[PATCH v4] sched/deadline: Fix the intention to re-evalute tick dependency for offline cpu

[Wanpeng Li]

From: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>

The dl task will be replenished after dl task timer fire and start a 
new period. It will be enqueued and to re-evaluate its dependency on 
the tick in order to restart it. However, if cpu is hot-unplug, 
irq_work_queue will splash since the target cpu is offline.

As a result:

    WARNING: CPU: 2 PID: 0 at kernel/irq_work.c:69 irq_work_queue_on+0xad/0xe0
    Call Trace:
     dump_stack+0x99/0xd0
     __warn+0xd1/0xf0
     warn_slowpath_null+0x1d/0x20
     irq_work_queue_on+0xad/0xe0
     tick_nohz_full_kick_cpu+0x44/0x50
     tick_nohz_dep_set_cpu+0x74/0xb0
     enqueue_task_dl+0x226/0x480
     activate_task+0x5c/0xa0
     dl_task_timer+0x19b/0x2c0
     ? push_dl_task.part.31+0x190/0x190
  
This can be triggered by hot-unplug the full dynticks cpu which dl 
task is running on. 

We enqueue the dl task on the offline CPU, because we need to do 
replenish for start_dl_timer(). So, as Juri pointed out, we would 
need to do is calling replenish_dl_entity() directly, instead of 
enqueue_task_dl(). pi_se shouldn't be a problem as the task shouldn't 
be boosted if it was throttled.

This patch fixes it by avoiding the whole enqueue+dequeue+enqueue story, by 
first migrating (set_task_cpu) and then doing 1 enqueue.

Suggested-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Juri Lelli <juri.lelli@xxxxxxx>
Cc: Luca Abeni <luca.abeni@xxxxxxxx>
Cc: Frederic Weisbecker <fweisbec@xxxxxxxxx>
Signed-off-by: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
---
v3 -> v4:
 * first migrating (set_task_cpu) and then doing 1 enqueue
v2 -> v3:
 * move rq->online check under CONFIG_SMP
v1 -> v2:
 * replenish dl entity

 kernel/sched/deadline.c | 46 ++++++++++++++++++----------------------------
 1 file changed, 18 insertions(+), 28 deletions(-)

diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index 1ce8867..efe2acd 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -243,10 +243,8 @@ static struct rq *find_lock_later_rq(struct task_struct *task, struct rq *rq);
 static struct rq *dl_task_offline_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;
 
@@ -254,7 +252,6 @@ static struct rq *dl_task_offline_migration(struct rq *rq, struct task_struct *p
 		 * 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) {
 			/*
@@ -274,16 +271,7 @@ static struct rq *dl_task_offline_migration(struct rq *rq, struct task_struct *p
 		double_lock_balance(rq, later_rq);
 	}
 
-	/*
-	 * By now the task is replenished and enqueued; migrate it.
-	 */
-	deactivate_task(rq, p, 0);
 	set_task_cpu(p, later_rq->cpu);
-	activate_task(later_rq, p, 0);
-
-	if (!fallback)
-		resched_curr(later_rq);
-
 	double_unlock_balance(later_rq, rq);
 
 	return later_rq;
@@ -641,29 +629,31 @@ static enum hrtimer_restart dl_task_timer(struct hrtimer *timer)
 		goto unlock;
 	}
 
-	enqueue_task_dl(rq, p, ENQUEUE_REPLENISH);
-	if (dl_task(rq->curr))
-		check_preempt_curr_dl(rq, p, 0);
-	else
-		resched_curr(rq);
-
 #ifdef CONFIG_SMP
-	/*
-	 * Perform balancing operations here; after the replenishments.  We
-	 * cannot drop rq->lock before this, otherwise the assertion in
-	 * start_dl_timer() about not missing updates is not true.
-	 *
-	 * If we find that the rq the task was on is no longer available, we
-	 * need to select a new rq.
-	 *
-	 * XXX figure out if select_task_rq_dl() deals with offline cpus.
-	 */
 	if (unlikely(!rq->online)) {
+		/*
+		 * If the runqueue is no longer available, migrate the
+		 * task elsewhere. This necessarily changes rq.
+		 */
 		lockdep_unpin_lock(&rq->lock, rf.cookie);
 		rq = dl_task_offline_migration(rq, p);
 		rf.cookie = lockdep_pin_lock(&rq->lock);
+
+		/*
+		 * Now that the task has been migrated to the new RQ and we
+		 * have that locked, proceed as normal and enqueue the task
+		 * there.
+		 */
 	}
+#endif
 
+	enqueue_task_dl(rq, p, ENQUEUE_REPLENISH);
+	if (dl_task(rq->curr))
+		check_preempt_curr_dl(rq, p, 0);
+	else
+		resched_curr(rq);
+
+#ifdef CONFIG_SMP
 	/*
 	 * Queueing this task back might have overloaded rq, check if we need
 	 * to kick someone away.
-- 
1.9.1