Re: [PATCH v13] sched/deadline: support dl task migration during cpu hotplug

[Juri Lelli]

Hi,

On 25/03/2015 23:47, 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.
> 
> Suggested-and-acked-by: Juri Lelli <juri.lelli@xxxxxxx>
> Signed-off-by: Wanpeng Li <wanpeng.li@xxxxxxxxxxxxxxx>
> ---
> v12 -> 13:
>  * move hotplug stuff to CONFIG_SMP in order to fix the error reported by kbuild test robot
> v11 -> v12:
>  * s/WARN_ON/BUG_ON
> v10 -> v11:
>  * fix codes comments
>  * tsk_cpus_allowed(p) shouldn't be on separate lines
>  * introduce a helper function to fold dl task migration during cpu hotplug support
> v9 -> v10:
>  * fix the "WARNING: line over 80 characters"
>  * handle no admission control
> v8 -> v9:
>  * align tsk_cpus_allowed(p) to cpu_active_mask
>  * add WARN_ON(1)
>  * don't resched_curr if later_rq come from the cpumask_any_and()
> 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 | 59 +++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 59 insertions(+)
> 
> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> index 24c18dc..60b7e90 100644
> --- a/kernel/sched/deadline.c
> +++ b/kernel/sched/deadline.c
> @@ -218,6 +218,54 @@ static inline void set_post_schedule(struct rq *rq)
>  	rq->post_schedule = has_pushable_dl_tasks(rq);
>  }
>  
> +static struct rq *find_lock_later_rq(struct task_struct *task, struct rq *rq);
> +
> +static void dl_task_migration(struct rq *rq, struct task_struct *p)
> +{
> +	struct rq *later_rq = NULL;
> +	bool fallback = false;
> +
> +	later_rq = find_lock_later_rq(p, rq);
> +
> +	if (!later_rq) {
> +		int cpu;
> +
> +		/*
> +		 * If we cannot preempt any rq, fall back to pick any
> +		 * online cpu.
> +		 */
> +		fallback = true;
> +		cpu = cpumask_any_and(cpu_active_mask, tsk_cpus_allowed(p));
> +		if (cpu >= nr_cpu_ids) {
> +			if (dl_bandwidth_enabled()) {
> +				/*
> +				 * Fail to find any suitable cpu.
> +				 * The task will never come back!
> +				 */
> +				BUG_ON(1);
> +				return;
> +			}
> +			/*
> +			 * If admission control is disabled we
> +			 * try a little harder to let the task
> +			 * run.
> +			 */
> +			cpu = cpumask_any(cpu_active_mask);
> +		}
> +		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);
> +
> +	if (!fallback)
> +		resched_curr(later_rq);
> +
> +	double_unlock_balance(rq, later_rq);
> +}
> +
>  #else
>  
>  static inline
> @@ -536,6 +584,17 @@ static enum hrtimer_restart dl_task_timer(struct hrtimer *timer)
>  	sched_clock_tick();
>  	update_rq_clock(rq);
>  
> +#ifdef CONFIG_SMP
> +	/*
> +	 * 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)) {
> +		dl_task_migration(rq, p);
> +		goto unlock;
> +	}
> +#endif
> +
>  	/*
>  	 * If the throttle happened during sched-out; like:
>  	 *
> 

I think we could have better naming and the BUG_ON part could be
simplified. I'd propose something like this (sorry, it is based
on one of your previous versions.. I'm a bit struggling keeping
up with the pace ;)).

Thanks!

---
 kernel/sched/deadline.c | 20 +++++++++-----------
 1 file changed, 9 insertions(+), 11 deletions(-)

diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index 1820d1b..2b260a9 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -518,7 +518,7 @@ static int start_dl_timer(struct sched_dl_entity *dl_se, bool boosted)
 
 static struct rq *find_lock_later_rq(struct task_struct *task, struct rq *rq);
 
-static void dl_task_migration(struct rq *rq, struct task_struct *p)
+static void dl_task_offline_migration(struct rq *rq, struct task_struct *p)
 {
 	struct rq *later_rq = NULL;
 	bool fallback = false;
@@ -535,14 +535,12 @@ static void dl_task_migration(struct rq *rq, struct task_struct *p)
 		fallback = true;
 		cpu = cpumask_any_and(cpu_active_mask, tsk_cpus_allowed(p));
 		if (cpu >= nr_cpu_ids) {
-			if (dl_bandwidth_enabled()) {
-				/*
-				 * Fail to find any suitable cpu.
-				 * The task will never come back!
-				 */
-				WARN_ON(1);
-				return;
-			}
+			/*
+			 * Fail to find any suitable cpu.
+			 * The task will never come back!
+			 */
+			BUG_ON(dl_bandwidth_enabled());
+
 			/*
 			 * If admission control is disabled we
 			 * try a little harder to let the task
@@ -609,11 +607,11 @@ 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
+	 * 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)) {
-		dl_task_migration(rq, p);
+		dl_task_offline_migration(rq, p);
 		goto unlock;
 	}
 
-- 
2.3.0

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