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

Qi Zheng
2020-08-12T00:46:27Z

On 2020/8/12 上午4:16, Valentin Schneider wrote:
>
> 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.
>

Okay, I will reply to the old patch and add your email to it.
Thanks for your comments.

Yours,
Qi Zheng