[PATCH] sched/core: Fix kick offline cpu to do nohz idle load balance

[Wanpeng Li <kernellwp@gmail.com>]

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

 WARNING: CPU: 0 PID: 3404 at arch/x86/kernel/smp.c:125 native_smp_send_reschedule+0x3f/0x50
 CPU: 0 PID: 3404 Comm: qemu-system-x86 Not tainted 4.8.0+ #21
 Call Trace:
   __warn+0xd1/0xf0
   warn_slowpath_null+0x1d/0x20
   native_smp_send_reschedule+0x3f/0x50
   trigger_load_balance+0x29c/0x4a0
   ? trigger_load_balance+0x72/0x4a0
   scheduler_tick+0x9f/0xd0
   ? tick_sched_do_timer+0x50/0x50
   update_process_times+0x47/0x60
   tick_sched_handle.isra.24+0x25/0x60
   tick_sched_timer+0x3d/0x70
   __hrtimer_run_queues+0xf4/0x510
   hrtimer_interrupt+0xb7/0x1d0
   local_apic_timer_interrupt+0x35/0x60
   smp_apic_timer_interrupt+0x3d/0x50
   apic_timer_interrupt+0x96/0xa0

If there is a need to kick the idle load balancer, an ILB will be selected 
to perform nohz idle load balance, however, if the selected ILB is in the 
process of offline, smp_sched_reschedule() which generates a sched IPI will 
splat as above.

           CPU0                      CPU1 
                         
                                 find_new_ilb() 
    set_rq_offline() 
                                 smp_sched_reschedule()  Oops
    nohz_balance_exit_idle() 

This patch fix it by exiting nohz idle balance before set cpu offline.

Cc: Ingo Molnar <mingo@kernel.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Wanpeng Li <wanpeng.li@hotmail.com>
---
 kernel/sched/core.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 94732d1..7c83f99 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -7412,6 +7412,7 @@ int sched_cpu_dying(unsigned int cpu)
 
 	/* Handle pending wakeups and then migrate everything off */
 	sched_ttwu_pending();
+	nohz_balance_exit_idle(cpu);
 	raw_spin_lock_irqsave(&rq->lock, flags);
 	if (rq->rd) {
 		BUG_ON(!cpumask_test_cpu(cpu, rq->rd->span));
@@ -7422,7 +7423,6 @@ int sched_cpu_dying(unsigned int cpu)
 	raw_spin_unlock_irqrestore(&rq->lock, flags);
 	calc_load_migrate(rq);
 	update_max_interval();
-	nohz_balance_exit_idle(cpu);
 	hrtick_clear(rq);
 	return 0;
 }
-- 
1.9.1