linux-kernel - Re: [PATCH v2 1/5] sched/fair: Reorder enqueue/dequeue_task

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Message-ID: <c18ab89e-d635-e370-6cbb-6015b404d906@arm.com>
Date:   Wed, 19 Feb 2020 12:07:14 +0100
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Vincent Guittot <vincent.guittot@...aro.org>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     Ingo Molnar <mingo@...hat.com>, Juri Lelli <juri.lelli@...hat.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Phil Auld <pauld@...hat.com>, Parth Shah <parth@...ux.ibm.com>,
        Valentin Schneider <valentin.schneider@....com>,
        Hillf Danton <hdanton@...a.com>
Subject: Re: [PATCH v2 1/5] sched/fair: Reorder enqueue/dequeue_task_fair path

On 18/02/2020 15:15, Vincent Guittot wrote:
> On Tue, 18 Feb 2020 at 14:22, Peter Zijlstra <peterz@...radead.org> wrote:
>>
>> On Tue, Feb 18, 2020 at 01:37:37PM +0100, Dietmar Eggemann wrote:
>>> On 14/02/2020 16:27, Vincent Guittot wrote:
>>>> The walk through the cgroup hierarchy during the enqueue/dequeue of a task
>>>> is split in 2 distinct parts for throttled cfs_rq without any added value
>>>> but making code less readable.
>>>>
>>>> Change the code ordering such that everything related to a cfs_rq
>>>> (throttled or not) will be done in the same loop.
>>>>
>>>> In addition, the same steps ordering is used when updating a cfs_rq:
>>>> - update_load_avg
>>>> - update_cfs_group
>>>> - update *h_nr_running
>>>
>>> Is this code change really necessary? You pay with two extra goto's. We
>>> still have the two for_each_sched_entity(se)'s because of 'if
>>> (se->on_rq); break;'.
>>
>> IIRC he relies on the presented ordering in patch #5 -- adding the
>> running_avg metric.
> 
> Yes, that's the main reason, updating load_avg before h_nr_running

My hunch is you refer to the new function:

static inline void se_update_runnable(struct sched_entity *se)
{
        if (!entity_is_task(se))
                se->runnable_weight = se->my_q->h_nr_running;
}

I don't see the dependency to the 'update_load_avg -> h_nr_running'
order since it operates on se->my_q, not cfs_rq = cfs_rq_of(se), i.e.
se->cfs_rq.

What do I miss here?