,
        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>,
        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 Thu, 5 Mar 2020 at 02:14, 王贇 <yun.wang@...ux.alibaba.com> wrote:
>
>
>
> On 2020/3/5 上午2:47, bsegall@...gle.com wrote:
> [snip]
> >> Argh, because A->cfs_rq.load.weight is B->se.load.weight which is
> >> B->shares/nr_cpus.
> >>
> >>> 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?
> >
> > I think instead we should probably scale_load_down(tg_shares) and
> > scale_load(load_avg). tg_shares is always a scaled integer, so just
> > moving the source of the scaling in the multiply should do the job.
> >
> > ie
> >
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index fcc968669aea..6d7a9d72d742 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -3179,9 +3179,9 @@ static long calc_group_shares(struct cfs_rq *cfs_rq)
> >         long tg_weight, tg_shares, load, shares;
> >         struct task_group *tg = cfs_rq->tg;
> >
> > -       tg_shares = READ_ONCE(tg->shares);
> > +       tg_shares = scale_load_down(READ_ONCE(tg->shares));
> >
> > -       load = max(scale_load_down(cfs_rq->load.weight), cfs_rq->avg.load_avg);
> > +       load = max(cfs_rq->load.weight, scale_load(cfs_rq->avg.load_avg));
> >
> >         tg_weight = atomic_long_read(&tg->load_avg);
>
> Get the point, but IMHO fix scale_load_down() sounds better, to
> cover all the similar cases, let's first try that way see if it's
> working :-)

The problem with this solution is that the avg.load_avg of gse or
cfs_rq might stay to 0 because it uses
scale_load_down(se/cfs_rq->load.weight)

>
> Regards,
> Michael Wang
>
> >
> >
> >