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

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Message-ID: <CAKfTPtB3qudK8aMq2cx==4RW8t1pz6ymz1Ti0r8oO4TefWzMRw@mail.gmail.com>
Date:   Tue, 18 Feb 2020 15:15:22 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Dietmar Eggemann <dietmar.eggemann@....com>,
        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 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