Update comments in Horizontal Pod Autoscaler Controller.

pkg/controller/podautoscaler/horizontal.go

@@ -179,7 +179,9 @@ func (a *HorizontalController) enqueueHPA(obj interface{}) {
 		return
 	}
 
-	// always add rate-limited so we don't fetch metrics more that once per resync interval
+	// Requests are always added to queue with resyncPeriod delay.  If there's already
+	// request for the HPA in the queue then a new request is always dropped. Requests spend resync
+	// interval in queue so HPAs are processed every resync interval.
 	a.queue.AddRateLimited(key)
 }
 
@@ -211,10 +213,15 @@ func (a *HorizontalController) processNextWorkItem() bool {
 	if err != nil {
 		utilruntime.HandleError(err)
 	}
-	// Add request processing HPA after resync interval just in case last resync didn't insert
-	// request into the queue. Request is not inserted into queue by resync if previous one wasn't processed yet.
-	// This happens quite often because requests from previous resync are removed from the queue at the same moment
-	// as next resync inserts new requests.
+	// Add request processing HPA to queue with resyncPeriod delay.
+	// Requests are always added to queue with resyncPeriod delay. If there's already request
+	// for the HPA in the queue then a new request is always dropped. Requests spend resyncPeriod
+	// in queue so HPAs are processed every resyncPeriod.
+	// Request is added here just in case last resync didn't insert request into the queue. This
+	// happens quite often because there is race condition between adding request after resyncPeriod
+	// and removing them from queue. Request can be added by resync before previous request is
+	// removed from queue. If we didn't add request here then in this case one request would be dropped
+	// and HPA would processed after 2 x resyncPeriod.
 	if !deleted {
 		a.queue.AddRateLimited(key)
 	}