Date:	Wed, 12 Nov 2014 08:12:15 +0800
From:	Wanpeng Li <wanpeng.li@...ux.intel.com>
To:	Kirill Tkhai <ktkhai@...allels.com>
Cc:	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Juri Lelli <juri.lelli@....com>, linux-kernel@...r.kernel.org,
	Wanpeng Li <wanpeng.li@...ux.intel.com>
Subject: Re: [PATCH v4] sched/deadline: support dl task migration during cpu
 hotplug

On Wed, Nov 12, 2014 at 07:53:20AM +0800, Wanpeng Li wrote:
>Hi Kirill,
>On Tue, Nov 11, 2014 at 04:09:14PM +0300, Kirill Tkhai wrote:
>>В Вт, 11/11/2014 в 21:07 +0800, Wanpeng Li пишет:
>>> Hi Kirill,
>>> On 11/11/14, 7:10 PM, Kirill Tkhai wrote:
>>> > В Вт, 11/11/2014 в 10:30 +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 fix it by push the task to another cpu in dl_task_timer() if
>>> >> rq is offline.
>>> >>
>>> >> Signed-off-by: Wanpeng Li <wanpeng.li@...ux.intel.com>
>>> > I'm still thinking we don't have to guarantee any "deadlines" during cpu hotplug...
>>> > But, if speaking about this way:
>>> >
>>> >> ---
>>> >> 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 | 51 ++++++++++++++++++++++++++++++++++++++++++++++---
>>> >>   1 file changed, 48 insertions(+), 3 deletions(-)
>>> >>
>>> >> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
>>> >> index 00324af..e0fbba4 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 +539,46 @@ 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 (!rq->online) {
>>> >> +		struct rq *later_rq = NULL;
>>> >> +
>>> >> +		raw_spin_unlock(&rq->lock);
>>> >> +
>>> >> +		later_rq = find_lock_later_rq(p, rq);
>>> > find_lock_later_rq() expects that rq is locked.
>>> >
>>> > The comment near its head confuses a reader. It locks newly found rq.
>>> 
>>> Sorry for my bad, what's you think should be changed?
>>
>>raw_spin_unlock(&rq->lock) is wrong here. It's not need.
>>
>
>The machine down after remove this.

Just solve it, sorry for the noise. 

Regards,
Wanpeng Li

>
>Regards,
>Wanpeng Li 
>
>>> 
>>> >
>>> >> +
>>> >> +		if (!later_rq) {
>>> >> +			int cpu;
>>> >> +
>>> >> +			/*
>>> >> +			 * If cannot preempt any rq, fallback to pick any
>>> >> +			 * online cpu.
>>> >> +			 */
>>> >> +			for_each_cpu(cpu, tsk_cpus_allowed(p))
>>> >> +				if (cpu_online(cpu))
>>> >> +					later_rq = cpu_rq(cpu);
>>> >> +			if (!later_rq) {
>>> >> +				pr_warn("fail to find any online and task "
>>> >> +				    "will never come back to us\n");
>>> >> +				goto out;
>>> >> +			}
>>> >> +		}
>>> >> +
>>> >> +		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);
>>> > double_unlock_balance() unlocks later_rq only.
>>> >
>>> >> +
>>> >> +		goto out;
>>> >> +	}
>>> >> +
>>> >>   	if (task_on_rq_queued(p)) {
>>> >>   		enqueue_task_dl(rq, p, ENQUEUE_REPLENISH);
>>> >>   		if (dl_task(rq->curr))
>>> >> @@ -555,7 +596,7 @@ again:
>>> >>   	}
>>> >>   unlock:
>>> >>   	raw_spin_unlock(&rq->lock);
>>> >> -
>>> >> +out:
>>> >>   	return HRTIMER_NORESTART;
>>> >>   }
>>> >>   
>>> >> @@ -1185,8 +1226,12 @@ 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);
>>> >> +	if (likely(task_rq(task)->online)) {
>>> >> +		cpumask_copy(later_mask, task_rq(task)->rd->span);
>>> >> +		cpumask_and(later_mask, later_mask, cpu_active_mask);
>>> >> +	} else
>>> >> +		/* for offline cpus we have a singleton rd */
>>> >> +		cpumask_copy(later_mask, cpu_active_mask);
>>> >>   	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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