cpu/hotplug: Let upcoming cpu bring itself fully up

Let the upcoming cpu kick the hotplug thread and let itself complete the
bringup. That way the controll side can just wait for the completion or later
when we made the hotplug machinery async not care at all.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-arch@vger.kernel.org
Cc: Rik van Riel <riel@redhat.com>
Cc: Rafael Wysocki <rafael.j.wysocki@intel.com>
Cc: "Srivatsa S. Bhat" <srivatsa@mit.edu>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Arjan van de Ven <arjan@linux.intel.com>
Cc: Sebastian Siewior <bigeasy@linutronix.de>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Paul McKenney <paulmck@linux.vnet.ibm.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul Turner <pjt@google.com>
Link: http://lkml.kernel.org/r/20160226182341.697655464@linutronix.de
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

kernel/sched/idle.c

@@ -4,6 +4,7 @@
 #include <linux/sched.h>
 #include <linux/cpu.h>
 #include <linux/cpuidle.h>
+#include <linux/cpuhotplug.h>
 #include <linux/tick.h>
 #include <linux/mm.h>
 #include <linux/stackprotector.h>
@@ -291,5 +292,6 @@ void cpu_startup_entry(enum cpuhp_state state)
 	boot_init_stack_canary();
 #endif
 	arch_cpu_idle_prepare();
+	cpuhp_online_idle(state);
 	cpu_idle_loop();
 }