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Subject[PATCH 5/5] idle: Add more comments to the code
Date
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>
---
kernel/sched/idle.c | 37 +++++++++++++++++++++++++++++++++----
1 file changed, 33 insertions(+), 4 deletions(-)

diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
index 72b5926..36ff1a7 100644
--- a/kernel/sched/idle.c
+++ b/kernel/sched/idle.c
@@ -86,19 +86,34 @@ static int cpuidle_idle_call(void)
if (cpu_idle_force_poll || tick_check_broadcast_expired())
return cpu_idle_poll();

+ /*
+ * Check if the idle task must 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();

- /* Ask the governor for the next state, this call can fail for
- * different reasons: cpuidle is not enabled or an idle state
- * fulfilling the constraints was not found. In this case, we fall
- * back to the default idle function
+ /*
+ * Ask the governor to choose an idle state it thinks it is
+ * convenient to go to. There is *always* a convenient idle
+ * state but the call could fail if cpuidle is not enabled
*/
next_state = cpuidle_select(drv, dev);
if (next_state < 0) {
@@ -106,6 +121,10 @@ static int cpuidle_idle_call(void)
goto out;
}

+ /*
+ * 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 (need_resched()) {
dev->last_residency = 0;
/* give the governor an opportunity to reflect on the outcome */
@@ -116,6 +135,12 @@ static int cpuidle_idle_call(void)

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);

trace_cpu_idle_rcuidle(PWR_EVENT_EXIT, dev->cpu);
@@ -152,6 +177,10 @@ static void cpu_idle_loop(void)
local_irq_disable();
arch_cpu_idle_enter();

+ /*
+ * It is up to the underlying functions to
+ * enable the local interrupts again
+ */
cpuidle_idle_call();

arch_cpu_idle_exit();
--
1.7.9.5


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