On 02/11/2014 06:51 PM, Nicolas Pitre wrote:
On Tue, 11 Feb 2014, Daniel Lezcano wrote:
The idle main function is a complex and a critical function. Added more
comments to the code.
Signed-off-by: Daniel Lezcano <[email protected]>
Few questions below. In any case,:
Acked-by: Nicolas Pitre <[email protected]>
Thanks for the review Nico !
Answer below.
---
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,
s/must/must be/
+ * 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 */
Is this if block really necessary? We already have need_resched() being
monitored in the outer loop. Are cpuidle_select() or rcu_idle_enter()
likely to spend a significant amount of time justifying a recheck here?
That's a question I have been always asking myself.
The cpuidle_select function could spend some time for:
1. reflecting the idle time for the statistics of the previous idle
period. This processing is post-poned when exiting an idle state via the
'need_update' field in the cpuidle structure. I guess, this is because
it can take a while and we want to exit asap to reduce the wakeup latency.
2. there are some processing to choose the idle state.
I don't know what is the rational here to use need_resched at this place
except to 'abort' an idle state arbitrarily after some experimentation
for better reactivity. I am wondering if the multiple need_resched() we
find in the call stack for some idle states makes really sense and
doesn't denote a lack of control of what is happening in the idle path
vs system activity or a lack of confidence in the idle duration prediction.
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