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

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Date:   Wed, 19 Feb 2020 17:26:11 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Dietmar Eggemann <dietmar.eggemann@....com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        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 Wed, 19 Feb 2020 at 12:07, Dietmar Eggemann <dietmar.eggemann@....com> wrote:
>
> 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?

update_load_avg() updates both se and cfs_rq so if you update
cfs_rq->h_nr_running before calling update_load_avg() like in the 2nd
for_each_sched_entity, you will update cfs_rq runnable_avg for the
past time slot with the new h_nr_running value instead of the previous
value.