sched/idle: Add more comments to the code

The idle main function is a complex and a critical function. Added more
comments to the code.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Acked-by: Nicolas Pitre <nico@linaro.org>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Cc: tglx@linutronix.de
Cc: rjw@rjwysocki.net
Cc: preeti@linux.vnet.ibm.com
Link: http://lkml.kernel.org/r/1393832934-11625-5-git-send-email-daniel.lezcano@linaro.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

kernel/sched/idle.c

@@ -76,21 +76,49 @@ static int cpuidle_idle_call(void)
 	int next_state, entered_state, ret;
 	bool broadcast;
 
+	/*
+	 * Check if the idle task must be rescheduled. If it is the
+	 * case, exit the function after re-enabling the local irq and
+	 * set again the polling flag
+	 */
 	if (current_clr_polling_and_test()) {
 		local_irq_enable();
 		__current_set_polling();
 		return 0;
 	}
 
+	/*
+	 * During the idle period, stop measuring the disabled irqs
+	 * critical sections latencies
+	 */
 	stop_critical_timings();
+
+	/*
+	 * Tell the RCU framework we are entering an idle section,
+	 * so no more rcu read side critical sections and one more
+	 * step to the grace period
+	 */
 	rcu_idle_enter();
 
+	/*
+	 * Check if the cpuidle framework is ready, otherwise fallback
+	 * to the default arch specific idle method
+	 */
 	ret = cpuidle_enabled(drv, dev);
 
 	if (!ret) {
-		/* ask the governor for the next state */
+		/*
+		 * Ask the governor to choose an idle state it thinks
+		 * it is convenient to go to. There is *always* a
+		 * convenient idle state
+		 */
 		next_state = cpuidle_select(drv, dev);
 
+		/*
+		 * The idle task must be scheduled, it is pointless to
+		 * go to idle, just update no idle residency and get
+		 * out of this function
+		 */
 		if (current_clr_polling_and_test()) {
 			dev->last_residency = 0;
 			entered_state = next_state;
@@ -100,13 +128,29 @@ static int cpuidle_idle_call(void)
 				       CPUIDLE_FLAG_TIMER_STOP);
 
 			if (broadcast)
+				/*
+				 * Tell the time framework to switch
+				 * to a broadcast timer because our
+				 * local timer will be shutdown. If a
+				 * local timer is used from another
+				 * cpu as a broadcast timer, this call
+				 * may fail if it is not available
+				 */
 				ret = clockevents_notify(
 					CLOCK_EVT_NOTIFY_BROADCAST_ENTER,
 					&dev->cpu);
 
 			if (!ret) {
 				trace_cpu_idle_rcuidle(next_state, dev->cpu);
 
+				/*
+				 * Enter the idle state previously
+				 * returned by the governor
+				 * decision. This function will block
+				 * until an interrupt occurs and will
+				 * take care of re-enabling the local
+				 * interrupts
+				 */
 				entered_state = cpuidle_enter(drv, dev,
 							      next_state);
 
@@ -118,17 +162,28 @@ static int cpuidle_idle_call(void)
 						CLOCK_EVT_NOTIFY_BROADCAST_EXIT,
 						&dev->cpu);
 
-				/* give the governor an opportunity to reflect on the outcome */
+				/*
+				 * Give the governor an opportunity to reflect on the
+				 * outcome
+				 */
 				cpuidle_reflect(dev, entered_state);
 			}
 		}
 	}
 
+	/*
+	 * We can't use the cpuidle framework, let's use the default
+	 * idle routine
+	 */
 	if (ret)
 		arch_cpu_idle();
 
 	__current_set_polling();
 
+	/*
+	 * It is up to the idle functions to enable back the local
+	 * interrupt
+	 */
 	if (WARN_ON_ONCE(irqs_disabled()))
 		local_irq_enable();