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From:   Benjamin Segall <bsegall@google.com>
To:     Abel Wu <wuyun.abel@bytedance.com>
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Subject: Re: [PATCH] sched/fair: fix pick_eevdf to always find the correct se
In-Reply-To: <e2a42ff2-d0f9-4963-bed7-229224ee8287@bytedance.com> (Abel Wu's
        message of "Fri, 13 Oct 2023 11:46:24 +0800")
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        <20230531124603.931005524@infradead.org>
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        <6b606049-3412-437f-af25-a4c33139e2d8@bytedance.com>
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        <699cc8b1-f341-4af7-9c47-fee961c5c4b7@bytedance.com>
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        <e2a42ff2-d0f9-4963-bed7-229224ee8287@bytedance.com>
Date:   Fri, 13 Oct 2023 09:51:48 -0700
Message-ID: <xm26bkd2h3dn.fsf@bsegall-linux.svl.corp.google.com>
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Abel Wu <wuyun.abel@bytedance.com> writes:

> 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.)
>

Ah, yes, in terms of optimizing performance rather than marginal
fairness, that would help.