linux-kernel - Re: [PATCH v8] sched/deadline: support dl task migration during cpu hotplug

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Date:	Tue, 3 Mar 2015 06:59:25 +0800
From:	Wanpeng Li <wanpeng.li@...ux.intel.com>
To:	Juri Lelli <juri.lelli@....com>
Cc:	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org,
	Wanpeng Li <wanpeng.li@...ux.intel.com>
Subject: Re: [PATCH v8] sched/deadline: support dl task migration during cpu
 hotplug

Hi Juri,
On Mon, Mar 02, 2015 at 12:11:48PM +0000, Juri Lelli wrote:
>Hi,
>
>On 25/02/2015 11:50, 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>
>> ---
>> v7 -> v8:
>>  * remove rd->span related modification since Pang's commit 16b269436b72 
>>    (sched/deadline: Modify cpudl::free_cpus to reflect rd->online) merged 
>>    upstream, which Juri pointed out can handle the exclusive cpusets.
>>  * rebase 
>> v6 -> v7:
>>  * rebase
>> v5 -> v6:
>>  * add double_lock_balance in the fallback path
>> 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 | 38 ++++++++++++++++++++++++++++++++++++++
>>  1 file changed, 38 insertions(+)
>> 
>> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
>> index 3fa8fa6..49f92c8 100644
>> --- a/kernel/sched/deadline.c
>> +++ b/kernel/sched/deadline.c
>> @@ -492,6 +492,7 @@ 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
>> @@ -537,6 +538,43 @@ static enum hrtimer_restart dl_task_timer(struct hrtimer *timer)
>>  	update_rq_clock(rq);
>>  
>>  	/*
>> +	 * 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));
>
>Please align this to cpu_active_mask above.

Ok.

>
>> +			if (cpu >= nr_cpu_ids) {
>> +				pr_warn("fail to find any online cpu and task will never come back\n");
>
>Wouldn't be better a WARN_ON(1) here? It is a pretty
>serious situation.

Good idea.

>
>> +				goto unlock;
>> +			}
>> +			later_rq = cpu_rq(cpu);
>> +			double_lock_balance(rq, later_rq);
>> +		}
>> +
>> +		deactivate_task(rq, p, 0);
>> +		set_task_cpu(p, later_rq->cpu);
>> +		activate_task(later_rq, p, ENQUEUE_REPLENISH);
>> +
>> +		resched_curr(later_rq);
>
>Your later_rq can also come from the cpumask_any_and(), we
>should check if we need a resched here.

I will add the check in next version, great thanks for your review. ;-)

Regards,
Wanpeng Li 

>
>Best,
>
>- Juri
>
>> +
>> +		double_unlock_balance(rq, later_rq);
>> +
>> +		goto unlock;
>> +	}
>> +
>> +	/*
>>  	 * If the throttle happened during sched-out; like:
>>  	 *
>>  	 *   schedule()
>> 
--
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