Re: Re: [PATCH] sched/fair: fix pick_eevdf to always find the correct se

[Abel Wu]

On 10/13/23 1:51 AM, Benjamin Segall Wrote:
> Abel Wu <wuyun.abel@xxxxxxxxxxxxx> writes:
>
> > On 10/12/23 5:01 AM, Benjamin Segall Wrote:
> > > Abel Wu <wuyun.abel@xxxxxxxxxxxxx> writes:
> > >
> > > > On 9/30/23 8:09 AM, Benjamin Segall Wrote:
> > > > > +	/*
> > > > > +	 * Now best_left and all of its children are eligible, and we are just
> > > > > +	 * looking for deadline == min_deadline
> > > > > +	 */
> > > > > +	node = &best_left->run_node;
> > > > > +	while (node) {
> > > > > +		struct sched_entity *se = __node_2_se(node);
> > > > > +
> > > > > +		/* min_deadline is the current node */
> > > > > +		if (se->deadline == se->min_deadline)
> > > > > +			return se;
> > > >
> > > > IMHO it would be better tiebreak on vruntime by moving this hunk to ..
> > > >
> > > > > +
> > > > > +		/* min_deadline is in the left branch */
> > > > >     		if (node->rb_left &&
> > > > >     		    __node_2_se(node->rb_left)->min_deadline == se->min_deadline) {
> > > > >     			node = node->rb_left;
> > > > >     			continue;
> > > > >     		}
> > > >
> > > > .. here, thoughts?
> > > Yeah, that should work and be better on the tiebreak (and my test code
> > > agrees). There's an argument that the tiebreak will never really come up
> > > and it's better to avoid the potential one extra cache line from
> > > "__node_2_se(node->rb_left)->min_deadline" though.
> >
> > I see. Then probably do the same thing in the first loop?
>
> We effectively do that already sorta by accident almost always -
> computing best and best_left via deadline_gt rather than gte prioritizes
> earlier elements, which always have a better vruntime.

Sorry for not clarifying clearly about the 'same thing'. What I meant
was to avoid touch left if the node itself has the min deadline.

@@ -894,6 +894,9 @@ static struct sched_entity *__pick_eevdf(struct cfs_rq *cfs_rq)
                if (!best || deadline_gt(deadline, best, se))
                        best = se;

+               if (se->deadline == se->min_deadline)
+                       break;
+
                /*
                 * Every se in a left branch is eligible, keep track of the
                 * branch with the best min_deadline
@@ -913,10 +916,6 @@ static struct sched_entity *__pick_eevdf(struct cfs_rq *cfs_rq)
                                break;
                }

-               /* min_deadline is at this node, no need to look right */
-               if (se->deadline == se->min_deadline)
-                       break;
-
                /* else min_deadline is in the right branch. */
                node = node->rb_right;
        }

(But still thanks for the convincing explanation on fairness.)

Best,
	Abel

> Then when we do the best_left->min_deadline vs best->deadline
> computation, we prioritize best_left, which is the one case it can be
> wrong, we'd need an additional
> "if (se->min_deadline == best->deadline &&
> (s64)(se->vruntime - best->vruntime) > 0) return best;" check at the end
> of the second loop.
>
> (Though again I don't know how much this sort of never-going-to-happen
> slight fairness improvement is worth compared to the extra bit of
> overhead)