linux-kernel - Re: [PATCH v2 02/12] sched/deadline: Less agressive dl

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Message-ID: <aJJ_yaD7R2oLsyKv@debian.local>
Date: Tue, 5 Aug 2025 23:03:53 +0100
From: Chris Bainbridge <chris.bainbridge@...il.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: mingo@...hat.com, juri.lelli@...hat.com, vincent.guittot@...aro.org,
	dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
	mgorman@...e.de, vschneid@...hat.com, clm@...a.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 02/12] sched/deadline: Less agressive dl_server
 handling

On Wed, Jul 02, 2025 at 01:49:26PM +0200, Peter Zijlstra wrote:
> Chris reported that commit 5f6bd380c7bd ("sched/rt: Remove default
> bandwidth control") caused a significant dip in his favourite
> benchmark of the day. Simply disabling dl_server cured things.
> 
> His workload hammers the 0->1, 1->0 transitions, and the
> dl_server_{start,stop}() overhead kills it -- fairly obviously a bad
> idea in hind sight and all that.
> 
> Change things around to only disable the dl_server when there has not
> been a fair task around for a whole period. Since the default period
> is 1 second, this ensures the benchmark never trips this, overhead
> gone.
> 
> Fixes: 557a6bfc662c ("sched/fair: Add trivial fair server")
> Reported-by: Chris Mason <clm@...a.com>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> Link: https://lkml.kernel.org/r/20250520101727.507378961@infradead.org

This commit causes almost every boot of my laptop (which is booted from
USB flash/SSD drive) to log "sched: DL replenish lagged too much" around
7 seconds in to the boot. Is this expected? Just asking as this is a
change in behaviour - I haven't seen this warning before in several
years of using this laptop.