linux-kernel - Re: [PATCH] sched/core: add unlikely in group_has

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Date:   Fri, 7 Aug 2020 10:47:54 +0800
From:   Qi Zheng <arch0.zheng@...il.com>
To:     Ingo Molnar <mingo@...nel.org>
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/core: add unlikely in group_has_capacity()

Yeah, because of the following two points, I also think
the probability is 0%:
a) the sd is protected by rcu lock, and load_balance()
    func is between rcu_read_lock() and rcu_read_unlock().
b) the sgs is a local variable.
	
So in the group_classify(), the env->sd->imbalance_pct and
the sgs will not be changed. May I remove the duplicate check
from group_has_capacity() and resubmit a patch?

Yours,
Qi Zheng

On 2020/8/6 下午10:45, Ingo Molnar wrote:
> 
> * Qi Zheng <arch0.zheng@...il.com> wrote:
> 
>> 1. The group_has_capacity() function is only called in
>>     group_classify().
>> 2. Before calling the group_has_capacity() function,
>>     group_is_overloaded() will first judge the following
>>     formula, if it holds, the group_classify() will directly
>>     return the group_overloaded.
>>
>> 	(sgs->group_capacity * imbalance_pct) <
>>                          (sgs->group_runnable * 100)
>>
>> Therefore, when the group_has_capacity() is called, the
>> probability that the above formalu holds is very small. Hint
>> compilers about that.
>>
>> Signed-off-by: Qi Zheng <arch0.zheng@...il.com>
>> ---
>>   kernel/sched/fair.c | 4 ++--
>>   1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> index 2ba8f230feb9..9074fd5e23b2 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -8234,8 +8234,8 @@ 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))
>> +	if (unlikely((sgs->group_capacity * imbalance_pct) <
>> +			(sgs->group_runnable * 100)))
>>   		return false;
> 
> Isn't the probability that this second check will match around 0%?
> 
> I.e. wouldn't the right fix be to remove the duplicate check from
> group_has_capacity(), because it's already been checked in
> group_classify()? Maybe while leaving a comment in place?
> 
> Thanks,
> 
> 	Ingo
>