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

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Date:   Wed, 26 Feb 2020 17:30:15 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Tao Zhou <zhout@...aldi.net>
Cc:     Dietmar Eggemann <dietmar.eggemann@....com>,
        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>, "T. Zhou" <t1zhou@....com>
Subject: Re: [PATCH v2 1/5] sched/fair: Reorder enqueue/dequeue_task_fair path

Hi Tao,

On Sat, 22 Feb 2020 at 16:23, Tao Zhou <zhout@...aldi.net> wrote:
>
> Hi,
>
> On Thu, Feb 20, 2020 at 02:38:59PM +0100, Dietmar Eggemann wrote:
> > On 19/02/2020 17:26, Vincent Guittot wrote:
> > > 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.
> >
> > Ah, now I see:
> >
> > update_load_avg()
> >   update_cfs_rq_load_avg()
> >     __update_load_avg_cfs_rq()
> >        ___update_load_sum(..., cfs_rq->h_nr_running, ...)
> >
> >                                ^^^^^^^^^^^^^^^^^^^^
>
> throttle/unthrottle_cfs_rq() update h_nr_running also. Maybe need
> to consider call se_update_runnable there.

yes you're right, group entity which are not enqueued/dequeued, which
updates runnable_avg and se->runnable_weight, during the
throttling/unthrottling are not updated. I'm going to send a fix on
top of sched/core

> And the name update_se_runnable seems to be consistent with others.

In fact, I have reused the same name as with runnable_load_avg

Thanks and sorry for the late reply

>
> Thanks,
> Tao
>
> > Not really obvious IMHO, since the code is introduced only in 4/5.
> >
> > Could you add a comment to this patch header?
> >
> > I see you mentioned this dependency already in v1 discussion
> >
> > https://lore.kernel.org/r/CAKfTPtAM=kgF7Fz-JKFY+s_k5KFirs-8Bub3s1Eqtq7P0NMa0w@mail.gmail.com
> >
> > "... But the following patches make PELT using h_nr_running ...".
> >
> > IMHO it would be helpful to have this explanation in the 1/5 patch
> > header so people stop wondering why this is necessary.