sched/fair: Plug hole between hotplug and active_load_balance()

The load balancer applies cpu_active_mask to whatever sched_domains it
finds, however in the case of active_balance there is a hole between
setting rq->{active_balance,push_cpu} and running the stop_machine
work doing the actual migration.

The @push_cpu can go offline in this window, which would result in us
moving a task onto a dead cpu, which is a fairly bad thing.

Double check the active mask before the stop work does the migration.

  CPU0					CPU1

  <SoftIRQ>
					stop_machine(takedown_cpu)
    load_balance()			cpu_stopper_thread()
      ...				  work = multi_cpu_stop
      stop_one_cpu_nowait(		    /* wait for CPU0 */
	.func = active_load_balance_cpu_stop
      );
  </SoftIRQ>

  cpu_stopper_thread()
    work = multi_cpu_stop
      /* sync with CPU1 */
					    take_cpu_down()
					<idle>
					  play_dead();

    work = active_load_balance_cpu_stop
      set_task_cpu(p, CPU1); /* oops!! */

Reported-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20170907150614.044460912@infradead.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

kernel/sched/fair.c

@@ -8560,6 +8560,13 @@ static int active_load_balance_cpu_stop(void *data)
 	struct rq_flags rf;
 
 	rq_lock_irq(busiest_rq, &rf);
+	/*
+	 * Between queueing the stop-work and running it is a hole in which
+	 * CPUs can become inactive. We should not move tasks from or to
+	 * inactive CPUs.
+	 */
+	if (!cpu_active(busiest_cpu) || !cpu_active(target_cpu))
+		goto out_unlock;
 
 	/* make sure the requested cpu hasn't gone down in the meantime */
 	if (unlikely(busiest_cpu != smp_processor_id() ||