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

[Abel Wu <wuyun.abel@bytedance.com>]

On 10/13/23 1:51 AM, Benjamin Segall Wrote:
> Abel Wu <wuyun.abel@bytedance.com> writes:
> 
>> On 10/12/23 5:01 AM, Benjamin Segall Wrote:
>>> Abel Wu <wuyun.abel@bytedance.com> 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)