Re: [RFC PATCH v2] sched/deadline: support dl task migration during cpu hotplug

[Kirill Tkhai <ktkhai@parallels.com>]

Hi, all,

В Ср, 05/11/2014 в 16:51 +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.
> 
> Note: dl task can be migrated successfully if rq is offline currently, however, 
> I'm still not sure why task_rq(task)->rd->span just include the cpu which the dl 
> task previous running on, so cpu_active_mask is used in the patch. 
> 
> Peterz, Juri?
> 
> Signed-off-by: Wanpeng Li <wanpeng.li@linux.intel.com>
> ---
> v1 -> v2:
>  * push the task to another cpu in dl_task_timer() if rq is offline.
> 
>  kernel/sched/deadline.c | 39 ++++++++++++++++++++++++++++++++++++---
>  1 file changed, 36 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> index 04c2cbb..233e482 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 +539,39 @@ 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;
> +
> +		/* We will release rq lock */
> +		get_task_struct(p);
> +
> +		raw_spin_unlock(&rq->lock);
> +
> +		later_rq = find_lock_later_rq(p, rq);
> +
> +		if (!later_rq) {
> +			put_task_struct(p);
> +			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);
> +
> +		put_task_struct(p);
> +
> +		goto out;
> +	}
> +
>  	if (task_on_rq_queued(p)) {
>  		enqueue_task_dl(rq, p, ENQUEUE_REPLENISH);
>  		if (dl_task(rq->curr))
> @@ -555,7 +589,7 @@ again:
>  	}
>  unlock:
>  	raw_spin_unlock(&rq->lock);
> -
> +out:
>  	return HRTIMER_NORESTART;
>  }
>  
> @@ -1182,8 +1216,7 @@ 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);
> +	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);

isn't this too complicated?

Can't we simply queue throttled tasks in rq_offline_dl() (without clearing
of dl_throttled() status)? migrate_tasks() will do the migration right.

This case the patch from the topic may be simply transformed in:

diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index f3d7776..5059aa8 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -531,7 +531,7 @@ static enum hrtimer_restart dl_task_timer(struct hrtimer *timer)
 	 * in the runqueue or is going to be enqueued back anyway.
 	 */
 	if (!dl_task(p) || dl_se->dl_new ||
-	    dl_se->dl_boosted || !dl_se->dl_throttled)
+	    dl_se->dl_boosted || !dl_se->dl_throttled || on_dl_rq(dl_se))
 		goto unlock;
 
 	sched_clock_tick();