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

[Kirill Tkhai <ktkhai@parallels.com>]

В Чт, 13/11/2014 в 18:19 +0800, Wanpeng Li пишет:
> 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().

Yeah, but nobody will move this task to alive rq.

> 
> 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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