linux-kernel - Re: [PATCH] sched/fair: Remove the duplicate check from group_has

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Date:   Wed, 12 Aug 2020 08:46:16 +0800
From:   Qi Zheng <arch0.zheng@...il.com>
To:     Valentin Schneider <valentin.schneider@....com>
Cc:     mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sched/fair: Remove the duplicate check from
 group_has_capacity()

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