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

[Wanpeng Li]

Hi Kirill,
On Wed, Nov 12, 2014 at 07:27:06PM +0300, Kirill Tkhai wrote:
>Ð ÐÑ, 12/11/2014 Ð 09:06 +0800, Wanpeng Li ÐÐÑÐÑ:
>> 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@xxxxxxxxxxxxxxx>
>> ---
>> 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?

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);
>
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