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

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Date:   Fri, 24 Jul 2020 18:18:04 +0200
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Daniel Bristot de Oliveira <bristot@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Peng Liu <iwtbavbm@...il.com>
Cc:     linux-kernel@...r.kernel.org, mingo@...hat.com,
        peterz@...radead.org, vincent.guittot@...aro.org,
        rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de
Subject: Re: [PATCH] sched/deadline: dome some cleanup for push_dl_task()

On 24/07/2020 17:31, Daniel Bristot de Oliveira wrote:
> On 7/24/20 9:14 AM, Juri Lelli wrote:
>> 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".
> 
> Looks good to me!
> 
> Acked-by: Daniel Bristot de Oliveira <bristot@...hat.com>

Yes, makes sense to me!

Reviewed-by: Dietmar Eggemann <dietmar.eggemann@....com>