Vincent Guittot <vincent.guit...@linaro.org> writes:
> On Tue, 12 May 2020 at 20:59, <bseg...@google.com> wrote:
>>
>> Vincent Guittot <vincent.guit...@linaro.org> writes:
>>
>> > Although not exactly identical, unthrottle_cfs_rq() and enqueue_task_fair()
>> > are quite close and follow the same sequence for enqueuing an entity in the
>> > cfs hierarchy. Modify unthrottle_cfs_rq() to use the same pattern as
>> > enqueue_task_fair(). This fixes a problem already faced with the latter and
>> > add an optimization in the last for_each_sched_entity loop.
>> >
>> > Fixes: fe61468b2cb (sched/fair: Fix enqueue_task_fair warning)
>> > Reported-by Tao Zhou <zohooou...@zoho.com.cn>
>> > Signed-off-by: Vincent Guittot <vincent.guit...@linaro.org>
>> > ---
>> >
>> > This path applies on top of
>> > 20200507203612.gf19...@lorien.usersys.redhat.com
>> > and fixes similar problem for unthrottle_cfs_rq()
>> >
>> > kernel/sched/fair.c | 37 ++++++++++++++++++++++++++++---------
>> > 1 file changed, 28 insertions(+), 9 deletions(-)
>> >
>> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> > index e2450c2e0747..4b73518aa25c 100644
>> > --- a/kernel/sched/fair.c
>> > +++ b/kernel/sched/fair.c
>> > @@ -4803,26 +4803,44 @@ void unthrottle_cfs_rq(struct cfs_rq *cfs_rq)
>> > idle_task_delta = cfs_rq->idle_h_nr_running;
>> > for_each_sched_entity(se) {
>> > if (se->on_rq)
>> > - enqueue = 0;
>> > + break;
>> > + cfs_rq = cfs_rq_of(se);
>> > + enqueue_entity(cfs_rq, se, ENQUEUE_WAKEUP);
>> >
>> > + cfs_rq->h_nr_running += task_delta;
>> > + cfs_rq->idle_h_nr_running += idle_task_delta;
>> > +
>> > + /* end evaluation on encountering a throttled cfs_rq */
>> > + if (cfs_rq_throttled(cfs_rq))
>> > + goto unthrottle_throttle;
>> > + }
>> > +
>> > + for_each_sched_entity(se) {
>> > cfs_rq = cfs_rq_of(se);
>> > - if (enqueue) {
>> > - enqueue_entity(cfs_rq, se, ENQUEUE_WAKEUP);
>> > - } else {
>> > - update_load_avg(cfs_rq, se, 0);
>> > - se_update_runnable(se);
>> > - }
>> > +
>> > + update_load_avg(cfs_rq, se, UPDATE_TG);
>> > + se_update_runnable(se);
>> >
>> > cfs_rq->h_nr_running += task_delta;
>> > cfs_rq->idle_h_nr_running += idle_task_delta;
>> >
>> > +
>> > + /* end evaluation on encountering a throttled cfs_rq */
>> > if (cfs_rq_throttled(cfs_rq))
>> > - break;
>> > + goto unthrottle_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);
>> > }
>> >
>> > if (!se)
>>
>> The if is no longer necessary, unlike in enqueue, where the skip goto
>
> Yes. Good point
>
>> goes to this if statement rather than past (but enqueue could be changed
>> to match this). Also in general if we are making these loops absolutely
>
> There is a patch on mailing that skip the if statement. I'm going to
> update it to remove the if
>
>> identical we should probably pull them out to a common function (ideally
>> including the goto target and following loop as well).
>
> I tried that but was not convinced by the result which generated a lot
> of arguments. I didn't want to delay the fix for such cleanup but I
> will have a closer look after. Also the same kind identical sequence
> and clean up can be done with dequeue_task_fair and throtthle_cfs_rq.
> But Those don't have the problem we are fixing here
>
>>
>> > add_nr_running(rq, task_delta);
>> >
>> > +unthrottle_throttle:
>> > /*
>> > * The cfs_rq_throttled() breaks in the above iteration can result in
>> > * incomplete leaf list maintenance, resulting in triggering the
>> > @@ -4831,7 +4849,8 @@ void unthrottle_cfs_rq(struct cfs_rq *cfs_rq)
>> > for_each_sched_entity(se) {
>> > cfs_rq = cfs_rq_of(se);
>> >
>> > - list_add_leaf_cfs_rq(cfs_rq);
>> > + if (list_add_leaf_cfs_rq(cfs_rq))
>> > + break;
>>
>> Do we also need to handle the case of tg_unthrottle_up followed by early exit
>> from unthrottle_cfs_rq? I do not have enough of an idea what
>> list_add_leaf_cfs_rq is doing to say.
>
> If you are speaking about the 'if (!cfs_rq->load.weight) return;"
> after walk_tg_tree_from(). I also thought it was needed but after more
> analyses, I concluded that if cfs_rq->load.weight == 0 , no child has
> been added in the leaf_cfs_rq_list in such case
Hmm, yes, if load.weight is 0 it should not have done anything there.
>
>
>>
>> > }
>> >
>> > assert_list_leaf_cfs_rq(rq);
