Re: [PATCH v4] sched/deadline: support dl task migration during cpu hotplug

[Wanpeng Li]

Hi Kirill,
On Tue, Nov 11, 2014 at 04:09:14PM +0300, Kirill Tkhai wrote:
>Ð ÐÑ, 11/11/2014 Ð 21:07 +0800, Wanpeng Li ÐÐÑÐÑ:
>> Hi Kirill,
>> On 11/11/14, 7:10 PM, Kirill Tkhai wrote:
>> > Ð ÐÑ, 11/11/2014 Ð 10:30 +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 fix it by push the task to another cpu in dl_task_timer() if
>> >> rq is offline.
>> >>
>> >> Signed-off-by: Wanpeng Li <wanpeng.li@xxxxxxxxxxxxxxx>
>> > 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.

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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