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

[Juri Lelli]

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@xxxxxxxxxxxxxxx>
> ---
> 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.

> +			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.

> +				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.

Best,

- Juri

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

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