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

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Message-ID: <9425382c-2a42-57ca-512d-c93c589dc701@gmail.com>
Date:   Tue, 11 Aug 2020 11:39:52 +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 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cpumask are no longer active at the == case,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?

> 
> 
> For the
> 
>      sgs->sum_nr_running > sgs->group_weight
> 
> case I agree with your patch, there just is that oddity at the == case.
> 
>> So just remove the duplicate check from group_has_capacity().
>>
>> Signed-off-by: Qi Zheng <arch0.zheng@...il.com>
>> ---
>>   kernel/sched/fair.c | 4 ----
>>   1 file changed, 4 deletions(-)
>>
>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> index 2ba8f230feb9..a41903fb327a 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -8234,10 +8234,6 @@ group_has_capacity(unsigned int imbalance_pct, struct sg_lb_stats *sgs)
>>        if (sgs->sum_nr_running < sgs->group_weight)
>>                return true;
>>
>> -	if ((sgs->group_capacity * imbalance_pct) <
>> -			(sgs->group_runnable * 100))
>> -		return false;
>> -
>>        if ((sgs->group_capacity * 100) >
>>                        (sgs->group_util * imbalance_pct))
>>                return true;