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From:   bsegall@google.com
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     =?utf-8?B?546L6LSH?= <yun.wang@linux.alibaba.com>,
        Ingo Molnar <mingo@redhat.com>,
        Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        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>
Subject: Re: [RFC PATCH] sched: fix the nonsense shares when load of cfs_rq is too, small
References: <44fa1cee-08db-e4ab-e5ab-08d6fbd421d7@linux.alibaba.com>
        <20200303195245.GF2596@hirez.programming.kicks-ass.net>
Date:   Wed, 04 Mar 2020 10:47:12 -0800
In-Reply-To: <20200303195245.GF2596@hirez.programming.kicks-ass.net> (Peter
        Zijlstra's message of "Tue, 3 Mar 2020 20:52:45 +0100")
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Peter Zijlstra <peterz@infradead.org> writes:

> On Tue, Mar 03, 2020 at 10:17:03PM +0800, =E7=8E=8B=E8=B4=87 wrote:
>> During our testing, we found a case that shares no longer
>> working correctly, the cgroup topology is like:
>>=20
>>   /sys/fs/cgroup/cpu/A		(shares=3D102400)
>>   /sys/fs/cgroup/cpu/A/B	(shares=3D2)
>>   /sys/fs/cgroup/cpu/A/B/C	(shares=3D1024)
>>=20
>>   /sys/fs/cgroup/cpu/D		(shares=3D1024)
>>   /sys/fs/cgroup/cpu/D/E	(shares=3D1024)
>>   /sys/fs/cgroup/cpu/D/E/F	(shares=3D1024)
>>=20
>> The same benchmark is running in group C & F, no other tasks are
>> running, the benchmark is capable to consumed all the CPUs.
>>=20
>> We suppose the group C will win more CPU resources since it could
>> enjoy all the shares of group A, but it's F who wins much more.
>>=20
>> The reason is because we have group B with shares as 2, which make
>> the group A 'cfs_rq->load.weight' very small.
>>=20
>> And in calc_group_shares() we calculate shares as:
>>=20
>>   load =3D max(scale_load_down(cfs_rq->load.weight), cfs_rq->avg.load_av=
g);
>>   shares =3D (tg_shares * load) / tg_weight;
>>=20
>> 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.
>
>> While the se of D on root cfs_rq is far more bigger than 2, so it
>> wins the battle.
>>=20
>> 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.
>>=20
>> Signed-off-by: Michael Wang <yun.wang@linux.alibaba.com>
>> ---
>>  kernel/sched/fair.c | 2 ++
>>  1 file changed, 2 insertions(+)
>>=20
>> 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_r=
q)
>>  	tg_shares =3D READ_ONCE(tg->shares);
>>=20
>>  	load =3D max(scale_load_down(cfs_rq->load.weight), cfs_rq->avg.load_av=
g);
>> +	if (!load && cfs_rq->load.weight)
>> +		load =3D MIN_SHARES;
>>=20
>>  	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 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 =3D cfs_rq->tg;
=20
-       tg_shares =3D READ_ONCE(tg->shares);
+       tg_shares =3D scale_load_down(READ_ONCE(tg->shares));
=20
-       load =3D max(scale_load_down(cfs_rq->load.weight), cfs_rq->avg.load=
_avg);
+       load =3D max(cfs_rq->load.weight, scale_load(cfs_rq->avg.load_avg));
=20
        tg_weight =3D atomic_long_read(&tg->load_avg);
=20