kubelet: Avoid sending no-op patches

In an e2e run, out of 1857 pod status updates executed by the
Kubelet 453 (25%) were no-ops - they only contained the UID of
the pod and no status changes. If the patch is a no-op we can
avoid invoking the server and continue.

pkg/kubelet/status/status_manager.go

@@ -541,15 +541,19 @@ func (m *manager) syncPod(uid types.UID, status versionedPodStatus) {
 	}
 
 	oldStatus := pod.Status.DeepCopy()
-	newPod, patchBytes, err := statusutil.PatchPodStatus(m.kubeClient, pod.Namespace, pod.Name, pod.UID, *oldStatus, mergePodStatus(*oldStatus, status.status))
+	newPod, patchBytes, unchanged, err := statusutil.PatchPodStatus(m.kubeClient, pod.Namespace, pod.Name, pod.UID, *oldStatus, mergePodStatus(*oldStatus, status.status))
 	klog.V(3).Infof("Patch status for pod %q with %q", format.Pod(pod), patchBytes)
 	if err != nil {
 		klog.Warningf("Failed to update status for pod %q: %v", format.Pod(pod), err)
 		return
 	}
-	pod = newPod
+	if unchanged {
+		klog.V(3).Infof("Status for pod %q is up-to-date: (%d)", format.Pod(pod), status.version)
+	} else {
+		klog.V(3).Infof("Status for pod %q updated successfully: (%d, %+v)", format.Pod(pod), status.version, status.status)
+		pod = newPod
+	}
 
-	klog.V(3).Infof("Status for pod %q updated successfully: (%d, %+v)", format.Pod(pod), status.version, status.status)
 	m.apiStatusVersions[kubetypes.MirrorPodUID(pod.UID)] = status.version
 
 	// We don't handle graceful deletion of mirror pods.

pkg/kubelet/status/status_manager_test.go

@@ -96,6 +96,7 @@ func getRandomPodStatus() v1.PodStatus {
 }
 
 func verifyActions(t *testing.T, manager *manager, expectedActions []core.Action) {
+	t.Helper()
 	manager.consumeUpdates()
 	actions := manager.kubeClient.(*fake.Clientset).Actions()
 	defer manager.kubeClient.(*fake.Clientset).ClearActions()
@@ -401,17 +402,17 @@ func TestStaleUpdates(t *testing.T) {
 	t.Logf("Nothing left in the channel to sync")
 	verifyActions(t, m, []core.Action{})
 
-	t.Log("Unchanged status should not send an update.")
+	t.Log("Unchanged status should not send an update")
 	m.SetPodStatus(pod, status)
 	verifyUpdates(t, m, 0)
 
-	t.Log("... unless it's stale.")
+	t.Log("... even if it's stale as long as nothing changes")
 	mirrorPodUID := kubetypes.MirrorPodUID(pod.UID)
 	m.apiStatusVersions[mirrorPodUID] = m.apiStatusVersions[mirrorPodUID] - 1
 
 	m.SetPodStatus(pod, status)
 	m.syncBatch()
-	verifyActions(t, m, []core.Action{getAction(), patchAction()})
+	verifyActions(t, m, []core.Action{getAction()})
 
 	t.Logf("Nothing stuck in the pipe.")
 	verifyUpdates(t, m, 0)
@@ -821,8 +822,9 @@ func TestReconcilePodStatus(t *testing.T) {
 	t.Logf("If the pod status is the same, a reconciliation is not needed and syncBatch should do nothing")
 	syncer.podManager.UpdatePod(testPod)
 	if syncer.needsReconcile(testPod.UID, podStatus) {
-		t.Errorf("Pod status is the same, a reconciliation is not needed")
+		t.Fatalf("Pod status is the same, a reconciliation is not needed")
 	}
+	syncer.SetPodStatus(testPod, podStatus)
 	syncer.syncBatch()
 	verifyActions(t, syncer, []core.Action{})
 
@@ -835,17 +837,19 @@ func TestReconcilePodStatus(t *testing.T) {
 	testPod.Status.StartTime = &normalizedStartTime
 	syncer.podManager.UpdatePod(testPod)
 	if syncer.needsReconcile(testPod.UID, podStatus) {
-		t.Errorf("Pod status only differs for timestamp format, a reconciliation is not needed")
+		t.Fatalf("Pod status only differs for timestamp format, a reconciliation is not needed")
 	}
+	syncer.SetPodStatus(testPod, podStatus)
 	syncer.syncBatch()
 	verifyActions(t, syncer, []core.Action{})
 
 	t.Logf("If the pod status is different, a reconciliation is needed, syncBatch should trigger an update")
-	testPod.Status = getRandomPodStatus()
+	changedPodStatus := getRandomPodStatus()
 	syncer.podManager.UpdatePod(testPod)
-	if !syncer.needsReconcile(testPod.UID, podStatus) {
-		t.Errorf("Pod status is different, a reconciliation is needed")
+	if !syncer.needsReconcile(testPod.UID, changedPodStatus) {
+		t.Fatalf("Pod status is different, a reconciliation is needed")
 	}
+	syncer.SetPodStatus(testPod, changedPodStatus)
 	syncer.syncBatch()
 	verifyActions(t, syncer, []core.Action{getAction(), patchAction()})
 }

pkg/util/pod/pod.go

@@ -17,6 +17,7 @@ limitations under the License.
 package pod
 
 import (
+	"bytes"
 	"context"
 	"encoding/json"
 	"fmt"
@@ -28,26 +29,29 @@ import (
 	clientset "k8s.io/client-go/kubernetes"
 )
 
-// PatchPodStatus patches pod status.
-func PatchPodStatus(c clientset.Interface, namespace, name string, uid types.UID, oldPodStatus, newPodStatus v1.PodStatus) (*v1.Pod, []byte, error) {
-	patchBytes, err := preparePatchBytesForPodStatus(namespace, name, uid, oldPodStatus, newPodStatus)
+// PatchPodStatus patches pod status. It returns true and avoids an update if the patch contains no changes.
+func PatchPodStatus(c clientset.Interface, namespace, name string, uid types.UID, oldPodStatus, newPodStatus v1.PodStatus) (*v1.Pod, []byte, bool, error) {
+	patchBytes, unchanged, err := preparePatchBytesForPodStatus(namespace, name, uid, oldPodStatus, newPodStatus)
 	if err != nil {
-		return nil, nil, err
+		return nil, nil, false, err
+	}
+	if unchanged {
+		return nil, patchBytes, true, nil
 	}
 
 	updatedPod, err := c.CoreV1().Pods(namespace).Patch(context.TODO(), name, types.StrategicMergePatchType, patchBytes, metav1.PatchOptions{}, "status")
 	if err != nil {
-		return nil, nil, fmt.Errorf("failed to patch status %q for pod %q/%q: %v", patchBytes, namespace, name, err)
+		return nil, nil, false, fmt.Errorf("failed to patch status %q for pod %q/%q: %v", patchBytes, namespace, name, err)
 	}
-	return updatedPod, patchBytes, nil
+	return updatedPod, patchBytes, false, nil
 }
 
-func preparePatchBytesForPodStatus(namespace, name string, uid types.UID, oldPodStatus, newPodStatus v1.PodStatus) ([]byte, error) {
+func preparePatchBytesForPodStatus(namespace, name string, uid types.UID, oldPodStatus, newPodStatus v1.PodStatus) ([]byte, bool, error) {
 	oldData, err := json.Marshal(v1.Pod{
 		Status: oldPodStatus,
 	})
 	if err != nil {
-		return nil, fmt.Errorf("failed to Marshal oldData for pod %q/%q: %v", namespace, name, err)
+		return nil, false, fmt.Errorf("failed to Marshal oldData for pod %q/%q: %v", namespace, name, err)
 	}
 
 	newData, err := json.Marshal(v1.Pod{
@@ -55,12 +59,12 @@ func preparePatchBytesForPodStatus(namespace, name string, uid types.UID, oldPod
 		Status:     newPodStatus,
 	})
 	if err != nil {
-		return nil, fmt.Errorf("failed to Marshal newData for pod %q/%q: %v", namespace, name, err)
+		return nil, false, fmt.Errorf("failed to Marshal newData for pod %q/%q: %v", namespace, name, err)
 	}
 
 	patchBytes, err := strategicpatch.CreateTwoWayMergePatch(oldData, newData, v1.Pod{})
 	if err != nil {
-		return nil, fmt.Errorf("failed to CreateTwoWayMergePatch for pod %q/%q: %v", namespace, name, err)
+		return nil, false, fmt.Errorf("failed to CreateTwoWayMergePatch for pod %q/%q: %v", namespace, name, err)
 	}
-	return patchBytes, nil
+	return patchBytes, bytes.Equal(patchBytes, []byte(fmt.Sprintf(`{"metadata":{"uid":%q}}`, uid))), nil
 }

pkg/util/pod/pod_test.go

@@ -44,11 +44,13 @@ func TestPatchPodStatus(t *testing.T) {
 	testCases := []struct {
 		description        string
 		mutate             func(input v1.PodStatus) v1.PodStatus
+		expectUnchanged    bool
 		expectedPatchBytes []byte
 	}{
 		{
 			"no change",
 			func(input v1.PodStatus) v1.PodStatus { return input },
+			true,
 			[]byte(fmt.Sprintf(`{"metadata":{"uid":"myuid"}}`)),
 		},
 		{
@@ -57,6 +59,7 @@ func TestPatchPodStatus(t *testing.T) {
 				input.Message = "random message"
 				return input
 			},
+			false,
 			[]byte(fmt.Sprintf(`{"metadata":{"uid":"myuid"},"status":{"message":"random message"}}`)),
 		},
 		{
@@ -65,6 +68,7 @@ func TestPatchPodStatus(t *testing.T) {
 				input.Conditions[0].Status = v1.ConditionFalse
 				return input
 			},
+			false,
 			[]byte(fmt.Sprintf(`{"metadata":{"uid":"myuid"},"status":{"$setElementOrder/conditions":[{"type":"Ready"},{"type":"PodScheduled"}],"conditions":[{"status":"False","type":"Ready"}]}}`)),
 		},
 		{
@@ -78,17 +82,23 @@ func TestPatchPodStatus(t *testing.T) {
 				}
 				return input
 			},
+			false,
 			[]byte(fmt.Sprintf(`{"metadata":{"uid":"myuid"},"status":{"initContainerStatuses":[{"image":"","imageID":"","lastState":{},"name":"init-container","ready":true,"restartCount":0,"state":{}}]}}`)),
 		},
 	}
 	for _, tc := range testCases {
-		_, patchBytes, err := PatchPodStatus(client, ns, name, uid, getPodStatus(), tc.mutate(getPodStatus()))
-		if err != nil {
-			t.Errorf("unexpected error: %v", err)
-		}
-		if !reflect.DeepEqual(patchBytes, tc.expectedPatchBytes) {
-			t.Errorf("for test case %q, expect patchBytes: %q, got: %q\n", tc.description, tc.expectedPatchBytes, patchBytes)
-		}
+		t.Run(tc.description, func(t *testing.T) {
+			_, patchBytes, unchanged, err := PatchPodStatus(client, ns, name, uid, getPodStatus(), tc.mutate(getPodStatus()))
+			if err != nil {
+				t.Errorf("unexpected error: %v", err)
+			}
+			if unchanged != tc.expectUnchanged {
+				t.Errorf("unexpected change: %t", unchanged)
+			}
+			if !reflect.DeepEqual(patchBytes, tc.expectedPatchBytes) {
+				t.Errorf("expect patchBytes: %q, got: %q\n", tc.expectedPatchBytes, patchBytes)
+			}
+		})
 	}
 }