[PATCH RESEND v10] 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 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>
---
 kernel/sched/deadline.c | 54 +++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 54 insertions(+)

diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index 5cb5c9c..db457b9 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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