Re: [PATCH v2 02/12] sched/deadline: Less agressive dl_server handling

[Chris Bainbridge <chris.bainbridge@gmail.com>]

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@meta.com>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.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.