linux-kernel - Re: [tip:sched/core] [sched/fair] 089d84203a: pts.schbench.32.usec,_99.9th_latency

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Message-ID: <20251218102020.GO3707891@noisy.programming.kicks-ass.net>
Date: Thu, 18 Dec 2025 11:20:20 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Shrikanth Hegde <sshegde@...ux.ibm.com>
Cc: kernel test robot <oliver.sang@...el.com>, oe-lkp@...ts.linux.dev,
	lkp@...el.com, linux-kernel@...r.kernel.org, x86@...nel.org,
	Ingo Molnar <mingo@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Juri Lelli <juri.lelli@...hat.com>, Mel Gorman <mgorman@...e.de>,
	Valentin Schneider <vschneid@...hat.com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	aubrey.li@...ux.intel.com, yu.c.chen@...el.com
Subject: Re: [tip:sched/core] [sched/fair] 089d84203a:
 pts.schbench.32.usec,_99.9th_latency_percentile 52.4% regression

On Thu, Dec 18, 2025 at 03:41:55PM +0530, Shrikanth Hegde wrote:
> On 12/18/25 2:07 PM, Peter Zijlstra wrote:
> > On Thu, Dec 18, 2025 at 12:59:53PM +0800, kernel test robot wrote:
> > > 
> > > 
> > > Hello,
> > > 
> > > kernel test robot noticed a 52.4% regression of pts.schbench.32.usec,_99.9th_latency_percentile on:
> > > 
> > > 
> > > commit: 089d84203ad42bc8fd6dbf41683e162ac6e848cd ("sched/fair: Fold the sched_avg update")
> > > https://git.kernel.org/cgit/linux/kernel/git/tip/tip.git sched/core
> > 
> > Well, that obviously wasn't the intention. Let me pull that patch :/
> 
> Is it possible because it missed scaling by se_weight(se) ??

>  static inline void
>  enqueue_load_avg(struct cfs_rq *cfs_rq, struct sched_entity *se)
>  {
> -       cfs_rq->avg.load_avg += se->avg.load_avg;
> -       cfs_rq->avg.load_sum += se_weight(se) * se->avg.load_sum;
> +       __update_sa(&cfs_rq->avg, load, se->avg.load_avg, se->avg.load_sum);
>  }

Ah, indeed, something like so then? Can the robot (Oliver/Philip)
verify?

(I was going to shelf it and look at it after the holidays, but if this
is it, we can get it fixed before I dissapear).

---
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 76f5e4b78b30..7377f9117501 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -3775,13 +3775,15 @@ account_entity_dequeue(struct cfs_rq *cfs_rq, struct sched_entity *se)
 static inline void
 enqueue_load_avg(struct cfs_rq *cfs_rq, struct sched_entity *se)
 {
-	__update_sa(&cfs_rq->avg, load, se->avg.load_avg, se->avg.load_sum);
+	__update_sa(&cfs_rq->avg, load, se->avg.load_avg,
+		    se_weight(se) * se->avg.load_sum);
 }
 
 static inline void
 dequeue_load_avg(struct cfs_rq *cfs_rq, struct sched_entity *se)
 {
-	__update_sa(&cfs_rq->avg, load, -se->avg.load_avg, -se->avg.load_sum);
+	__update_sa(&cfs_rq->avg, load, -se->avg.load_avg,
+		    se_weight(se) * -se->avg.load_sum);
 }
 
 static void place_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags);