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

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Date:   Tue, 11 Aug 2020 21:12:17 +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 2020/8/11 下午8:48, Valentin Schneider wrote:
> 
> On 11/08/20 12:44, Qi Zheng wrote:
>> 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cpumask are no longer active at the == case,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?
> 
> 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?