linux-kernel - Re: [PATCH v5 8/9] sched/fair: defer task sched_avg attach to enqueue

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Message-ID: <Yv4W5u4+CXir3bjZ@worktop.programming.kicks-ass.net>
Date:   Thu, 18 Aug 2022 12:39:34 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Chengming Zhou <zhouchengming@...edance.com>
Cc:     vincent.guittot@...aro.org, dietmar.eggemann@....com,
        mingo@...hat.com, rostedt@...dmis.org, bsegall@...gle.com,
        vschneid@...hat.com, linux-kernel@...r.kernel.org, tj@...nel.org
Subject: Re: [PATCH v5 8/9] sched/fair: defer task sched_avg attach to
 enqueue_entity()

On Thu, Aug 18, 2022 at 11:43:42AM +0800, Chengming Zhou wrote:
> When wake_up_new_task(), we would use post_init_entity_util_avg()
> to init util_avg/runnable_avg based on cpu's util_avg at that time,
> then attach task sched_avg to cfs_rq.
> 
> Since enqueue_entity() would always attach any unattached task entity,
> so we can defer this work to enqueue_entity().
> 
> post_init_entity_util_avg(p)
>   attach_entity_cfs_rq()  --> (1)
> activate_task(rq, p)
>   enqueue_task() := enqueue_task_fair()
>   enqueue_entity()
>     update_load_avg(cfs_rq, se, UPDATE_TG | DO_ATTACH)
>       if (!se->avg.last_update_time && (flags & DO_ATTACH))
>         attach_entity_load_avg()  --> (2)
> 
> This patch defer attach from (1) to (2)
> 
> Signed-off-by: Chengming Zhou <zhouchengming@...edance.com>
> ---
>  kernel/sched/fair.c | 4 ----
>  1 file changed, 4 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index e0d34ecdabae..aacf38a72714 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -799,8 +799,6 @@ void init_entity_runnable_average(struct sched_entity *se)
>  	/* when this task enqueue'ed, it will contribute to its cfs_rq's load_avg */
>  }
>  
> -static void attach_entity_cfs_rq(struct sched_entity *se);
> -
>  /*
>   * With new tasks being created, their initial util_avgs are extrapolated
>   * based on the cfs_rq's current util_avg:
> @@ -863,8 +861,6 @@ void post_init_entity_util_avg(struct task_struct *p)
>  		se->avg.last_update_time = cfs_rq_clock_pelt(cfs_rq);
>  		return;
>  	}
> -
> -	attach_entity_cfs_rq(se);
>  }

There are comments with update_cfs_rq_load_avg() and
remove_entity_load_avg() that seem to rely on post_init_entity_util()
doing this attach.

If that is no longer true; at the very least those comments need to be
updated, but also, I don't immediately see why that's no longer the
case, so please explain.