Re: [PATCH v10] sched/deadline: support dl task migration during cpu hotplug

[Wanpeng Li]

Ping Juri, your reviewed-by/acked-by is a great appreciated. ;-)
On Thu, Mar 05, 2015 at 10:12:13AM +0800, 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-by: Juri Lelli <juri.lelli@xxxxxxx>
>Signed-off-by: Wanpeng Li <wanpeng.li@xxxxxxxxxxxxxxx>
>---
>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 | 54 +++++++++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 54 insertions(+)
>
>diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
>index 08766a3..1ed6928 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 +538,59 @@ 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;
>+		bool fallback = false;
>+
>+		later_rq = find_lock_later_rq(p, rq);
>+
>+		if (!later_rq) {
>+			int cpu;
>+
>+			/*
>+			 * If cannot preempt any rq, fallback 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!
>+					 */
>+					WARN_ON(1);
>+					goto unlock;
>+				} else {
>+					/*
>+					 * 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);
>+
>+		goto unlock;
>+	}
>+
>+	/*
> 	 * If the throttle happened during sched-out; like:
> 	 *
> 	 *   schedule()
>-- 
>1.9.1
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