linux-kernel - Re: [PATCH 2/2] sched/deadline: fix start high-res preemption tick for a non-running task

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Message-ID: <5469C503.9010007@arm.com>
Date:	Mon, 17 Nov 2014 09:50:59 +0000
From:	Juri Lelli <juri.lelli@....com>
To:	Wanpeng Li <kernellwp@...il.com>,
	Wanpeng Li <wanpeng.li@...ux.intel.com>,
	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>
CC:	Kirill Tkhai <ktkhai@...allels.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"juri.lelli@...il.com" <juri.lelli@...il.com>
Subject: Re: [PATCH 2/2] sched/deadline: fix start high-res preemption tick
 for a non-running task

Hi,

On 13/11/14 10:30, Wanpeng Li wrote:
> Hi Juri,
> On 11/13/14, 5:37 PM, Juri Lelli wrote:
>> Hi,
>>
>> not sure I understand what the problem is here.
>>
>> On 11/11/14 01:52, Wanpeng Li wrote:
>>> Queued ticks are scheduled to match the budget, which means the budget
>>> is overall consumed and the dl task should be throttled.
>> ... enforce the budget? It means that when the budget is consumed the
>> task has to be throttled...
> 
> Agreed.
> 
>>
>>> Dl task will
>>> be replenished immediately if fail to start a dl timer.
>>>
>>> However, the curr maybe not the left most dl task in the rb tree any
>>> more after this immediately replenished and reschedule is needed.
>>> Start high-res preemption tick for this upcoming rescheduled dl task
>>> is not correct.
>>>
>> So, the task that is going to preempt curr is picked by
>> pick_next_task_dl(), that correctly starts the hrtick for this new task.
>>
>> Maybe you can add more information about what you are seeing? A callpath
>> maybe?
> 
> The parameter of task_tick_dl() queued == 1 means that hrtick is fired. 
> hrtick() => task_tick_dl( , ,1), so p->dl.runtime should be <= 0 if 
> queued == 1. What I see is queued == 1 && p->dl.runtime > 0 && p is not 
> the left most task and hrtick is start for this task.
> 

Oh, right, now it makes sense. Good catch! :)

Could you please post a second version with a more explanatory changelog
and maybe a comment just above the check?

Thanks,

- Juri

> Regards,
> Wanpeng Li
> 
>>
>> Thanks,
>>
>> - Juri
>>
>>> This patch fix it by not starting high-res preemption tick for a
>>> non-running dl task.
>>>
>>> Signed-off-by: Wanpeng Li <wanpeng.li@...ux.intel.com>
>>> ---
>>>   kernel/sched/deadline.c | 3 ++-
>>>   1 file changed, 2 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
>>> index 56674f6..2a6a5bb 100644
>>> --- a/kernel/sched/deadline.c
>>> +++ b/kernel/sched/deadline.c
>>> @@ -1090,7 +1090,8 @@ static void task_tick_dl(struct rq *rq, struct task_struct *p, int queued)
>>>   {
>>>   	update_curr_dl(rq);
>>>   
>>> -	if (hrtick_enabled(rq) && queued && p->dl.runtime > 0)
>>> +	if (hrtick_enabled(rq) && queued && p->dl.runtime > 0 &&
>>> +	    is_leftmost(p, &rq->dl))
>>>   		start_hrtick_dl(rq, p);
>>>   }
>>>   
>>>
>> --
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> 
> 

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