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Message-ID: <CAKfTPtADKqVSvUFmLRPCU_XGVvf3wme0EnnvPf1Q39HEs=VqRw@mail.gmail.com>
Date: Mon, 30 May 2022 09:52:36 +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, vschneid@...hat.com,
odin@...d.al, linux-kernel@...r.kernel.org,
duanxiongchun@...edance.com, songmuchun@...edance.com
Subject: Re: [PATCH v3 2/2] sched/fair: optimize and simplify rq leaf_cfs_rq_list
On Thu, 26 May 2022 at 12:40, Chengming Zhou
<zhouchengming@...edance.com> 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>
> ---
> v3:
> - fix !CONFIG_FAIR_GROUP_SCHED build error, reported by
> kernel test robot <lkp@...el.com>
>
> v2:
> - move throttled_hierarchy() outside list_add_leaf_cfs_rq(),
> suggested by Vincent.
> ---
> kernel/sched/fair.c | 81 ++++++++++++---------------------------------
> 1 file changed, 22 insertions(+), 59 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index c6da204f3068..48ff4330d68a 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -3114,6 +3114,8 @@ void reweight_task(struct task_struct *p, int prio)
> load->inv_weight = sched_prio_to_wmult[prio];
> }
>
> +static inline int throttled_hierarchy(struct cfs_rq *cfs_rq);
> +
> #ifdef CONFIG_FAIR_GROUP_SCHED
> #ifdef CONFIG_SMP
> /*
> @@ -3224,8 +3226,6 @@ 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.
> @@ -4338,16 +4338,11 @@ 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())
> - list_add_leaf_cfs_rq(cfs_rq);
> -
> - if (cfs_rq->nr_running == 1)
> + if (cfs_rq->nr_running == 1) {
> check_enqueue_throttle(cfs_rq);
> + if (!throttled_hierarchy(cfs_rq))
> + list_add_leaf_cfs_rq(cfs_rq);
> + }
> }
>
> static void __clear_buddies_last(struct sched_entity *se)
> @@ -4962,11 +4957,18 @@ 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;
> @@ -5004,31 +5006,12 @@ 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: */
> @@ -5683,13 +5666,6 @@ 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*/
> @@ -5713,21 +5689,6 @@ 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);
> @@ -11257,9 +11218,10 @@ static inline bool vruntime_normalized(struct task_struct *p)
> */
> static void propagate_entity_cfs_rq(struct sched_entity *se)
> {
> - struct cfs_rq *cfs_rq;
> + struct cfs_rq *cfs_rq = cfs_rq_of(se);
>
> - list_add_leaf_cfs_rq(cfs_rq_of(se));
> + if (!throttled_hierarchy(cfs_rq))
> + list_add_leaf_cfs_rq(cfs_rq);
>
> /* Start to propagate at parent */
> se = se->parent;
> @@ -11268,7 +11230,8 @@ static void propagate_entity_cfs_rq(struct sched_entity *se)
> cfs_rq = cfs_rq_of(se);
>
you can break if the cfs is throttled because it's sched_entity has
been dequeued. In this case we check if the cfs is throttled not if
the hierarchy is throttled
+ if (cfs_rq_throttled(cfs_rq))
+ break;
> update_load_avg(cfs_rq, se, UPDATE_TG);
> - list_add_leaf_cfs_rq(cfs_rq);
> + if (!throttled_hierarchy(cfs_rq))
> + list_add_leaf_cfs_rq(cfs_rq);
> }
> }
> #else
> --
> 2.36.1
>
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