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

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Date:   Tue, 18 Feb 2020 14:22:03 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Dietmar Eggemann <dietmar.eggemann@....com>
Cc:     Vincent Guittot <vincent.guittot@...aro.org>, mingo@...hat.com,
        juri.lelli@...hat.com, rostedt@...dmis.org, bsegall@...gle.com,
        mgorman@...e.de, linux-kernel@...r.kernel.org, pauld@...hat.com,
        parth@...ux.ibm.com, valentin.schneider@....com, hdanton@...a.com
Subject: Re: [PATCH v2 1/5] sched/fair: Reorder enqueue/dequeue_task_fair path

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.