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Date:   Fri, 13 Dec 2019 13:13:04 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Peng Wang <rocking@linux.alibaba.com>
Cc:     mingo@redhat.com, juri.lelli@redhat.com,
        vincent.guittot@linaro.org, dietmar.eggemann@arm.com,
        rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2] schied/fair: Skip calculating @contrib without load
Message-ID: <20191213121304.GC2844@hirez.programming.kicks-ass.net>
References: <1575648862-12095-1-git-send-email-rocking@linux.alibaba.com>
 <1576208740-35609-1-git-send-email-rocking@linux.alibaba.com>
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On Fri, Dec 13, 2019 at 11:45:40AM +0800, Peng Wang wrote:
> 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;

I've made that:

--- a/kernel/sched/pelt.c
+++ b/kernel/sched/pelt.c
@@ -129,8 +129,20 @@ accumulate_sum(u64 delta, struct sched_a
 		 * Step 2
 		 */
 		delta %= 1024;
-		contrib = __accumulate_pelt_segments(periods,
-				1024 - sa->period_contrib, delta);
+		if (load) {
+			/*
+			 * This relies on the:
+			 *
+			 * if (!load)
+			 *	runnable = running = 0;
+			 *
+			 * clause from ___update_load_sum(); this results in
+			 * the below usage of @contrib to dissapear entirely,
+			 * so no point in calculating it.
+			 */
+			contrib = __accumulate_pelt_segments(periods,
+					1024 - sa->period_contrib, delta);
+		}
 	}
 	sa->period_contrib = delta;
 
@@ -205,7 +217,9 @@ ___update_load_sum(u64 now, struct sched
 	 * This means that weight will be 0 but not running for a sched_entity
 	 * but also for a cfs_rq if the latter becomes idle. As an example,
 	 * this happens during idle_balance() which calls
-	 * update_blocked_averages()
+	 * update_blocked_averages().
+	 *
+	 * Also see the comment in accumulate_sum().
 	 */
 	if (!load)
 		runnable = running = 0;