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

Valentin Schneider
2024-08-02T14:39:15Z

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.

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