linux-kernel - Re: [PATCH] sched/deadline: dome some cleanup for push_dl

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Date:   Fri, 24 Jul 2020 09:14:26 +0200
From:   Juri Lelli <juri.lelli@...hat.com>
To:     Peng Liu <iwtbavbm@...il.com>
Cc:     linux-kernel@...r.kernel.org, mingo@...hat.com,
        peterz@...radead.org, vincent.guittot@...aro.org,
        dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
        mgorman@...e.de, Daniel Bristot de Oliveira <bristot@...hat.com>
Subject: Re: [PATCH] sched/deadline: dome some cleanup for push_dl_task()

Hi,

On 07/07/20 00:04, Peng Liu wrote:
> 'commit 840d719604b0 ("sched/deadline: Update rq_clock of later_rq when pushing a task")'
> introduced the update_rq_clock() to fix the "used-before-update" bug.
> 
> 'commit f4904815f97a ("sched/deadline: Fix double accounting of rq/running bw in push & pull")'
> took away the bug source(add_running_bw()).
> 
> We no longer need to update rq_clock in advance, let activate_task()
> worry about that.
> 
> Signed-off-by: Peng Liu <iwtbavbm@...il.com>
> ---
>  kernel/sched/deadline.c | 8 +-------
>  1 file changed, 1 insertion(+), 7 deletions(-)
> 
> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> index 504d2f51b0d6..c3fa11f84d93 100644
> --- a/kernel/sched/deadline.c
> +++ b/kernel/sched/deadline.c
> @@ -2104,13 +2104,7 @@ static int push_dl_task(struct rq *rq)
>  
>  	deactivate_task(rq, next_task, 0);
>  	set_task_cpu(next_task, later_rq->cpu);
> -
> -	/*
> -	 * Update the later_rq clock here, because the clock is used
> -	 * by the cpufreq_update_util() inside __add_running_bw().
> -	 */
> -	update_rq_clock(later_rq);
> -	activate_task(later_rq, next_task, ENQUEUE_NOCLOCK);
> +	activate_task(later_rq, next_task, 0);
>  	ret = 1;

The change looks good to me, since now add_running_bw() is called later
by enqueue_task_dl(), but rq_clock has already been updated by core's
enqueue_task().

Daniel, Dietmar, a second pair of eyes (since you authored the commits
above)?

I'd chage subject to something like "sched/deadline: Stop updating
rq_clock before pushing a task".

Thanks,

Juri