linux-kernel - Re: [PATCH v2 09/10] sched/fair: stop load tracking when task switched_from

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Message-ID: <27f0675e-8ac3-2200-749f-7290a256e3d9@arm.com>
Date:   Thu, 14 Jul 2022 14:33:21 +0200
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Chengming Zhou <zhouchengming@...edance.com>, mingo@...hat.com,
        peterz@...radead.org, vincent.guittot@...aro.org,
        rostedt@...dmis.org, bsegall@...gle.com, vschneid@...hat.com
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 09/10] sched/fair: stop load tracking when task
 switched_from_fair()

On 13/07/2022 06:04, Chengming Zhou wrote:
> The same reason as the previous commit, if we don't reset the
> sched_avg last_update_time to 0, after a while in switched_to_fair():
> 
> switched_to_fair
>   attach_task_cfs_rq
>     attach_entity_cfs_rq
>       update_load_avg
>         __update_load_avg_se(now, cfs_rq, se)
> 
> The delta (now - sa->last_update_time) will wrongly contribute/decay
> sched_avg depends on the task running/runnable status at that time.

IMHO, a queued !fair task when switching back to fair will already be
enqueued (attached) as a fair task in __sched_setscheduler() prior to
the check_class_changed() call.

I can't see how this will work with your proposed change in using
last_update_time=0 for fair->!fair->fair class changes?

> This patch reset it's sched_avg last_update_time to 0, stop load
> tracking for !fair task, later in switched_to_fair() ->
> update_load_avg(), we can use its saved sched_avg.
> 
> Signed-off-by: Chengming Zhou <zhouchengming@...edance.com>
> ---
>  kernel/sched/fair.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 50f65a2ede32..576028f5a09e 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -11552,6 +11552,11 @@ static void attach_task_cfs_rq(struct task_struct *p)
>  static void switched_from_fair(struct rq *rq, struct task_struct *p)
>  {
>  	detach_task_cfs_rq(p);
> +
> +#ifdef CONFIG_SMP
> +	/* Stop load tracking for !fair task */
> +	p->se.avg.last_update_time = 0;
> +#endif
>  }
>  
>  static void switched_to_fair(struct rq *rq, struct task_struct *p)