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

Peter Zijlstra
2024-08-02T14:59:52Z

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.