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

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Message-ID: <20200806144533.GA2123716@gmail.com>
Date:   Thu, 6 Aug 2020 16:45:33 +0200
From:   Ingo Molnar <mingo@...nel.org>
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/core: add unlikely in group_has_capacity()


* 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