Use a fake client when evicting pods by individual strategies to accumulate the evictions

Currently, when the descheduler is running with the --dry-run on, no strategy actually
evicts a pod so every strategy always starts with a complete list of
pods. E.g. when the PodLifeTime strategy evicts few pods, the RemoveDuplicatePods
strategy still takes into account even the pods eliminated by the PodLifeTime
strategy. Which does not correspond to the real case scenarios as the
same pod can be evicted multiple times. Instead, use a fake client and
evict/delete the pods from its cache so the strategies evict each pod
at most once as it would be normally done in a real cluster.

pkg/descheduler/descheduler.go

@@ -21,11 +21,20 @@ import (
 	"fmt"
 
 	v1 "k8s.io/api/core/v1"
+	policy "k8s.io/api/policy/v1beta1"
+	metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
+	"k8s.io/apimachinery/pkg/labels"
+	"k8s.io/apimachinery/pkg/runtime"
 	"k8s.io/apimachinery/pkg/util/wait"
 	"k8s.io/client-go/informers"
 	clientset "k8s.io/client-go/kubernetes"
+	fakeclientset "k8s.io/client-go/kubernetes/fake"
+	core "k8s.io/client-go/testing"
 	"k8s.io/klog/v2"
 
+	corev1informers "k8s.io/client-go/informers/core/v1"
+	schedulingv1informers "k8s.io/client-go/informers/scheduling/v1"
+
 	"sigs.k8s.io/descheduler/cmd/descheduler/app/options"
 	"sigs.k8s.io/descheduler/metrics"
 	"sigs.k8s.io/descheduler/pkg/api"
@@ -67,10 +76,92 @@ func Run(rs *options.DeschedulerServer) error {
 
 type strategyFunction func(ctx context.Context, client clientset.Interface, strategy api.DeschedulerStrategy, nodes []*v1.Node, podEvictor *evictions.PodEvictor, getPodsAssignedToNode podutil.GetPodsAssignedToNodeFunc)
 
+func cachedClient(
+	realClient clientset.Interface,
+	podInformer corev1informers.PodInformer,
+	nodeInformer corev1informers.NodeInformer,
+	namespaceInformer corev1informers.NamespaceInformer,
+	priorityClassInformer schedulingv1informers.PriorityClassInformer,
+) (clientset.Interface, error) {
+	fakeClient := fakeclientset.NewSimpleClientset()
+	// simulate a pod eviction by deleting a pod
+	fakeClient.PrependReactor("create", "pods", func(action core.Action) (bool, runtime.Object, error) {
+		if action.GetSubresource() == "eviction" {
+			createAct, matched := action.(core.CreateActionImpl)
+			if !matched {
+				return false, nil, fmt.Errorf("unable to convert action to core.CreateActionImpl")
+			}
+			eviction, matched := createAct.Object.(*policy.Eviction)
+			if !matched {
+				return false, nil, fmt.Errorf("unable to convert action object into *policy.Eviction")
+			}
+			if err := fakeClient.Tracker().Delete(action.GetResource(), eviction.GetNamespace(), eviction.GetName()); err != nil {
+				return false, nil, fmt.Errorf("unable to delete pod %v/%v: %v", eviction.GetNamespace(), eviction.GetName(), err)
+			}
+			return true, nil, nil
+		}
+		// fallback to the default reactor
+		return false, nil, nil
+	})
+
+	klog.V(3).Infof("Pulling resources for the cached client from the cluster")
+	pods, err := podInformer.Lister().List(labels.Everything())
+	if err != nil {
+		return nil, fmt.Errorf("unable to list pods: %v", err)
+	}
+
+	for _, item := range pods {
+		if _, err := fakeClient.CoreV1().Pods(item.Namespace).Create(context.TODO(), item, metav1.CreateOptions{}); err != nil {
+			return nil, fmt.Errorf("unable to copy pod: %v", err)
+		}
+	}
+
+	nodes, err := nodeInformer.Lister().List(labels.Everything())
+	if err != nil {
+		return nil, fmt.Errorf("unable to list nodes: %v", err)
+	}
+
+	for _, item := range nodes {
+		if _, err := fakeClient.CoreV1().Nodes().Create(context.TODO(), item, metav1.CreateOptions{}); err != nil {
+			return nil, fmt.Errorf("unable to copy node: %v", err)
+		}
+	}
+
+	namespaces, err := namespaceInformer.Lister().List(labels.Everything())
+	if err != nil {
+		return nil, fmt.Errorf("unable to list namespaces: %v", err)
+	}
+
+	for _, item := range namespaces {
+		if _, err := fakeClient.CoreV1().Namespaces().Create(context.TODO(), item, metav1.CreateOptions{}); err != nil {
+			return nil, fmt.Errorf("unable to copy node: %v", err)
+		}
+	}
+
+	priorityClasses, err := priorityClassInformer.Lister().List(labels.Everything())
+	if err != nil {
+		return nil, fmt.Errorf("unable to list priorityclasses: %v", err)
+	}
+
+	for _, item := range priorityClasses {
+		if _, err := fakeClient.SchedulingV1().PriorityClasses().Create(context.TODO(), item, metav1.CreateOptions{}); err != nil {
+			return nil, fmt.Errorf("unable to copy priorityclass: %v", err)
+		}
+	}
+
+	return fakeClient, nil
+}
+
 func RunDeschedulerStrategies(ctx context.Context, rs *options.DeschedulerServer, deschedulerPolicy *api.DeschedulerPolicy, evictionPolicyGroupVersion string, stopChannel chan struct{}) error {
 	sharedInformerFactory := informers.NewSharedInformerFactory(rs.Client, 0)
 	nodeInformer := sharedInformerFactory.Core().V1().Nodes()
 	podInformer := sharedInformerFactory.Core().V1().Pods()
+	namespaceInformer := sharedInformerFactory.Core().V1().Namespaces()
+	priorityClassInformer := sharedInformerFactory.Scheduling().V1().PriorityClasses()
+
+	// create the informers
+	namespaceInformer.Informer()
+	priorityClassInformer.Informer()
 
 	getPodsAssignedToNode, err := podutil.BuildGetPodsAssignedToNodeFunc(podInformer)
 	if err != nil {
@@ -138,8 +229,39 @@ func RunDeschedulerStrategies(ctx context.Context, rs *options.DeschedulerServer
 			return
 		}
 
+		var podEvictorClient clientset.Interface
+		// When the dry mode is enable, collect all the relevant objects (mostly pods) under a fake client.
+		// So when evicting pods while running multiple strategies in a row have the cummulative effect
+		// as is when evicting pods for real.
+		if rs.DryRun {
+			klog.V(3).Infof("Building a cached client from the cluster for the dry run")
+			// Create a new cache so we start from scratch without any leftovers
+			fakeClient, err := cachedClient(rs.Client, podInformer, nodeInformer, namespaceInformer, priorityClassInformer)
+			if err != nil {
+				klog.Error(err)
+				return
+			}
+
+			fakeSharedInformerFactory := informers.NewSharedInformerFactory(fakeClient, 0)
+			getPodsAssignedToNode, err = podutil.BuildGetPodsAssignedToNodeFunc(fakeSharedInformerFactory.Core().V1().Pods())
+			if err != nil {
+				klog.Errorf("build get pods assigned to node function error: %v", err)
+				return
+			}
+
+			fakeCtx, cncl := context.WithCancel(context.TODO())
+			defer cncl()
+			fakeSharedInformerFactory.Start(fakeCtx.Done())
+			fakeSharedInformerFactory.WaitForCacheSync(fakeCtx.Done())
+
+			podEvictorClient = fakeClient
+		} else {
+			podEvictorClient = rs.Client
+		}
+
+		klog.V(3).Infof("Building a pod evictor")
 		podEvictor := evictions.NewPodEvictor(
-			rs.Client,
+			podEvictorClient,
 			evictionPolicyGroupVersion,
 			rs.DryRun,
 			deschedulerPolicy.MaxNoOfPodsToEvictPerNode,