[RFC PATCH v3] sched/deadline: support dl task migration during cpu hotplug

[Wanpeng Li]

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@xxxxxxxxxxxxxxx>
---
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 f3d7776..b1660c8 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);
+
+		if (!later_rq) {
+			int cpu;
+
+			/*
+			 * If cannot preempt any rq, fallback to pick any
+			 * online cpu.
+			 */
+			for_each_cpu(cpu, &p->cpus_allowed)
+				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);
+
+		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);
-- 
1.9.1

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