Date:	Wed, 19 Nov 2014 13:49:20 +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>
Subject: Re: [PATCH v5] sched/deadline: support dl task migration during cpu
 hotplug

Hi,

On 19/11/14 12:30, Wanpeng Li wrote:
> Hi Juri,
> On 11/19/14, 6:13 PM, Juri Lelli wrote:
>> Hi,
>>
>> On 18/11/14 23:18, Wanpeng Li wrote:
>>> Hi Juri,
>>> On 11/12/14, 11:08 PM, Juri Lelli wrote:
>>>> Hi,
>>>>
>>>> On 12/11/14 01:06, Wanpeng Li wrote:
>>>>> I observe that dl task can't be migrated to other cpus during cpu hotplug,
>>>>> in addition, task may/may not be running again if cpu is added back. The
>>>>> root cause which I found is that dl task will be throtted and removed from
>>>>> dl rq after comsuming all budget, which leads to stop task can't pick it up
>>>>> from dl rq and migrate to other cpus during hotplug.
>>>>>
>>>>> The method to reproduce:
>>>>> schedtool -E -t 50000:100000 -e ./test
>>>>> Actually test is just a simple for loop. Then observe which cpu the test
>>>>> task is on.
>>>>> echo 0 > /sys/devices/system/cpu/cpuN/online
>>>>>
>>>>> This patch adds the dl task migration during cpu hotplug by finding a most
>>>>> suitable later deadline rq after dl timer fire if current rq is offline,
>>>>> if fail to find a suitable later deadline rq then fallback to any eligible
>>>>> online cpu in order that the deadline task will come back to us, and the
>>>>> push/pull mechanism should then move it around properly.
>>>>>
>>>>> Signed-off-by: Wanpeng Li <wanpeng.li@...ux.intel.com>
>>>>> ---
>>>>> v4 -> v5:
>>>>>    * remove raw_spin_unlock(&rq->lock)
>>>>>    * cleanup codes, spotted by Peterz
>>>>>    * cleanup patch description
>>>>> v3 -> v4:
>>>>>    * use tsk_cpus_allowed wrapper
>>>>>    * fix compile error
>>>>> v2 -> v3:
>>>>>    * don't get_task_struct
>>>>>    * if cannot preempt any rq, fallback to pick any online cpus
>>>>>    * use cpu_active_mask as original later_mask if cpu is offline
>>>>> v1 -> v2:
>>>>>    * push the task to another cpu in dl_task_timer() if rq is offline.
>>>>>
>>>>>    kernel/sched/deadline.c | 43 +++++++++++++++++++++++++++++++++++++++++--
>>>>>    1 file changed, 41 insertions(+), 2 deletions(-)
>>>>>
>>>>> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
>>>>> index f3d7776..7c31906 100644
>>>>> --- a/kernel/sched/deadline.c
>>>>> +++ b/kernel/sched/deadline.c
>>>>> @@ -487,6 +487,7 @@ static int start_dl_timer(struct sched_dl_entity *dl_se, bool boosted)
>>>>>    	return hrtimer_active(&dl_se->dl_timer);
>>>>>    }
>>>>>    
>>>>> +static struct rq *find_lock_later_rq(struct task_struct *task, struct rq *rq);
>>>>>    /*
>>>>>     * This is the bandwidth enforcement timer callback. If here, we know
>>>>>     * a task is not on its dl_rq, since the fact that the timer was running
>>>>> @@ -538,6 +539,43 @@ again:
>>>>>    	update_rq_clock(rq);
>>>>>    	dl_se->dl_throttled = 0;
>>>>>    	dl_se->dl_yielded = 0;
>>>>> +
>>>>> +	/*
>>>>> +	 * So if we find that the rq the task was on is no longer
>>>>> +	 * available, we need to select a new rq.
>>>>> +	 */
>>>>> +	if (unlikely(!rq->online)) {
>>>>> +		struct rq *later_rq = NULL;
>>>>> +
>>>>> +		later_rq = find_lock_later_rq(p, rq);
>>>>> +
>>>>> +		if (!later_rq) {
>>>>> +			int cpu;
>>>>> +
>>>>> +			/*
>>>>> +			 * If cannot preempt any rq, fallback to pick any
>>>>> +			 * online cpu.
>>>>> +			 */
>>>>> +			cpu = cpumask_any_and(cpu_active_mask,
>>>>> +					tsk_cpus_allowed(p));
>>>>> +			if (cpu >= nr_cpu_ids) {
>>>>> +				pr_warn("fail to find any online cpu and task will never come back\n");
>>>>> +				goto unlock;
>>>>> +			}
>>>>> +			later_rq = cpu_rq(cpu);
>>>>> +		}
>>>>> +
>>>>> +		deactivate_task(rq, p, 0);
>>>>> +		set_task_cpu(p, later_rq->cpu);
>>>>> +		activate_task(later_rq, p, 0);
>>>>> +
>>>>> +		resched_curr(later_rq);
>>>>> +
>>>>> +		double_unlock_balance(rq, later_rq);
>>>>> +
>>>>> +		goto unlock;
>>>>> +	}
>>>>> +
>>>>>    	if (task_on_rq_queued(p)) {
>>>>>    		enqueue_task_dl(rq, p, ENQUEUE_REPLENISH);
>>>>>    		if (dl_task(rq->curr))
>>>>> @@ -1185,8 +1223,9 @@ static int find_later_rq(struct task_struct *task)
>>>>>    	 * We have to consider system topology and task affinity
>>>>>    	 * first, then we can look for a suitable cpu.
>>>>>    	 */
>>>>> -	cpumask_copy(later_mask, task_rq(task)->rd->span);
>>>>> -	cpumask_and(later_mask, later_mask, cpu_active_mask);
>>>>> +	cpumask_copy(later_mask, cpu_active_mask);
>>>>> +	if (likely(task_rq(task)->online))
>>>>> +		cpumask_and(later_mask, later_mask, task_rq(task)->rd->span);
>>>> So, here you consider the span only when the task_rq is online,
>>>> but there might be others cpus still online belonging to the same
>>>> rd->span. And you have to consider them when migrating. Actually,
>>>> migration must still be restricted to the online cpus of task's
>>>> original rd->span, or I fear you can break clustered scheduling.
>>> Sorry, what's clustered scheduling?
>>>
>> It's a scheduling configuration in which you restrict tasks to run in
>> disjoint subsets of system CPUs. Translated to what we have, it's what
>> you get when you create exclusive cpusets (each one gets a rd) and
>> associate tasks to them.
>>
>> My concern in what above is that you may end up breaking this setup
>> if you don't consider the sd->span when one of the CPUs of your
>> cpuset is off. But, Pang Xunlei patches may solve this, I still have to
>> check :/.
> 
> Thanks for your explanation. Could you point out which one of Pang's 
> patchset solve this? ;-)
> 

https://lkml.org/lkml/2014/11/17/443 may help with this, although I
still have to properly look at it.

Best,

- Juri

> Regards,
> Wanpeng Li
> 
>>
>> Thanks,
>>
>> - Juri
>>
>>> Regards,
>>> Wanpeng Li
>>>
>>>> Thanks,
>>>>
>>>> - Juri
>>>>
>>>>>    	cpumask_and(later_mask, later_mask, &task->cpus_allowed);
>>>>>    	best_cpu = cpudl_find(&task_rq(task)->rd->cpudl,
>>>>>    			task, later_mask);
>>>>>
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>>>
> 
> 

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