linux-kernel - Re: [PATCH] sched/pelt: use shift operation instead of division operation

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Message-ID: <20200224093215.GC14897@hirez.programming.kicks-ass.net>
Date:   Mon, 24 Feb 2020 10:32:15 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     qiwuchen55@...il.com
Cc:     mingo@...hat.com, 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, chenqiwu <chenqiwu@...omi.com>
Subject: Re: [PATCH] sched/pelt: use shift operation instead of division
 operation

On Mon, Feb 24, 2020 at 11:30:55AM +0800, qiwuchen55@...il.com wrote:
> From: chenqiwu <chenqiwu@...omi.com>
> 
> Use shift operation to calculate the periods instead of division,
> since shift operation is more efficient than division operation.
> 
> Signed-off-by: chenqiwu <chenqiwu@...omi.com>
> ---
>  kernel/sched/pelt.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/pelt.c b/kernel/sched/pelt.c
> index bd006b7..ac79f8e 100644
> --- a/kernel/sched/pelt.c
> +++ b/kernel/sched/pelt.c
> @@ -114,7 +114,7 @@ static u32 __accumulate_pelt_segments(u64 periods, u32 d1, u32 d3)
>  	u64 periods;
>  
>  	delta += sa->period_contrib;
> -	periods = delta / 1024; /* A period is 1024us (~1ms) */
> +	periods = delta >> 10; /* A period is 1024us (~1ms) */

Find me a compiler that stupid.