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From:   Vincent Guittot <vincent.guittot@linaro.org>
Date:   Wed, 4 Mar 2020 10:43:51 +0100
Message-ID: <CAKfTPtCnwUKCNbmGR-oErNrF+H+D0FPZPVS=d4m3mvr8Hc7ivQ@mail.gmail.com>
Subject: Re: [RFC PATCH] sched: fix the nonsense shares when load of cfs_rq is
 too, small
To:     =?UTF-8?B?546L6LSH?= <yun.wang@linux.alibaba.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>,
        Juri Lelli <juri.lelli@redhat.com>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
        "open list:SCHEDULER" <linux-kernel@vger.kernel.org>
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On Wed, 4 Mar 2020 at 09:47, Vincent Guittot <vincent.guittot@linaro.org> w=
rote:
>
> On Wed, 4 Mar 2020 at 02:19, =E7=8E=8B=E8=B4=87 <yun.wang@linux.alibaba.c=
om> wrote:
> >
> >
> >
> > On 2020/3/4 =E4=B8=8A=E5=8D=883:52, 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 =3D max(scale_load_down(cfs_rq->load.weight), cfs_rq->avg.loa=
d_avg);
> > >>   shares =3D (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@linux.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 *c=
fs_rq)
> > >>      tg_shares =3D READ_ONCE(tg->shares);
> > >>
> > >>      load =3D max(scale_load_down(cfs_rq->load.weight), cfs_rq->avg.=
load_avg);
> > >> +    if (!load && cfs_rq->load.weight)
> > >> +            load =3D MIN_SHARES;
> > >>
> > >>      tg_weight =3D 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 does=
n'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)) >> SCH=
ED_FIXEDPOINT_SHIFT)
>
> you will add +1 of nice prio for each device

Of course, it's not prio but only weight which is different

>
> should we use instead
> # define scale_load_down(w) ((w >> SCHED_FIXEDPOINT_SHIFT) ? (w >>
> SCHED_FIXEDPOINT_SHIFT) : MIN_SHARES)
>
> Regards,
> Vincent
>
> >
> > Regards,
> > Michael Wang
> >
> > >