sched/idle: Simplify wake_up_idle_cpu()

Now that rq->idle's polling bit is a reliable indication that the cpu is
polling, use it.

Signed-off-by: Andy Lutomirski <luto@amacapital.net>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Cc: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Cc: nicolas.pitre@linaro.org
Cc: daniel.lezcano@linaro.org
Cc: umgwanakikbuti@gmail.com
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: linux-kernel@vger.kernel.org
Link: http://lkml.kernel.org/r/922f00761445a830ebb23d058e2ae53956ce2d73.1401902905.git.luto@amacapital.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>

kernel/sched/core.c

@@ -628,26 +628,7 @@ static void wake_up_idle_cpu(int cpu)
 	if (cpu == smp_processor_id())
 		return;
 
-	/*
-	 * This is safe, as this function is called with the timer
-	 * wheel base lock of (cpu) held. When the CPU is on the way
-	 * to idle and has not yet set rq->curr to idle then it will
-	 * be serialized on the timer wheel base lock and take the new
-	 * timer into account automatically.
-	 */
-	if (rq->curr != rq->idle)
-		return;
-
-	/*
-	 * We can set TIF_RESCHED on the idle task of the other CPU
-	 * lockless. The worst case is that the other CPU runs the
-	 * idle task through an additional NOOP schedule()
-	 */
-	set_tsk_need_resched(rq->idle);
-
-	/* NEED_RESCHED must be visible before we test polling */
-	smp_mb();
-	if (!tsk_is_polling(rq->idle))
+	if (set_nr_and_not_polling(rq->idle))
 		smp_send_reschedule(cpu);
 	else
 		trace_sched_wake_idle_without_ipi(cpu);