[PATCH] sched: fix the intention to re-evalute tick dependency for offline cpu

[Wanpeng Li <kernellwp@gmail.com>]

From: Wanpeng Li <wanpeng.li@hotmail.com>

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. 

Actually we don't need to restart the tick since the target cpu is offline 
and nothing need scheduler tick. This patch fix it by not intend to re-evaluate 
tick dependency if the cpu is offline.

Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Juri Lelli <juri.lelli@arm.com>
Cc: Luca Abeni <luca.abeni@unitn.it>
Signed-off-by: Wanpeng Li <wanpeng.li@hotmail.com>
---
 kernel/sched/core.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 7f2cae4..43b494f 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -628,6 +628,9 @@ bool sched_can_stop_tick(struct rq *rq)
 {
 	int fifo_nr_running;
 
+	if (unlikely(!rq->online))
+		return true;
+
 	/* Deadline tasks, even if single, need the tick */
 	if (rq->dl.dl_nr_running)
 		return false;
-- 
1.9.1