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

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Date:	Tue, 11 Nov 2014 14:10:09 +0300
From:	Kirill Tkhai <ktkhai@...allels.com>
To:	Wanpeng Li <wanpeng.li@...ux.intel.com>
CC:	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Juri Lelli <juri.lelli@....com>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4] sched/deadline: support dl task migration during cpu
 hotplug

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.

> +
> +		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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