Introduce a defaultpreemption PostFilter plugin

- Add a defaultpreemption PostFilter plugin
- Make g.Preempt() stateless
    - make g.Preempt() stateless
    - make g.getLowerPriorityNominatedPods() stateless
    - make g.processPreemptionWithExtenders() stateless

pkg/scheduler/eventhandlers.go

@@ -30,6 +30,7 @@ import (
 	coreinformers "k8s.io/client-go/informers/core/v1"
 	"k8s.io/client-go/tools/cache"
 	"k8s.io/kubernetes/pkg/features"
+	kubefeatures "k8s.io/kubernetes/pkg/features"
 	"k8s.io/kubernetes/pkg/scheduler/internal/queue"
 	"k8s.io/kubernetes/pkg/scheduler/profile"
 )
@@ -466,6 +467,11 @@ func addAllEventHandlers(
 			AddFunc: sched.onStorageClassAdd,
 		},
 	)
+
+	// TODO(Huang-Wei): remove this hack when defaultpreemption plugin is enabled.
+	if utilfeature.DefaultFeatureGate.Enabled(kubefeatures.PodDisruptionBudget) {
+		informerFactory.Policy().V1beta1().PodDisruptionBudgets().Lister()
+	}
 }
 
 func nodeSchedulingPropertiesChange(newNode *v1.Node, oldNode *v1.Node) string {