linux-kernel - Re: [PATCH v2] sched/fair: Fix insertion in rq->leaf_cfs_rq

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Date:   Wed, 30 Jan 2019 15:27:12 +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>
Subject: Re: [PATCH v2] sched/fair: Fix insertion in rq->leaf_cfs_rq_list

On Wed, 30 Jan 2019 at 15:01, Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Wed, Jan 30, 2019 at 03:01:04PM +0100, Peter Zijlstra wrote:
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -282,13 +282,15 @@ static inline struct cfs_rq *group_cfs_r
> >       return grp->my_q;
> >  }
> >
> > -static inline void list_add_leaf_cfs_rq(struct cfs_rq *cfs_rq)
> > +static inline bool list_add_leaf_cfs_rq(struct cfs_rq *cfs_rq)
> >  {
> >       struct rq *rq = rq_of(cfs_rq);
> >       int cpu = cpu_of(rq);
> >
> >       if (cfs_rq->on_list)
> > -             return;
> > +             return rq->tmp_alone_branch == &rq->leaf_cfs_rq_list;
>
> And I'm almost certain that can be: return true, but got my brain in a
> twist.

Yes this can return true

If cfs_rq->on_list) then a child not already on the list used the path :

if (cfs_rq->tg->parent &&
           cfs_rq->tg->parent->cfs_rq[cpu]->on_list) {

which does rq->tmp_alone_branch = &rq->leaf_cfs_rq_list;

>
> > +
> > +     cfs_rq->on_list = 1;
> >
> >       /*
> >        * Ensure we either appear before our parent (if already