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

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Date:   Tue, 11 Aug 2020 21:16:26 +0100
From:   Valentin Schneider <valentin.schneider@....com>
To:     Qi Zheng <arch0.zheng@...il.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 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.