[PATCH] sched/rt: Fix possible warn when push_rt_task

[Hui Tang]

A warn may be triggered during reboot, as follows:

reboot
  ->kernel_restart
    ->machine_restart
      ->smp_send_stop --- ipi handler set_cpu_online(cpu, false)

balance_callback
-> __balance_callback
  ->push_rt_task
    -> find_lock_lowest_rq  --- offline cpu in vec->mask not be cleared
      -> find_lowest_rq
        -> cpupri_find
          -> cpupri_find_fitness
            -> __cpupri_find [cpumask_and(..., vec->mask)]
    -> set_task_cpu(next_task, lowest_rq->cpu) --- WARN_ON(!oneline(cpu)

So add !cpu_online(lowest_rq->cpu) check before set_task_cpu().
The fix does not completely fix the problem, since cpu_online_mask may
be cleared after check.

Fixes: 4ff9083b8a9a8 ("sched/core: WARN() when migrating to an offline CPU")
Signed-off-by: Hui Tang <tanghui20@xxxxxxxxxx>
---
 kernel/sched/rt.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index 00e0e5074115..852ef18b6a50 100644
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -2159,6 +2159,9 @@ static int push_rt_task(struct rq *rq, bool pull)
 		goto retry;
 	}
 
+	if (unlikely(!cpu_online(lowest_rq->cpu)))
+		goto out;
+
 	deactivate_task(rq, next_task, 0);
 	set_task_cpu(next_task, lowest_rq->cpu);
 	activate_task(lowest_rq, next_task, 0);
-- 
2.17.1