sched: Start stopper early

Ensure the stopper thread is active 'early', because the load balancer
pretty much assumes that its available. And when 'online && active' the
load-balancer is fully available.

Not only the numa balancing stop_two_cpus() caller relies on it, but
also the self migration stuff does, and at CPU_ONLINE time the cpu
really is 'free' to run anything.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Rik van Riel <riel@redhat.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: heiko.carstens@de.ibm.com
Link: http://lkml.kernel.org/r/20151009160054.GA10176@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

kernel/cpu.c

@@ -475,7 +475,6 @@ static int smpboot_thread_call(struct notifier_block *nfb,
 
 	case CPU_DOWN_FAILED:
 	case CPU_ONLINE:
-		stop_machine_unpark(cpu);
 		smpboot_unpark_threads(cpu);
 		break;
 

kernel/sched/core.c

@@ -5545,21 +5545,27 @@ static void set_cpu_rq_start_time(void)
 static int sched_cpu_active(struct notifier_block *nfb,
 				      unsigned long action, void *hcpu)
 {
+	int cpu = (long)hcpu;
+
 	switch (action & ~CPU_TASKS_FROZEN) {
 	case CPU_STARTING:
 		set_cpu_rq_start_time();
 		return NOTIFY_OK;
+
 	case CPU_ONLINE:
 		/*
 		 * At this point a starting CPU has marked itself as online via
 		 * set_cpu_online(). But it might not yet have marked itself
 		 * as active, which is essential from here on.
-		 *
-		 * Thus, fall-through and help the starting CPU along.
 		 */
+		set_cpu_active(cpu, true);
+		stop_machine_unpark(cpu);
+		return NOTIFY_OK;
+
 	case CPU_DOWN_FAILED:
-		set_cpu_active((long)hcpu, true);
+		set_cpu_active(cpu, true);
 		return NOTIFY_OK;
+
 	default:
 		return NOTIFY_DONE;
 	}