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Message-ID: <CAKfTPtDpqECw_mo1+iPN8OPb3cBf7ppJsLSTAJwPRLr-N3auqg@mail.gmail.com>
Date: Thu, 12 May 2022 16:02:49 +0200
From: Vincent Guittot <vincent.guittot@...aro.org>
To: Chengming Zhou <zhouchengming@...edance.com>
Cc: mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com,
dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
mgorman@...e.de, bristot@...hat.com, linux-kernel@...r.kernel.org,
duanxiongchun@...edance.com, songmuchun@...edance.com,
zhengqi.arch@...edance.com
Subject: Re: [PATCH] sched/fair: optimize and simplify rq leaf_cfs_rq_list
Hi Chengming,
Your patch is on my list but I have been busy on other stuff and we
had enough warnings and problems with this part that I want to
carefully review that all the cases are covered. I will review it soon
Vincent
On Wed, 11 May 2022 at 14:05, Chengming Zhou
<zhouchengming@...edance.com> wrote:
>
> Hello, friendly ping...
>
>
> On 2022/4/28 00:05, Chengming Zhou wrote:
> > We notice the rq leaf_cfs_rq_list has two problems when do bugfix
> > backports and some test profiling.
> >
> > 1. cfs_rqs under throttled subtree could be added to the list, and
> > make their fully decayed ancestors on the list, even though not needed.
> >
> > 2. #1 also make the leaf_cfs_rq_list management complex and error prone,
> > this is the list of related bugfix so far:
> >
> > commit 31bc6aeaab1d ("sched/fair: Optimize update_blocked_averages()")
> > commit fe61468b2cbc ("sched/fair: Fix enqueue_task_fair warning")
> > commit b34cb07dde7c ("sched/fair: Fix enqueue_task_fair() warning some more")
> > commit 39f23ce07b93 ("sched/fair: Fix unthrottle_cfs_rq() for leaf_cfs_rq list")
> > commit 0258bdfaff5b ("sched/fair: Fix unfairness caused by missing load decay")
> > commit a7b359fc6a37 ("sched/fair: Correctly insert cfs_rq's to list on unthrottle")
> > commit fdaba61ef8a2 ("sched/fair: Ensure that the CFS parent is added after unthrottling")
> > commit 2630cde26711 ("sched/fair: Add ancestors of unthrottled undecayed cfs_rq")
> >
> > commit 31bc6aeaab1d ("sched/fair: Optimize update_blocked_averages()")
> > delete every cfs_rq under throttled subtree from rq->leaf_cfs_rq_list,
> > and delete the throttled_hierarchy() test in update_blocked_averages(),
> > which optimized update_blocked_averages().
> >
> > But those later bugfix add cfs_rqs under throttled subtree back to
> > rq->leaf_cfs_rq_list again, with their fully decayed ancestors, for
> > the integrity of rq->leaf_cfs_rq_list.
> >
> > This patch takes another method, skip all cfs_rqs under throttled
> > hierarchy when list_add_leaf_cfs_rq(), to completely make cfs_rqs
> > under throttled subtree off the leaf_cfs_rq_list.
> >
> > So we don't need to consider throttled related things in
> > enqueue_entity(), unthrottle_cfs_rq() and enqueue_task_fair(),
> > which simplify the code a lot. Also optimize update_blocked_averages()
> > since cfs_rqs under throttled hierarchy and their ancestors
> > won't be on the leaf_cfs_rq_list.
> >
> > Signed-off-by: Chengming Zhou <zhouchengming@...edance.com>
> > ---
> > kernel/sched/fair.c | 72 ++++++++++-----------------------------------
> > 1 file changed, 16 insertions(+), 56 deletions(-)
> >
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 1ad18b5cc1b8..083c3d32c899 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -309,6 +309,8 @@ const struct sched_class fair_sched_class;
> >
> > #ifdef CONFIG_FAIR_GROUP_SCHED
> >
> > +static inline int throttled_hierarchy(struct cfs_rq *cfs_rq);
> > +
> > /* Walk up scheduling entities hierarchy */
> > #define for_each_sched_entity(se) \
> > for (; se; se = se->parent)
> > @@ -331,7 +333,7 @@ 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)
> > + if (cfs_rq->on_list || throttled_hierarchy(cfs_rq))
> > return rq->tmp_alone_branch == &rq->leaf_cfs_rq_list;
> >
> > cfs_rq->on_list = 1;
> > @@ -3242,8 +3244,6 @@ static long calc_group_shares(struct cfs_rq *cfs_rq)
> > }
> > #endif /* CONFIG_SMP */
> >
> > -static inline int throttled_hierarchy(struct cfs_rq *cfs_rq);
> > -
> > /*
> > * Recomputes the group entity based on the current state of its group
> > * runqueue.
> > @@ -4356,16 +4356,10 @@ enqueue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
> > __enqueue_entity(cfs_rq, se);
> > se->on_rq = 1;
> >
> > - /*
> > - * When bandwidth control is enabled, cfs might have been removed
> > - * because of a parent been throttled but cfs->nr_running > 1. Try to
> > - * add it unconditionally.
> > - */
> > - if (cfs_rq->nr_running == 1 || cfs_bandwidth_used())
> > + if (cfs_rq->nr_running == 1) {
> > list_add_leaf_cfs_rq(cfs_rq);
> > -
> > - if (cfs_rq->nr_running == 1)
> > check_enqueue_throttle(cfs_rq);
> > + }
> > }
> >
> > static void __clear_buddies_last(struct sched_entity *se)
> > @@ -4980,11 +4974,18 @@ void unthrottle_cfs_rq(struct cfs_rq *cfs_rq)
> > /* update hierarchical throttle state */
> > walk_tg_tree_from(cfs_rq->tg, tg_nop, tg_unthrottle_up, (void *)rq);
> >
> > - /* Nothing to run but something to decay (on_list)? Complete the branch */
> > if (!cfs_rq->load.weight) {
> > - if (cfs_rq->on_list)
> > - goto unthrottle_throttle;
> > - return;
> > + if (!cfs_rq->on_list)
> > + return;
> > + /*
> > + * Nothing to run but something to decay (on_list)?
> > + * Complete the branch.
> > + */
> > + for_each_sched_entity(se) {
> > + if (list_add_leaf_cfs_rq(cfs_rq_of(se)))
> > + break;
> > + }
> > + goto unthrottle_throttle;
> > }
> >
> > task_delta = cfs_rq->h_nr_running;
> > @@ -5022,31 +5023,12 @@ void unthrottle_cfs_rq(struct cfs_rq *cfs_rq)
> > /* end evaluation on encountering a throttled cfs_rq */
> > if (cfs_rq_throttled(qcfs_rq))
> > 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(qcfs_rq))
> > - list_add_leaf_cfs_rq(qcfs_rq);
> > }
> >
> > /* At this point se is NULL and we are at root level*/
> > 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
> > - * assertion below.
> > - */
> > - for_each_sched_entity(se) {
> > - struct cfs_rq *qcfs_rq = cfs_rq_of(se);
> > -
> > - if (list_add_leaf_cfs_rq(qcfs_rq))
> > - break;
> > - }
> > -
> > assert_list_leaf_cfs_rq(rq);
> >
> > /* Determine whether we need to wake up potentially idle CPU: */
> > @@ -5701,13 +5683,6 @@ 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);
> > }
> >
> > /* At this point se is NULL and we are at root level*/
> > @@ -5731,21 +5706,6 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags)
> > update_overutilized_status(rq);
> >
> > enqueue_throttle:
> > - if (cfs_bandwidth_used()) {
> > - /*
> > - * When bandwidth control is enabled; the cfs_rq_throttled()
> > - * breaks in the above iteration can result in incomplete
> > - * leaf list maintenance, resulting in triggering the assertion
> > - * below.
> > - */
> > - for_each_sched_entity(se) {
> > - cfs_rq = cfs_rq_of(se);
> > -
> > - if (list_add_leaf_cfs_rq(cfs_rq))
> > - break;
> > - }
> > - }
> > -
> > assert_list_leaf_cfs_rq(rq);
> >
> > hrtick_update(rq);
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