Re: [PATCH v5] sched/deadline: support dl task migration during cpu hotplug

[Wanpeng Li <kernellwp@gmail.com>]

Hi Kirill,
On 11/13/14, 6:10 PM, Kirill Tkhai wrote:
> В Чт, 13/11/2014 в 06:56 +0800, Wanpeng Li пишет:
>> Hi Kirill,
>> On Wed, Nov 12, 2014 at 07:27:06PM +0300, Kirill Tkhai wrote:
>>> В Ср, 12/11/2014 в 09:06 +0800, Wanpeng Li пишет:
>>>> 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@linux.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);
>>> later_rq is not locked here, but you activate p on it and you do unlock below.
>> Great catch! How about add double_lock_balance(rq, later_rq); here?
> This sounds good.

I will do this in next version. ;-)

>
>> Regards,
>> Wanpeng Li
>>
>>>> +		}
>>>> +
>>>> +		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);
>>>>   	cpumask_and(later_mask, later_mask, &task->cpus_allowed);
>>>>   	best_cpu = cpudl_find(&task_rq(task)->rd->cpudl,
>>>>   			task, later_mask);
> Also, we should think about the following situation.
>
> DL task is left on dead rq. In your scheme it will be moved by the timer.
> But what will be if somebody changes the class of the task (before timer)?

I think timer will be cancelled in switched_from_dl().

Regards,
Wanpeng Li

> In this case the task still remains on dead rq.
>
> We should handle this situation in some way.
>
> Kirill
>
>
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