linux-kernel - Re: [PATCH 1/2] sched/fair: keep load_avg and load

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Message-ID: <YK+ZGlfPxK3JCySS@hirez.programming.kicks-ass.net>
Date:   Thu, 27 May 2021 15:05:30 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     mingo@...hat.com, juri.lelli@...hat.com, dietmar.eggemann@....com,
        rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
        bristot@...hat.com, linux-kernel@...r.kernel.org, odin@...d.al,
        cgroups@...r.kernel.org
Subject: Re: [PATCH 1/2] sched/fair: keep load_avg and load_sum synced

On Thu, May 27, 2021 at 02:29:15PM +0200, Vincent Guittot wrote:

> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -3509,7 +3509,8 @@ update_tg_cfs_load(struct cfs_rq *cfs_rq, struct sched_entity *se, struct cfs_rq
>  	se->avg.load_sum = runnable_sum;
>  	se->avg.load_avg = load_avg;
>  	add_positive(&cfs_rq->avg.load_avg, delta_avg);
> -	add_positive(&cfs_rq->avg.load_sum, delta_sum);
> +	cfs_rq->avg.load_sum = cfs_rq->avg.load_avg * divider;

If I'm not mistaken, this makes delta_sum unused, so we can remove it
entirely, see below.

> +

This extra blank space, we really need that? :-)

>  }

---
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 161b92aa1c79..2b99e687fe7a 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -3453,10 +3453,9 @@ update_tg_cfs_runnable(struct cfs_rq *cfs_rq, struct sched_entity *se, struct cf
 static inline void
 update_tg_cfs_load(struct cfs_rq *cfs_rq, struct sched_entity *se, struct cfs_rq *gcfs_rq)
 {
-	long delta_avg, running_sum, runnable_sum = gcfs_rq->prop_runnable_sum;
+	long running_sum, runnable_sum = gcfs_rq->prop_runnable_sum;
 	unsigned long load_avg;
 	u64 load_sum = 0;
-	s64 delta_sum;
 	u32 divider;
 
 	if (!runnable_sum)
@@ -3503,13 +3502,11 @@ update_tg_cfs_load(struct cfs_rq *cfs_rq, struct sched_entity *se, struct cfs_rq
 	load_sum = (s64)se_weight(se) * runnable_sum;
 	load_avg = div_s64(load_sum, divider);
 
-	delta_sum = load_sum - (s64)se_weight(se) * se->avg.load_sum;
-	delta_avg = load_avg - se->avg.load_avg;
-
 	se->avg.load_sum = runnable_sum;
 	se->avg.load_avg = load_avg;
-	add_positive(&cfs_rq->avg.load_avg, delta_avg);
-	add_positive(&cfs_rq->avg.load_sum, delta_sum);
+
+	add_positive(&cfs_rq->avg.load_avg, (long)(load_avg - se->avg.load_avg));
+	cfs_rq->avg.load_sum = cfs_rq->avg.load_avg * divider;
 }
 
 static inline void add_tg_cfs_propagate(struct cfs_rq *cfs_rq, long runnable_sum)