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Message-ID: <0575c014-6fe7-4118-bae8-cbb5b303a390@arm.com>
Date: Mon, 22 Jul 2024 08:34:02 +0200
From: Dietmar Eggemann <dietmar.eggemann@....com>
To: Chuyi Zhou <zhouchuyi@...edance.com>, mingo@...hat.com,
peterz@...radead.org, juri.lelli@...hat.com, vincent.guittot@...aro.org,
rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de, vschneid@...hat.com
Cc: chengming.zhou@...ux.dev, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] sched/fair: Sync se's load_avg with cfs_rq in
reweight_task
On 20/07/2024 07:12, Chuyi Zhou wrote:
> In reweight_task(), there are two situations:
>
> 1. The task was on_rq, then the task's load_avg is accurate because we
> synchronized it 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 update_load_avg() to synchronize the
> load_avg of se with cfs_rq. 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.
>
> Signed-off-by: Chuyi Zhou <zhouchuyi@...edance.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 | 3 +++
> 1 file changed, 3 insertions(+)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 9057584ec06d..b1e07ce90284 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -3835,12 +3835,15 @@ static void reweight_entity(struct cfs_rq *cfs_rq, struct sched_entity *se,
> }
> }
>
> +static inline void update_load_avg(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags);
> +
> void reweight_task(struct task_struct *p, const struct load_weight *lw)
> {
> struct sched_entity *se = &p->se;
> struct cfs_rq *cfs_rq = cfs_rq_of(se);
> struct load_weight *load = &se->load;
>
> + update_load_avg(cfs_rq, se, 0);
IIUC, you only want to sync the sleeping task with its cfs_rq. IMHO,
sync_entity_load_avg() should be used here instead of update_load_avg().
The latter is doing much more than this.
> reweight_entity(cfs_rq, se, lw->weight);
> load->inv_weight = lw->inv_weight;
> }
Maybe even do this in reweight_entity()?. You would have to do it under
'if (!se->on_rq) in reweight_task() anyway I assume.
-->8--
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 9057584ec06d..555392be4e82 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -3669,11 +3669,31 @@ 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 +3815,10 @@ 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)) {
+ sync_entity_load_avg(se);
}
+
dequeue_load_avg(cfs_rq, se);
if (se->on_rq) {
@@ -4033,12 +4056,6 @@ 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 +4790,6 @@ 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).
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