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

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Date:   Wed, 29 Mar 2023 10:22:35 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Josh Don <joshdon@...gle.com>
Cc:     mingo@...nel.org, vincent.guittot@...aro.org,
        linux-kernel@...r.kernel.org, juri.lelli@...hat.com,
        dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
        mgorman@...e.de, bristot@...hat.com, corbet@....net,
        qyousef@...alina.io, chris.hyser@...cle.com,
        patrick.bellasi@...bug.net, pjt@...gle.com, pavel@....cz,
        qperret@...gle.com, tim.c.chen@...ux.intel.com, timj@....org,
        kprateek.nayak@....com, yu.c.chen@...el.com,
        youssefesmat@...omium.org, joel@...lfernandes.org, efault@....de
Subject: Re: [PATCH 08/17] sched/fair: Implement an EEVDF like policy

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.
> 
> 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.