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Date: Fri, 10 Sep 2021 16:19:27 +0200
From: Vincent Guittot <vincent.guittot@...aro.org>
To: Michal Koutný <mkoutny@...e.com>
Cc: linux-kernel <linux-kernel@...r.kernel.org>,
Phil Auld <pauld@...hat.com>, Ingo Molnar <mingo@...hat.com>,
Peter Zijlstra <peterz@...radead.org>,
Juri Lelli <juri.lelli@...hat.com>,
Dietmar Eggemann <dietmar.eggemann@....com>,
Steven Rostedt <rostedt@...dmis.org>,
Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
Daniel Bristot de Oliveira <bristot@...hat.com>,
Odin Ugedal <odin@...d.al>, Rik van Riel <riel@...riel.com>,
Giovanni Gherdovich <ggherdovich@...e.cz>
Subject: Re: [RFC PATCH v2 4/5] sched/fair: Simplify load_cfs_rq_list maintenance
On Thu, 19 Aug 2021 at 19:50, Michal Koutný <mkoutny@...e.com> wrote:
>
> load_cfs_rq_list should contain cfs_rqs that have runnable entities in
> them. When we're operating under a throttled hierarchy we always
> update the load_cfs_rq_list in order not to break list_add_load_cfs_rq
> invariant of adding complete branches.
>
> This caused troubles when an entity became runnable (enqueue_entity)
> under a throttled hierarchy (see commit b34cb07dde7c ("sched/fair: Fix
> enqueue_task_fair() warning some more")). (Basically when we add to the
> load list, we have to ensure all the ancestors are added and when
> deleting we have to delete whole subtree.)
>
> This patch simplifies the code by no updates of load_cfs_rq_list when
> we're operating under the throttled hierarchy and defers the
> load_cfs_rq_list update to the point when whole hierarchy is unthrottled
> (tg_unthrottle_up). Specifically, subtrees of a throttled cfs_rq are not
> decaying their PELT when they're being throttled (but the parent of the
> throttled cfs_rq is decayed).
Your proposal looks interesting but I need more time to make sure that
all cases are covered. We have faced several odd corner cases and
sequences in the past that I need time to check that you don't put
some back
>
> The code is now simpler and load_cfs_rq_list contains only cfs_rqs that
> have load to decay.
>
> Signed-off-by: Michal Koutný <mkoutny@...e.com>
> ---
> kernel/sched/fair.c | 58 ++++++++++-----------------------------------
> 1 file changed, 12 insertions(+), 46 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 6f4d5d4dcdd9..9978485334ec 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -3086,6 +3086,8 @@ 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
> /*
> @@ -3196,8 +3198,6 @@ 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.
> @@ -4294,10 +4294,11 @@ enqueue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
>
> /*
> * 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.
> + * because of a parent been throttled. We'll add it later (with
> + * complete branch up to se->on_rq/cfs_eq->on_list) in
> + * tg_unthrottle_up() and unthrottle_cfs_rq().
> */
> - if (cfs_rq->nr_running == 1 || cfs_bandwidth_used())
> + if (cfs_rq->nr_running == 1 && !throttled_hierarchy(cfs_rq))
> list_add_load_cfs_rq(cfs_rq);
>
> if (cfs_rq->nr_running == 1)
> @@ -4936,31 +4937,13 @@ void unthrottle_cfs_rq(struct cfs_rq *cfs_rq)
> /* end evaluation on encountering a throttled cfs_rq */
> if (cfs_rq_throttled(cfs_rq))
> goto unthrottle_throttle;
> -
> - /*
> - * One parent has been throttled and cfs_rq removed from the
> - * list. Add it back to not break the load list.
> - */
> - if (throttled_hierarchy(cfs_rq))
> - list_add_load_cfs_rq(cfs_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 load list maintenance, resulting in triggering the
> - * assertion below.
> - */
> - for_each_sched_entity(se) {
> - cfs_rq = cfs_rq_of(se);
> -
> - if (list_add_load_cfs_rq(cfs_rq))
> - break;
> - }
> -
> + /* See enqueue_task_fair:enqueue_throttle */
> assert_list_load_cfs_rq(rq);
>
> /* Determine whether we need to wake up potentially idle CPU: */
> @@ -5600,13 +5583,6 @@ 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 load list.
> - */
> - if (throttled_hierarchy(cfs_rq))
> - list_add_load_cfs_rq(cfs_rq);
> }
>
> /* At this point se is NULL and we are at root level*/
> @@ -5630,21 +5606,11 @@ 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
> - * load list maintenance, resulting in triggering the assertion
> - * below.
> - */
> - for_each_sched_entity(se) {
> - cfs_rq = cfs_rq_of(se);
> -
> - if (list_add_load_cfs_rq(cfs_rq))
> - break;
> - }
> - }
> -
> + /*
> + * If we got here, subtree of a cfs_rq must have been throttled and
> + * therefore we did not modify load list or we climbed up to root (or
> + * joined to an ancestor cfs_rq with on_rq == 1 => on_list).
> + */
> assert_list_load_cfs_rq(rq);
>
> hrtick_update(rq);
> --
> 2.32.0
>
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