narrow down the scope of EnqueueExtensions to subscribe less cluster events

pkg/scheduler/framework/interface.go

@@ -350,8 +350,17 @@ type QueueSortPlugin interface {
 }
 
 // EnqueueExtensions is an optional interface that plugins can implement to efficiently
-// move unschedulable Pods in internal scheduling queues. Plugins
-// that fail pod scheduling (e.g., Filter plugins) are expected to implement this interface.
+// move unschedulable Pods in internal scheduling queues.
+// In the scheduler, Pods can be unschedulable by PreEnqueue, PreFilter, Filter, Reserve, and Permit plugins,
+// and Pods rejected by these plugins are requeued based on this extension point.
+// Failures from other extension points are regarded as temporal errors (e.g., network failure),
+// and the scheduler requeue Pods without this extension point - always requeue Pods to activeQ after backoff.
+// This is because such temporal errors cannot be resolved by specific cluster events,
+// and we have no choise but keep retrying scheduling until the failure is resolved.
+//
+// Plugins that make pod unschedulable (PreEnqueue, PreFilter, Filter, Reserve, and Permit plugins) should implement this interface,
+// otherwise the default implementation will be used, which is less efficient in requeueing Pods rejected by the plugin.
+// And, if plugins other than above extension points support this interface, they are just ignored.
 type EnqueueExtensions interface {
 	Plugin
 	// EventsToRegister returns a series of possible events that may cause a Pod

pkg/scheduler/framework/runtime/framework.go

@@ -545,7 +545,22 @@ func (f *frameworkImpl) expandMultiPointPlugins(logger klog.Logger, profile *con
 	return nil
 }
 
+func shouldHaveEnqueueExtensions(p framework.Plugin) bool {
+	switch p.(type) {
+	// Only PreEnqueue, PreFilter, Filter, Reserve, and Permit plugins can (should) have EnqueueExtensions.
+	// See the comment of EnqueueExtensions for more detailed reason here.
+	case framework.PreEnqueuePlugin, framework.PreFilterPlugin, framework.FilterPlugin, framework.ReservePlugin, framework.PermitPlugin:
+		return true
+	}
+	return false
+}
+
 func (f *frameworkImpl) fillEnqueueExtensions(p framework.Plugin) {
+	if !shouldHaveEnqueueExtensions(p) {
+		// Ignore EnqueueExtensions from plugin which isn't PreEnqueue, PreFilter, Filter, Reserve, and Permit.
+		return
+	}
+
 	ext, ok := p.(framework.EnqueueExtensions)
 	if !ok {
 		// If interface EnqueueExtensions is not implemented, register the default enqueue extensions