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
>
> > }
> >
> > assert_list_leaf_cfs_rq(rq);
