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Message-ID: <CAKfTPtA4VRDzvbKC11rhbkFECMNvbFdWkf2n4pk=AehBUgRxvQ@mail.gmail.com>
Date: Mon, 5 Aug 2024 15:41:15 +0200
From: Vincent Guittot <vincent.guittot@...aro.org>
To: Chuyi Zhou <zhouchuyi@...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, vschneid@...hat.com, chengming.zhou@...ux.dev,
linux-kernel@...r.kernel.org, Vishal Chourasia <vishalc@...ux.ibm.com>
Subject: Re: [PATCH v4] sched/fair: Sync se's load_avg with cfs_rq in reweight_task
On Mon, 29 Jul 2024 at 12:47, Chuyi Zhou <zhouchuyi@...edance.com> wrote:
>
> In reweight_task(), there are two situations:
>
> 1. The task was on_rq, then the task's load_avg is accurate because in
> __sched_setscheduler()/set_user_nice(), we would dequeue the on_rq tasks
> before doing reweight. The task's load_avg would be synchronized with
> cfs_rq through update_load_avg() in dequeue_task().
>
> 2. The task is sleeping, its load_avg might not have been updated for some
> time, which can result in inaccurate dequeue_load_avg() in
> reweight_entity().
>
> This patch solves this by using sync_entity_load_avg() to synchronize the
> load_avg of se with cfs_rq before dequeue_load_avg() in reweight_entity().
> For tasks were on_rq, since we already update load_avg to accurate values
> in dequeue_task(), this change will not have other effects due to the short
> time interval between the two updates.
>
> Suggested-by: Dietmar Eggemann <dietmar.eggemann@....com>
> Signed-off-by: Chuyi Zhou <zhouchuyi@...edance.com>
> Reviewed-by: Chengming Zhou <chengming.zhou@...ux.dev>
> Reviewed-by: Vishal Chourasia <vishalc@...ux.ibm.com>
> Reviewed-by: Dietmar Eggemann <dietmar.eggemann@....com>
Reviewed-by: Vincent Guittot <vincent.guittot@...aro.org>
>
> ---
> Changes in v4:
> - Fix the 'if else' code style issue.(Dietmar)
> - Add a description of __sched_setscheduler()/set_user_nice() in the commit
> log.(Dietmar)
> - Add comment before calling sync_entity_load_avg().(Qais)
> Changes in v3:
> - use sync_entity_load_avg() rather than update_load_avg() to sync the
> sleeping task with its cfs_rq suggested by Dietmar.
> - Link t0 v2: https://lore.kernel.org/lkml/20240720051248.59608-1-zhouchuyi@bytedance.com/
> Changes in v2:
> - change the description in commit log.
> - use update_load_avg() in reweight_task() rather than in reweight_entity
> suggested by chengming.
> - Link to v1: https://lore.kernel.org/lkml/20240716150840.23061-1-zhouchuyi@bytedance.com/
> ---
> kernel/sched/fair.c | 46 +++++++++++++++++++++++++++------------------
> 1 file changed, 28 insertions(+), 18 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 9057584ec06d..1e3c7c582541 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -3669,11 +3669,32 @@ dequeue_load_avg(struct cfs_rq *cfs_rq, struct sched_entity *se)
> cfs_rq->avg.load_sum = max_t(u32, cfs_rq->avg.load_sum,
> cfs_rq->avg.load_avg * PELT_MIN_DIVIDER);
> }
> +
> +static inline u64 cfs_rq_last_update_time(struct cfs_rq *cfs_rq)
> +{
> + return u64_u32_load_copy(cfs_rq->avg.last_update_time,
> + cfs_rq->last_update_time_copy);
> +}
> +
> +/*
> + * Synchronize entity load avg of dequeued entity without locking
> + * the previous rq.
> + */
> +static void sync_entity_load_avg(struct sched_entity *se)
> +{
> + struct cfs_rq *cfs_rq = cfs_rq_of(se);
> + u64 last_update_time;
> +
> + last_update_time = cfs_rq_last_update_time(cfs_rq);
> + __update_load_avg_blocked_se(last_update_time, se);
> +}
> +
> #else
> static inline void
> enqueue_load_avg(struct cfs_rq *cfs_rq, struct sched_entity *se) { }
> static inline void
> dequeue_load_avg(struct cfs_rq *cfs_rq, struct sched_entity *se) { }
> +static void sync_entity_load_avg(struct sched_entity *se) { }
> #endif
>
> static void reweight_eevdf(struct sched_entity *se, u64 avruntime,
> @@ -3795,7 +3816,14 @@ static void reweight_entity(struct cfs_rq *cfs_rq, struct sched_entity *se,
> if (!curr)
> __dequeue_entity(cfs_rq, se);
> update_load_sub(&cfs_rq->load, se->load.weight);
> + } else if (entity_is_task(se)) {
> + /*
> + * If the task is sleeping, we need to synchronize entity load avg
> + * before dequeue_load_avg().
> + */
> + sync_entity_load_avg(se);
> }
> +
> dequeue_load_avg(cfs_rq, se);
>
> if (se->on_rq) {
> @@ -4034,11 +4062,6 @@ static inline bool load_avg_is_decayed(struct sched_avg *sa)
> return true;
> }
>
> -static inline u64 cfs_rq_last_update_time(struct cfs_rq *cfs_rq)
> -{
> - return u64_u32_load_copy(cfs_rq->avg.last_update_time,
> - cfs_rq->last_update_time_copy);
> -}
> #ifdef CONFIG_FAIR_GROUP_SCHED
> /*
> * Because list_add_leaf_cfs_rq always places a child cfs_rq on the list
> @@ -4773,19 +4796,6 @@ static inline void update_load_avg(struct cfs_rq *cfs_rq, struct sched_entity *s
> }
> }
>
> -/*
> - * Synchronize entity load avg of dequeued entity without locking
> - * the previous rq.
> - */
> -static void sync_entity_load_avg(struct sched_entity *se)
> -{
> - struct cfs_rq *cfs_rq = cfs_rq_of(se);
> - u64 last_update_time;
> -
> - last_update_time = cfs_rq_last_update_time(cfs_rq);
> - __update_load_avg_blocked_se(last_update_time, se);
> -}
> -
> /*
> * Task first catches up with cfs_rq, and then subtract
> * itself from the cfs_rq (task must be off the queue now).
> --
> 2.20.1
>
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