linux-kernel - Re: [PATCH 17/24] sched/fair: Implement delayed dequeue

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Message-ID: <20240802145938.GK39708@noisy.programming.kicks-ass.net>
Date: Fri, 2 Aug 2024 16:59:38 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Valentin Schneider <vschneid@...hat.com>
Cc: mingo@...hat.com, juri.lelli@...hat.com, vincent.guittot@...aro.org,
	dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
	mgorman@...e.de, linux-kernel@...r.kernel.org,
	kprateek.nayak@....com, wuyun.abel@...edance.com,
	youssefesmat@...omium.org, tglx@...utronix.de, efault@....de
Subject: Re: [PATCH 17/24] sched/fair: Implement delayed dequeue

On Fri, Aug 02, 2024 at 04:39:08PM +0200, Valentin Schneider wrote:
> 
> On 27/07/24 12:27, Peter Zijlstra wrote:
> > Extend / fix 86bfbb7ce4f6 ("sched/fair: Add lag based placement") by
> > noting that lag is fundamentally a temporal measure. It should not be
> > carried around indefinitely.
> >
> > OTOH it should also not be instantly discarded, doing so will allow a
> > task to game the system by purposefully (micro) sleeping at the end of
> > its time quantum.
> >
> > Since lag is intimately tied to the virtual time base, a wall-time
> > based decay is also insufficient, notably competition is required for
> > any of this to make sense.
> >
> > Instead, delay the dequeue and keep the 'tasks' on the runqueue,
> > competing until they are eligible.
> >
> > Strictly speaking, we only care about keeping them until the 0-lag
> > point, but that is a difficult proposition, instead carry them around
> > until they get picked again, and dequeue them at that point.
> >
> 
> Question from a lazy student who just caught up to the current state of
> EEVDF...
> 
> IIUC this makes it so time spent sleeping increases an entity's lag, rather
> than it being frozen & restored via the place_entity() magic.
> 
> So entities with negative lag get closer to their 0-lag point, after which
> they can get picked & dequeued if still not runnable.

Right.

> However, don't entities with positive lag get *further* away from their
> 0-lag point?

Which is why we only delay de dequeue when !eligible, IOW when lag is
negative.

The next patch additionally truncates lag to 0 (for delayed entities),
so they can never earn extra time.