linux-kernel - Re: [PATCH v4] sched/fair: optimize and simplify rq leaf_cfs_rq

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Date:   Wed, 1 Jun 2022 17:41:27 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     Chengming Zhou <zhouchengming@...edance.com>, mingo@...hat.com,
        juri.lelli@...hat.com, dietmar.eggemann@....com,
        rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
        bristot@...hat.com, vschneid@...hat.com,
        linux-kernel@...r.kernel.org, duanxiongchun@...edance.com,
        songmuchun@...edance.com, zhengqi.arch@...edance.com
Subject: Re: [PATCH v4] sched/fair: optimize and simplify rq leaf_cfs_rq_list

On Wed, Jun 01, 2022 at 03:22:12PM +0200, Vincent Guittot wrote:
> On Wed, 1 Jun 2022 at 04:19, Chengming Zhou <zhouchengming@...edance.com> wrote:
> >
> > We notice the rq leaf_cfs_rq_list has two problems when do bugfix
> > backports and some test profiling.
> >
> > 1. cfs_rqs under throttled subtree could be added to the list, and
> >    make their fully decayed ancestors on the list, even though not needed.
> >
> > 2. #1 also make the leaf_cfs_rq_list management complex and error prone,
> >    this is the list of related bugfix so far:
> >
> >    commit 31bc6aeaab1d ("sched/fair: Optimize update_blocked_averages()")
> >    commit fe61468b2cbc ("sched/fair: Fix enqueue_task_fair warning")
> >    commit b34cb07dde7c ("sched/fair: Fix enqueue_task_fair() warning some more")
> >    commit 39f23ce07b93 ("sched/fair: Fix unthrottle_cfs_rq() for leaf_cfs_rq list")
> >    commit 0258bdfaff5b ("sched/fair: Fix unfairness caused by missing load decay")
> >    commit a7b359fc6a37 ("sched/fair: Correctly insert cfs_rq's to list on unthrottle")
> >    commit fdaba61ef8a2 ("sched/fair: Ensure that the CFS parent is added after unthrottling")
> >    commit 2630cde26711 ("sched/fair: Add ancestors of unthrottled undecayed cfs_rq")
> >
> > commit 31bc6aeaab1d ("sched/fair: Optimize update_blocked_averages()")
> > delete every cfs_rq under throttled subtree from rq->leaf_cfs_rq_list,
> > and delete the throttled_hierarchy() test in update_blocked_averages(),
> > which optimized update_blocked_averages().
> >
> > But those later bugfix add cfs_rqs under throttled subtree back to
> > rq->leaf_cfs_rq_list again, with their fully decayed ancestors, for
> > the integrity of rq->leaf_cfs_rq_list.
> >
> > This patch takes another method, skip all cfs_rqs under throttled
> > hierarchy when list_add_leaf_cfs_rq(), to completely make cfs_rqs
> > under throttled subtree off the leaf_cfs_rq_list.
> >
> > So we don't need to consider throttled related things in
> > enqueue_entity(), unthrottle_cfs_rq() and enqueue_task_fair(),
> > which simplify the code a lot. Also optimize update_blocked_averages()
> > since cfs_rqs under throttled hierarchy and their ancestors
> > won't be on the leaf_cfs_rq_list.
> >
> > Signed-off-by: Chengming Zhou <zhouchengming@...edance.com>
> 
> Reviewed-by: Vincent Guittot <vincent.guittot@...aro.org>

Thanks!