Re: [PATCH] sched/fair: Remove the duplicate check from group_has_capacity()

Qi Zheng
2020-08-11T03:40:01Z

On 2020/8/11 上午2:33, Valentin Schneider wrote:
>
> On 10/08/20 02:00, Qi Zheng wrote:
>> 1. The group_has_capacity() function is only called in
>> group_classify().
>> 2. The following inequality has already been checked in
>> group_is_overloaded() which was also called in
>> group_classify().
>>
>> (sgs->group_capacity * imbalance_pct) <
>> (sgs->group_runnable * 100)
>>
>
> Consider group_is_overloaded() returns false because of the first
> condition:
>
> if (sgs->sum_nr_running <= sgs->group_weight)
> return false;
>
> then group_has_capacity() would be the first place where the group_runnable
> vs group_capacity comparison would be done.
>
> Now in that specific case we'll actually only check it if
>
> sgs->sum_nr_running == sgs->group_weight
>
> and the only case where the runnable vs capacity check can fail here is if
> there's significant capacity pressure going on. TBH this capacity pressure
> could be happening even when there are fewer tasks than CPUs, so I'm not
> sure how intentional that corner case is.

Maybe some cpus in sg->cpumask are no longer active at the == case,
which causes the significant capacity pressure?

>
>
> For the
>
> sgs->sum_nr_running > sgs->group_weight
>
> case I agree with your patch, there just is that oddity at the == case.
>
>> So just remove the duplicate check from group_has_capacity().
>>
>> Signed-off-by: Qi Zheng <arch0.zheng@gmail.com>
>> ---
>> kernel/sched/fair.c | 4 ----
>> 1 file changed, 4 deletions(-)
>>
>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> index 2ba8f230feb9..a41903fb327a 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -8234,10 +8234,6 @@ group_has_capacity(unsigned int imbalance_pct, struct sg_lb_stats *sgs)
>> if (sgs->sum_nr_running < sgs->group_weight)
>> return true;
>>
>> - if ((sgs->group_capacity * imbalance_pct) <
>> - (sgs->group_runnable * 100))
>> - return false;
>> -
>> if ((sgs->group_capacity * 100) >
>> (sgs->group_util * imbalance_pct))
>> return true;