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

Valentin Schneider
2020-08-11T20:16:33Z

On 11/08/20 14:12, Qi Zheng wrote:
> On 2020/8/11 下午8:48, Valentin Schneider wrote:
>> On 11/08/20 12:44, Qi Zheng wrote:
>>> 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?
>>
>> The "unlikely" approach has the benefit of keeping all corner cases in
>> place. I was tempted to say it could still make sense to get rid of the
>> extra check entirely, given that it has an impact only when:
>>
>> - sum_nr_running == group_weight
>> - group capacity has been noticeably reduced
>>
>> If sum_nr_running < group_weight, we won't evaluate it.
>> If sum_nr_running > group_weight, we either won't call into
>> group_has_capacity() or we'll have checked it already in
>> group_overloaded().
>>
>> That said, it does make very much sense to check it in that ==
>> case. Vincent might have a different take on this, but right now I'd say
>> the unlikely approach is the safest one of the two.
>>
>
> So what should I do next? Do I resubmit a patch with unlikely() or
> add your email to the old patch([PATCH]sched/core: add unlikely in
> group_has_capacity())? Or continue to wait for suggestions from
> other maintainers?

I guess you can add a reply to the original thread where you had the
unlikely() to point out *removing* the check isn't 100% harmless.

Vincent might want to have a look at it, but AFAIA he's on holidays ATM.