spinwait for events in the near future (#1491)

* add spinwait

* simplify and reduce lateDelay

* maybe probably fix tests

* comments

Co-authored-by: Matthew Kennedy <makenne@microsoft.com>

firmware/controllers/system/timer/event_queue.cpp

@@ -24,11 +24,6 @@ extern bool verboseMode;
 
 uint32_t maxSchedulingPrecisionLoss = 0;
 
-EventQueue::EventQueue() {
-	head = nullptr;
-	setLateDelay(100);
-}
-
 bool EventQueue::checkIfPending(scheduling_s *scheduling) {
 	assertNotInListMethodBody(scheduling_s, head, scheduling, nextScheduling_s);
 }
@@ -100,10 +95,8 @@ efitime_t EventQueue::getNextEventTime(efitime_t nowX) const {
 			 *
 			 * looks like we end up here after 'writeconfig' (which freezes the firmware) - we are late
 			 * for the next scheduled event
-			 *
 			 */
-			efitime_t aBitInTheFuture = nowX + lateDelay;
-			return aBitInTheFuture;
+			return nowX + lateDelay;
 		} else {
 			return head->momentX;
 		}
@@ -139,13 +132,20 @@ int EventQueue::executeAll(efitime_t now) {
 			break;
 		}
 
-		// Only execute events that occured in the past.
-		// The list is sorted, so as soon as we see an event
-		// in the future, we're done.
-		if (current->momentX > now) {
+		// If the next event is far in the future, we'll reschedule
+		// and execute it next time.
+		// We do this when the next event is close enough that the overhead of
+		// resetting the timer and scheduling an new interrupt is greater than just
+		// waiting for the time to arrive.  On current CPUs, this is reasonable to set
+		// around 10 microseconds.
+		if (current->momentX > now + lateDelay) {
 			break;
 		}
 
+		// near future - spin wait for the event to happen and avoid the
+		// overhead of rescheduling the timer.
+		while (current->momentX > getTimeNowNt()) ;
+
 		executionCounter++;
 
 		// step the head forward, unlink this element, clear scheduled flag
@@ -187,10 +187,6 @@ void EventQueue::assertListIsSorted() const {
 	}
 }
 
-void EventQueue::setLateDelay(int value) {
-	lateDelay = value;
-}
-
 scheduling_s * EventQueue::getHead() {
 	return head;
 }

firmware/controllers/system/timer/event_queue.h

@@ -45,7 +45,10 @@
  */
 class EventQueue {
 public:
-	EventQueue();
+	// See comment in EventQueue::executeAll for info about lateDelay - it sets the 
+	// time gap between events for which we will wait instead of rescheduling the next
+	// event in a group of events near one another.
+	EventQueue(efitime_t lateDelay = 0) : lateDelay(lateDelay) {}
 
 	/**
 	 * O(size) - linear search in sorted linked list
@@ -58,7 +61,6 @@ public:
 	void clear(void);
 	int size(void) const;
 	scheduling_s *getElementAtIndexForUnitText(int index);
-	void setLateDelay(int value);
 	scheduling_s * getHead();
 	void assertListIsSorted() const;
 private:
@@ -66,7 +68,7 @@ private:
 	/**
 	 * this list is sorted
 	 */
-	scheduling_s *head;
-	efitime_t lateDelay;
+	scheduling_s *head = nullptr;
+	const efitime_t lateDelay;
 };
 

firmware/controllers/system/timer/single_timer_executor.cpp

@@ -49,12 +49,10 @@ void globalTimerCallback() {
 	___engine.executor.onTimerCallback();
 }
 
-SingleTimerExecutor::SingleTimerExecutor() {
-	/**
-	 * See comments in "getNextEventTime"
-	 * 10us is roughly double the cost of the interrupt + overhead of a single timer event
-	 */
-	queue.setLateDelay(US2NT(10));
+SingleTimerExecutor::SingleTimerExecutor()
+	// 10us is roughly double the cost of the interrupt + overhead of a single timer event
+	: queue(US2NT(10))
+{
 }
 
 void SingleTimerExecutor::scheduleForLater(scheduling_s *scheduling, int delayUs, action_s action) {