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

Qi Zheng
2020-08-11T11:44:46Z

On 2020/8/11 下午6:38, Valentin Schneider wrote:
>
> On 11/08/20 04:39, Qi Zheng wrote:
>> 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cpumask are no longer active at the == case,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?
>>
>
> That can only happen in that short window between deactivating a CPU and
> not having rebuilt the sched_domains yet, which sounds quite elusive.
>

In fact, at the beginning, I added unlikely() here to hint the compiler:

- if ((sgs->group_capacity * imbalance_pct) <
- (sgs->group_runnable * 100))
+ if (unlikely((sgs->group_capacity * imbalance_pct) <
+ (sgs->group_runnable * 100)))

The corresponding patch is as follows:

[PATCH]sched/core: add unlikely in group_has_capacity()

Do you think it is necessary?