linux-kernel - Re: [RFC PATCH] sched/deadline: Avoid dl_server boosting with expired deadline

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Message-ID: <20251101000803.GA2212249@noisy.programming.kicks-ass.net>
Date: Sat, 1 Nov 2025 01:08:03 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Gabriele Monaco <gmonaco@...hat.com>
Cc: Juri Lelli <juri.lelli@...hat.com>, linux-kernel@...r.kernel.org,
	Ingo Molnar <mingo@...hat.com>,
	Clark Williams <williams@...hat.com>, arighi@...dia.com
Subject: Re: [RFC PATCH] sched/deadline: Avoid dl_server boosting with
 expired deadline

On Fri, Oct 31, 2025 at 04:41:22PM +0100, Gabriele Monaco wrote:
> On Fri, 2025-10-31 at 16:20 +0100, Peter Zijlstra wrote:
> > On Fri, Oct 31, 2025 at 02:24:17PM +0100, Gabriele Monaco wrote:
> > > 
> > > Different scenario if I have the CPU busy with other tasks (e.g. RT
> > > policies), there I can see the server stopping and starting again.
> > > After I do this I seem to get a different behaviour (even some boosting
> > > after idle), I'm trying to understand what's going on.
> > > 
> 
> After running some heavy RT workload (stress-ng --cpu 10 --sched rr) I do see
> the server stopping and starting as the models would expect, but somehow it's
> always boosting as soon as it's started.
> 
> Apparently dl_defer_running is always 1 in that scenario. Perhaps running idle
> counts as running something too, so it never defers. But I can't really see how
> this happens..

The transition [4], will retain dl_defer_running, such that a timely
re-start of the dl_server can immediately run again.