linux-kernel - Re: scheduler performance regression since v6.11

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Message-ID: <20250513071551.GC25763@noisy.programming.kicks-ass.net>
Date: Tue, 13 May 2025 09:15:51 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Chris Mason <clm@...a.com>
Cc: linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
	dietmar.eggemann@....com, vschneid@...hat.com
Subject: Re: scheduler performance regression since v6.11

On Mon, May 12, 2025 at 06:35:24PM -0400, Chris Mason wrote:

> If I take 54a58a787791 or 54a58a787791 and turn off the
> DELAY_DEQUEUE/ZERO features at run time, I don't get the performance
> back.  But, if I patch them such that DELAY_DEQUEUE/ZERO default off
> 
> diff --git a/kernel/sched/features.h b/kernel/sched/features.h
> index 7fdeb5576188c..94409e9831e97 100644
> --- a/kernel/sched/features.h
> +++ b/kernel/sched/features.h
> @@ -37,8 +37,8 @@ SCHED_FEAT(CACHE_HOT_BUDDY, true)
>   *
>   * DELAY_ZERO clips the lag on dequeue (or wakeup) to 0.
>   */
> -SCHED_FEAT(DELAY_DEQUEUE, true)
> -SCHED_FEAT(DELAY_ZERO, true)
> +SCHED_FEAT(DELAY_DEQUEUE, false)
> +SCHED_FEAT(DELAY_ZERO, false)
> 
> It runs at 2M QPS again.  If I enable DELAY_QUEUE/ZERO, I go back to 1.95M

Well, that's odd... there should not be residual effects like that. I'll
see if I can spot anything.