On Fri, 8 Feb 2019 at 17:31, Peter Zijlstra <[email protected]> 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;
>
>
