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

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Date:   Tue, 11 Aug 2020 19:44:34 +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cpumask are no longer active at the == case,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?