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SubjectRe: [PATCH v2] sched/fair: Fix insertion in rq->leaf_cfs_rq_list
On Wed, 30 Jan 2019 at 15:01, Peter Zijlstra <peterz@infradead.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

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