Re: [RFC PATCH] sched: fix the nonsense shares when load of cfs_rq is too, small

[王贇 <yun.wang@linux.alibaba.com>]

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@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 *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)

Regards,
Michael Wang

>