[PATCH v3] 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 fix it by just replenish dl entity to avoid the intention 
to re-evaluate tick dependency if the cpu is offline.

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>
---
v2 -> v3:
 * move rq->online check under CONFIG_SMP
v1 -> v2:
 * replenish dl entity

 kernel/sched/deadline.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index d091f4a..ce0fb00 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -641,6 +641,11 @@ static enum hrtimer_restart dl_task_timer(struct hrtimer *timer)
 		goto unlock;
 	}
 
+#ifdef CONFIG_SMP
+	if (unlikely(!rq->online))
+		goto offline;
+#endif
+
 	enqueue_task_dl(rq, p, ENQUEUE_REPLENISH);
 	if (dl_task(rq->curr))
 		check_preempt_curr_dl(rq, p, 0);
@@ -648,6 +653,7 @@ static enum hrtimer_restart dl_task_timer(struct hrtimer *timer)
 		resched_curr(rq);
 
 #ifdef CONFIG_SMP
+offline:
 	/*
 	 * Perform balancing operations here; after the replenishments.  We
 	 * cannot drop rq->lock before this, otherwise the assertion in
@@ -659,6 +665,7 @@ static enum hrtimer_restart dl_task_timer(struct hrtimer *timer)
 	 * XXX figure out if select_task_rq_dl() deals with offline cpus.
 	 */
 	if (unlikely(!rq->online)) {
+		replenish_dl_entity(dl_se, dl_se);
 		lockdep_unpin_lock(&rq->lock, rf.cookie);
 		rq = dl_task_offline_migration(rq, p);
 		rf.cookie = lockdep_pin_lock(&rq->lock);
-- 
1.9.1