linux-kernel - Re: [PATCH] schied/fair: Skip updating "contrib" without load

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Date:   Mon, 9 Dec 2019 17:16:27 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Peng Wang <rocking@...ux.alibaba.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
Subject: Re: [PATCH] schied/fair: Skip updating "contrib" without load

On Sat, Dec 07, 2019 at 12:14:22AM +0800, Peng Wang wrote:
> We only update load_sum/runnable_load_sum/util_sum with
> decayed old sum when load is clear.

What you're saying is that because of the:

	if (!load)
		runnable = running = 0;

clause in ___update_load_sum(), all the actual users of @contrib in
accumulate_sum():

	if (load)
		sa->load_sum += load * contrib;
	if (runnable)
		sa->runnable_load_sum += runnable * contrib;
	if (running)
		sa->util_sum += contrib << SCHED_CAPACITY_SHIFT;

don't happen, and therefore we don't care what @contrib actually is and
calculating it is pointless.

I suppose that is so. did you happen to have performance numbers? Also,
I'm thinking this wants a comment.

> Signed-off-by: Peng Wang <rocking@...ux.alibaba.com>
> ---
>  kernel/sched/pelt.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/sched/pelt.c b/kernel/sched/pelt.c
> index a96db50..4392953 100644
> --- a/kernel/sched/pelt.c
> +++ b/kernel/sched/pelt.c
> @@ -129,8 +129,9 @@ static u32 __accumulate_pelt_segments(u64 periods, u32 d1, u32 d3)
>  		 * Step 2
>  		 */
>  		delta %= 1024;
> -		contrib = __accumulate_pelt_segments(periods,
> -				1024 - sa->period_contrib, delta);
> +		if (load)
> +			contrib = __accumulate_pelt_segments(periods,
> +					1024 - sa->period_contrib, delta);
>  	}
>  	sa->period_contrib = delta;
>  
> -- 
> 1.8.3.1
>