linux-kernel - Re: [PATCH RFC] sched/deadline: support dl task migrate during cpu hotplug

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Date:	Tue, 04 Nov 2014 21:33:44 +0800
From:	Wanpeng Li <kernellwp@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>,
	Wanpeng Li <wanpeng.li@...ux.intel.com>
CC:	Ingo Molnar <mingo@...hat.com>,
	Kirill Tkhai <ktkhai@...allels.com>,
	Juri Lelli <juri.lelli@....com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC] sched/deadline: support dl task migrate during cpu
 hotplug


On 14/11/4 下午9:30, Wanpeng Li wrote:
> Hi Peter,
> On 14/11/4 下午6:10, Peter Zijlstra wrote:
>> On Tue, Nov 04, 2014 at 04:23:45PM +0800, Wanpeng Li wrote:
>>> On Tue, Nov 04, 2014 at 09:32:25AM +0100, Peter Zijlstra wrote:
>>>> On Tue, Nov 04, 2014 at 07:57:48AM +0800, Wanpeng Li wrote:
>>>>> On Mon, Nov 03, 2014 at 11:41:11AM +0100, Peter Zijlstra wrote:
>>>>>> On Fri, Oct 31, 2014 at 03:28:17PM +0800, Wanpeng Li wrote:
>>>>>> So what is wrong with making dl_task_timer() deal with it? The timer
>>>>>> will still fire on the correct time, canceling it and or otherwise
>>>>>> messing with the CBS is wrong. Once it fires, all we need to do is
>>>>>> migrate it to another cpu (preferably one that is still online of 
>>>>>> course
>>>>>> :-).
>>>>> Do you mean what I need to do is push the task to another cpu in 
>>>>> dl_task_timer()
>>>>> if rq is offline?
>>>> That does indeed appear to be the sensible fix to me.
>>>>
>>>>> In addition, what will happen if dl task can't preempt on
>>>>> another cpu?
>>>> So if we find that the rq the task was on is no longer available, we
>>>> need to select a new rq, the 'right' rq would be the one running the
>>>> latest deadline.
>>>>
>>>> If it cannot preempt the latest (running) deadline, it was not 
>>>> eligible
>>>> for running in the first place so no worries, right?
>>> I think this will lead to this deadline task cannot running on any 
>>> rqs any more.
>>> If my understanding is not right, when it will be picked?
>> So you unconditionally place it on the rq with the latest deadline. If
>> it cannot preempt, at least its on an online cpu. It will get scheduled
>> whenever its deadline is one of the N earliest, with N the number of
>> online CPUs.
>
> If something like this make sense?
>
> (Will test it tomorrow and send out a formal one)
> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> index f3d7776..dac33d1 100644
> --- a/kernel/sched/deadline.c
> +++ b/kernel/sched/deadline.c
> @@ -553,6 +553,41 @@ again:
> push_dl_task(rq);
> #endif
> }
> +
> + /*
> + * So if we find that the rq the task was on is no longer
> + * available, we need to select a new rq, the 'right' rq
> + * would be the one running the latest deadline.
> + *
> + * If it cannot preempt, at least it's on an online cpu. It
> + * will get scheduled whenever its deadline is one of the N
> + * earliest, with N the number of online CPUs.
> + */
> +
> + if (!rq->online) {
> + struct rq *latest_rq = NULL;
> + int cpu;
> + u64 dmin = LONG_MAX;
> +
> + for_each_cpu(cpu, &p->cpus_allowed)
> + if (cpu_online(cpu) &&
> + cpu_rq(cpu)->dl.earliest_dl.curr < dmin) {
> + latest_rq = cpu_rq(cpu);
> + dmin = latest_rq->dl.earliest_dl.curr;
> + }
> +
> + if (!latest_rq)
> + goto unlock;
> +
> + raw_spin_lock(&latest_rq->lock);
> +
> + deactivate_task(rq, p, 0);
> + set_task_cpu(p, latest_rq->cpu);
> + activate_task(latest_rq, p, 0);
> +
> + raw_spin_unlock(&latest_rq->lock);
> + }
> +
> unlock:
> raw_spin_unlock(&rq->lock);

wow, sorry for mess format.

>
> Regards,
> Wanpeng Li
>
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