Re: [PATCH v2] sched/fair: Fix enqueue_task_fair warning some more

Vincent Guittot
2020-05-08T15:27:59Z

On Fri, 8 May 2020 at 17:12, Tao Zhou <zohooouoto@zoho.com.cn> wrote:
>
> Hi Phil,
>
> On Thu, May 07, 2020 at 04:36:12PM -0400, Phil Auld wrote:
> > sched/fair: Fix enqueue_task_fair warning some more
> >
> > The recent patch, fe61468b2cb (sched/fair: Fix enqueue_task_fair warning)
> > did not fully resolve the issues with the rq->tmp_alone_branch !=
> > &rq->leaf_cfs_rq_list warning in enqueue_task_fair. There is a case where
> > the first for_each_sched_entity loop exits due to on_rq, having incompletely
> > updated the list. In this case the second for_each_sched_entity loop can
> > further modify se. The later code to fix up the list management fails to do
> > what is needed because se no longer points to the sched_entity which broke
> > out of the first loop.
> >
>
> > Address this by calling leaf_add_rq_list if there are throttled parents while
> > doing the second for_each_sched_entity loop.
>
> Thanks for your trace imformation and explanation. I
> truely have learned from this and that.
>
> s/leaf_add_rq_list/list_add_leaf_cfs_rq/
>
> >
> > Suggested-by: Vincent Guittot <vincent.guittot@linaro.org>
> > Signed-off-by: Phil Auld <pauld@redhat.com>
> > Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
> > Cc: Vincent Guittot <vincent.guittot@linaro.org>
> > Cc: Ingo Molnar <mingo@kernel.org>
> > Cc: Juri Lelli <juri.lelli@redhat.com>
> > ---
> > kernel/sched/fair.c | 7 +++++++
> > 1 file changed, 7 insertions(+)
> >
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 02f323b85b6d..c6d57c334d51 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -5479,6 +5479,13 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags)
> > /* end evaluation on encountering a throttled cfs_rq */
> > if (cfs_rq_throttled(cfs_rq))
> > goto enqueue_throttle;
> > +
> > + /*
> > + * One parent has been throttled and cfs_rq removed from the
> > + * list. Add it back to not break the leaf list.
> > + */
> > + if (throttled_hierarchy(cfs_rq))
> > + list_add_leaf_cfs_rq(cfs_rq);
> > }
>
> I was confused by why the throttled cfs rq can be on list.
> It is possible when enqueue a task and thanks to the 'threads'.
> But I think the above comment does not truely put the right
> intention, right ?
> If throttled parent is onlist, the child cfs_rq is ignored
> to be added to the leaf cfs_rq list me think.
>
> unthrottle_cfs_rq() follows the same logic if i am not wrong.
> Is it necessary to add the above to it ?

When a cfs_rq is throttled, its sched group is dequeued and all child
cfs_rq are removed from leaf_cfs_rq list. But the sched group of the
child cfs_rq stay enqueued in the throttled cfs_rq so child sched
group->on_rq might be still set.

>
> Thanks,
> Tau
>
> >
> > enqueue_throttle:
> > --
> > 2.18.0
> >
> > V2 rework the fix based on Vincent's suggestion. Thanks Vincent.
> >
> >
> > Cheers,
> > Phil
> >
> > --
> >