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Date:   Fri, 8 Feb 2019 17:48:30 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>, Tejun Heo <tj@...nel.org>,
        Sargun Dhillon <sargun@...gun.me>,
        Xie XiuQi <xiexiuqi@...wei.com>, xiezhipeng1@...wei.com,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 1/2] sched/fair: optimization of update_blocked_averages()

On Fri, 8 Feb 2019 at 17:31, Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Wed, Feb 06, 2019 at 05:14:21PM +0100, Vincent Guittot wrote:
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -346,6 +346,18 @@ static inline bool list_add_leaf_cfs_rq(struct cfs_rq *cfs_rq)
> >  static inline void list_del_leaf_cfs_rq(struct cfs_rq *cfs_rq)
> >  {
> >       if (cfs_rq->on_list) {
> > +             struct rq *rq = rq_of(cfs_rq);
> > +
> > +             /*
> > +              * With cfs_rq being unthrottled/throttled during an enqueue,
> > +              * it can happen the tmp_alone_branch points the a leaf that
> > +              * we finally want to del. In this case, tmp_alone_branch moves
> > +              * to the prev element but it will point to rq->leaf_cfs_rq_list
> > +              * at the end of the enqueue.
> > +              */
> > +             if (rq->tmp_alone_branch == &cfs_rq->leaf_cfs_rq_list)
> > +                     rq->tmp_alone_branch = cfs_rq->leaf_cfs_rq_list.prev;
> > +
> >               list_del_rcu(&cfs_rq->leaf_cfs_rq_list);
> >               cfs_rq->on_list = 0;
> >       }
>
> So that is:
>
>   enqueue_task_fair()
>     enqueue_entity()
>       list_add_lead_cfs_rq()
>       check_enqueue_throttle()
>         throttle_cfs_rq()
>           walk_tg_tree_from()
>             tg_throttle_down()
>               list_del_leaf_cfs_rq()
>
> Which can try and remove a cfs_rq which we just added.
>
> And because the list is a bottom-up order, and the deletion is a
> downward operation, we must go back (prev) in the list.

yes exactly

>
> So far so good I suppose.
>
> > @@ -4449,8 +4465,10 @@ static int tg_throttle_down(struct task_group *tg, void *data)
> >       struct cfs_rq *cfs_rq = tg->cfs_rq[cpu_of(rq)];
> >
> >       /* group is entering throttled state, stop time */
> > -     if (!cfs_rq->throttle_count)
> > +     if (!cfs_rq->throttle_count) {
> >               cfs_rq->throttled_clock_task = rq_clock_task(rq);
> > +             list_del_leaf_cfs_rq(cfs_rq);
> > +     }
> >       cfs_rq->throttle_count++;
> >
> >       return 0;
>
>

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