Re: [PATCH 08/17] sched/fair: Implement an EEVDF like policy

[Josh Don]

On Wed, Mar 29, 2023 at 1:22 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> On Wed, Mar 29, 2023 at 10:06:46AM +0200, Peter Zijlstra wrote:
> > On Tue, Mar 28, 2023 at 06:26:51PM -0700, Josh Don wrote:
> > > > +static struct sched_entity *pick_eevdf(struct cfs_rq *cfs_rq)
> > > > +{
> > > > +       struct rb_node *node = cfs_rq->tasks_timeline.rb_root.rb_node;
> > > > +       struct sched_entity *curr = cfs_rq->curr;
> > > > +       struct sched_entity *best = NULL;
> > > > +
> > > > +       if (curr && (!curr->on_rq || !entity_eligible(cfs_rq, curr)))
> > > > +               curr = NULL;
> > > > +
> > > > +       while (node) {
> > > > +               struct sched_entity *se = __node_2_se(node);
> > > > +
> > > > +               /*
> > > > +                * If this entity is not eligible, try the left subtree.
> > > > +                */
> > > > +               if (!entity_eligible(cfs_rq, se)) {
> > > > +                       node = node->rb_left;
> > > > +                       continue;
> > > > +               }
> > > > +
> > > > +               /*
> > > > +                * If this entity has an earlier deadline than the previous
> > > > +                * best, take this one. If it also has the earliest deadline
> > > > +                * of its subtree, we're done.
> > > > +                */
> > > > +               if (!best || deadline_gt(deadline, best, se)) {
> > > > +                       best = se;
> > > > +                       if (best->deadline == best->min_deadline)
> > > > +                               break;
> > >
> > > Isn't it possible to have a child with less vruntime (ie. rb->left)
> > > but with the same deadline? Wouldn't it be preferable to choose the
> > > child instead since the deadlines are equivalent but the child has
> > > received less service time?
> >
> > Possible, yes I suppose. But given this is ns granular virtual time,
> > somewhat unlikely. You can modify the last (validation) patch and have
> > it detect the case, see if you can trigger it.

Agreed on unlikely, was just checking my understanding here, since
this becomes a question of tradeoff (likelihood of decent vs the ideal
scheduling decision). Leaving as-is seems fine, with potentially a
short comment.

> > Doing that will make the pick always do a full decent of the tree
> > through, which is a little more expensive. Not sure it's worth the
> > effort.
>
> Hmm, maybe not, if there is no smaller-or-equal deadline then the
> min_deadline of the child will be greater and we can terminate the
> decent right there.
>