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 <[e-mail]>
Signed-off-by: Peter Zijlstra (Intel) <[e-mail]>
Cc: Linus Torvalds <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Ingo Molnar <[e-mail]>

kernel/sched/fair.c

···
 	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() ||