linux-kernel - Re: [PATCH V3] sched/deadline: Update rq_clock of later

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Date:   Fri, 20 Jul 2018 17:36:06 +0200
From:   Daniel Bristot de Oliveira <bristot@...hat.com>
To:     Juri Lelli <juri.lelli@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org, Clark Williams <williams@...hat.com>,
        Luca Abeni <luca.abeni@...tannapisa.it>,
        Tommaso Cucinotta <tommaso.cucinotta@...tannapisa.it>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...nel.org>, stable@...r.kernel.org
Subject: Re: [PATCH V3] sched/deadline: Update rq_clock of later_rq when
 pushing a task

On 07/20/2018 02:53 PM, Juri Lelli wrote:
> On 20/07/18 14:48, Peter Zijlstra wrote:
>> On Fri, Jul 20, 2018 at 02:46:15PM +0200, Peter Zijlstra wrote:
>>> On Fri, Jul 20, 2018 at 11:16:30AM +0200, Daniel Bristot de Oliveira wrote:
>>>> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
>>>> index fbfc3f1d368a..8b50eea4b607 100644
>>>> --- a/kernel/sched/deadline.c
>>>> +++ b/kernel/sched/deadline.c
>>>> @@ -2090,8 +2090,14 @@ static int push_dl_task(struct rq *rq)
>>>>  	sub_rq_bw(&next_task->dl, &rq->dl);
>>>>  	set_task_cpu(next_task, later_rq->cpu);
>>>>  	add_rq_bw(&next_task->dl, &later_rq->dl);
>>>> +
>>>> +	/*
>>>> +	 * 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);
>>>>  	add_running_bw(&next_task->dl, &later_rq->dl);
>>>> -	activate_task(later_rq, next_task, 0);
>>>> +	activate_task(later_rq, next_task, ENQUEUE_NOCLOCK);
>>>>  	ret = 1;
>>>>  
>>>>  	resched_curr(later_rq);
>>>
>>> Why isn't push_rt_task() affected by the very same issue?
>>
>> Aah, I see, its the add_running_bw() think; for which RT doesn't have a
>> counter-part.
> 
> Right, but doesn't enqueue_top_rt_rq end-up being called by activate_
> task on lowest_rq? Mmm.


AFAICS we have:

push_rt_task() {
	activate_task() {
		enqueue_task(,,(flags=0)) {
		        if (!(flags & ENQUEUE_NOCLOCK))
	        	        update_rq_clock(rq);
			enqueue_task_rt() {
				enqueue_rt_entity() {
					enqueue_top_rt_rq();
			}
		}
	}
}

So we will have the clock updated already...

Am I missing something?

Thanks,
-- Daniel