func (kl *Kubelet) syncLoopIteration(configCh <-chan kubetypes.PodUpdate, handler SyncHandler,
	syncCh <-chan time.Time, housekeepingCh <-chan time.Time, plegCh <-chan *pleg.PodLifecycleEvent) bool {
	select {
	case u, open := <-configCh:
		// Update from a config source; dispatch it to the right handler
		// callback.
		if !open {
			glog.Errorf("Update channel is closed. Exiting the sync loop.")
			return false
		}

		switch u.Op {
		case kubetypes.ADD:
			glog.V(2).Infof("SyncLoop (ADD, %q): %q", u.Source, format.Pods(u.Pods))
			// After restarting, kubelet will get all existing pods through
			// ADD as if they are new pods. These pods will then go through the
			// admission process and *may* be rejected. This can be resolved
			// once we have checkpointing.
			handler.HandlePodAdditions(u.Pods)
		case kubetypes.UPDATE:
			glog.V(2).Infof("SyncLoop (UPDATE, %q): %q", u.Source, format.PodsWithDeletionTimestamps(u.Pods))
			handler.HandlePodUpdates(u.Pods)
		case kubetypes.REMOVE:
			glog.V(2).Infof("SyncLoop (REMOVE, %q): %q", u.Source, format.Pods(u.Pods))
			handler.HandlePodRemoves(u.Pods)
		case kubetypes.RECONCILE:
			glog.V(4).Infof("SyncLoop (RECONCILE, %q): %q", u.Source, format.Pods(u.Pods))
			handler.HandlePodReconcile(u.Pods)
		case kubetypes.DELETE:
			glog.V(2).Infof("SyncLoop (DELETE, %q): %q", u.Source, format.Pods(u.Pods))
			// DELETE is treated as a UPDATE because of graceful deletion.
			handler.HandlePodUpdates(u.Pods)
		case kubetypes.RESTORE:
			glog.V(2).Infof("SyncLoop (RESTORE, %q): %q", u.Source, format.Pods(u.Pods))
			// These are pods restored from the checkpoint. Treat them as new
			// pods.
			handler.HandlePodAdditions(u.Pods)
		case kubetypes.SET:
			// TODO: Do we want to support this?
			glog.Errorf("Kubelet does not support snapshot update")
		}

		if u.Op != kubetypes.RESTORE {
			// If the update type is RESTORE, it means that the update is from
			// the pod checkpoints and may be incomplete. Do not mark the
			// source as ready.

			// Mark the source ready after receiving at least one update from the
			// source. Once all the sources are marked ready, various cleanup
			// routines will start reclaiming resources. It is important that this
			// takes place only after kubelet calls the update handler to process
			// the update to ensure the internal pod cache is up-to-date.
			kl.sourcesReady.AddSource(u.Source)
		}
	case e := <-plegCh:
		if isSyncPodWorthy(e) {
			// PLEG event for a pod; sync it.
			if pod, ok := kl.podManager.GetPodByUID(e.ID); ok {
				glog.V(2).Infof("SyncLoop (PLEG): %q, event: %#v", format.Pod(pod), e)
				handler.HandlePodSyncs([]*v1.Pod{pod})
			} else {
				// If the pod no longer exists, ignore the event.
				glog.V(4).Infof("SyncLoop (PLEG): ignore irrelevant event: %#v", e)
			}
		}

		if e.Type == pleg.ContainerDied {
			if containerID, ok := e.Data.(string); ok {
				kl.cleanUpContainersInPod(e.ID, containerID)
			}
		}
	case <-syncCh:
		// Sync pods waiting for sync
		podsToSync := kl.getPodsToSync()
		if len(podsToSync) == 0 {
			break
		}
		glog.V(4).Infof("SyncLoop (SYNC): %d pods; %s", len(podsToSync), format.Pods(podsToSync))
		handler.HandlePodSyncs(podsToSync)
	case update := <-kl.livenessManager.Updates():
		if update.Result == proberesults.Failure {
			// The liveness manager detected a failure; sync the pod.

			// We should not use the pod from livenessManager, because it is never updated after
			// initialization.
			pod, ok := kl.podManager.GetPodByUID(update.PodUID)
			if !ok {
				// If the pod no longer exists, ignore the update.
				glog.V(4).Infof("SyncLoop (container unhealthy): ignore irrelevant update: %#v", update)
				break
			}
			glog.V(1).Infof("SyncLoop (container unhealthy): %q", format.Pod(pod))
			handler.HandlePodSyncs([]*v1.Pod{pod})
		}
	case <-housekeepingCh:
		if !kl.sourcesReady.AllReady() {
			// If the sources aren't ready or volume manager has not yet synced the states,
			// skip housekeeping, as we may accidentally delete pods from unready sources.
			glog.V(4).Infof("SyncLoop (housekeeping, skipped): sources aren't ready yet.")
		} else {
			glog.V(4).Infof("SyncLoop (housekeeping)")
			if err := handler.HandlePodCleanups(); err != nil {
				glog.Errorf("Failed cleaning pods: %v", err)
			}
		}
	}
	return true
}

// SyncHandler is an interface implemented by Kubelet, for testability
type SyncHandler interface {
	HandlePodAdditions(pods []*v1.Pod)
	HandlePodUpdates(pods []*v1.Pod)
	HandlePodRemoves(pods []*v1.Pod)
	HandlePodReconcile(pods []*v1.Pod)
	HandlePodSyncs(pods []*v1.Pod)
	HandlePodCleanups() error
}

// HandlePodAdditions is the callback in SyncHandler for pods being added from
// a config source.
func (kl *Kubelet) HandlePodAdditions(pods []*v1.Pod) {
	start := kl.clock.Now()
	sort.Sort(sliceutils.PodsByCreationTime(pods))
	for _, pod := range pods {
    ...
    }
}

for _, pod := range pods {
	// Responsible for checking limits in resolv.conf
	if kl.dnsConfigurer != nil && kl.dnsConfigurer.ResolverConfig != "" {
		kl.dnsConfigurer.CheckLimitsForResolvConf()
	}
	existingPods := kl.podManager.GetPods()
	// Always add the pod to the pod manager. Kubelet relies on the pod
	// manager as the source of truth for the desired state. If a pod does
	// not exist in the pod manager, it means that it has been deleted in
	// the apiserver and no action (other than cleanup) is required.
	kl.podManager.AddPod(pod)
    ...
}

if kubepod.IsMirrorPod(pod) {
	kl.handleMirrorPod(pod, start)
	continue
}

if !kl.podIsTerminated(pod) {
	// Only go through the admission process if the pod is not
	// terminated.

	// We failed pods that we rejected, so activePods include all admitted
	// pods that are alive.
	activePods := kl.filterOutTerminatedPods(existingPods)

	// Check if we can admit the pod; if not, reject it.
	if ok, reason, message := kl.canAdmitPod(activePods, pod); !ok {
		kl.rejectPod(pod, reason, message)
		continue
	}
}

mirrorPod, _ := kl.podManager.GetMirrorPodByPod(pod)
kl.dispatchWork(pod, kubetypes.SyncPodCreate, mirrorPod, start)

kl.probeManager.AddPod(pod)

// HandlePodUpdates is the callback in the SyncHandler interface for pods
// being updated from a config source.
func (kl *Kubelet) HandlePodUpdates(pods []*v1.Pod) {
	start := kl.clock.Now()
	for _, pod := range pods {
	...
	}
}

for _, pod := range pods {
	// Responsible for checking limits in resolv.conf
	if kl.dnsConfigurer != nil && kl.dnsConfigurer.ResolverConfig != "" {
		kl.dnsConfigurer.CheckLimitsForResolvConf()
	}
	kl.podManager.UpdatePod(pod)
    ...
}

if kubepod.IsMirrorPod(pod) {
	kl.handleMirrorPod(pod, start)
	continue
}

// TODO: Evaluate if we need to validate and reject updates.

mirrorPod, _ := kl.podManager.GetMirrorPodByPod(pod)
kl.dispatchWork(pod, kubetypes.SyncPodUpdate, mirrorPod, start)

// HandlePodRemoves is the callback in the SyncHandler interface for pods
// being removed from a config source.
func (kl *Kubelet) HandlePodRemoves(pods []*v1.Pod) {
	start := kl.clock.Now()
	for _, pod := range pods {
    ...
    }
}

for _, pod := range pods {
	kl.podManager.DeletePod(pod)
    ...
}

if kubepod.IsMirrorPod(pod) {
	kl.handleMirrorPod(pod, start)
	continue
}

// Deletion is allowed to fail because the periodic cleanup routine
// will trigger deletion again.
if err := kl.deletePod(pod); err != nil {
	glog.V(2).Infof("Failed to delete pod %q, err: %v", format.Pod(pod), err)
}

// deletePod deletes the pod from the internal state of the kubelet by:
// 1.  stopping the associated pod worker asynchronously
// 2.  signaling to kill the pod by sending on the podKillingCh channel
//
// deletePod returns an error if not all sources are ready or the pod is not
// found in the runtime cache.
func (kl *Kubelet) deletePod(pod *v1.Pod) error {
	if pod == nil {
		return fmt.Errorf("deletePod does not allow nil pod")
	}
	if !kl.sourcesReady.AllReady() {
		// If the sources aren't ready, skip deletion, as we may accidentally delete pods
		// for sources that haven't reported yet.
		return fmt.Errorf("skipping delete because sources aren't ready yet")
	}
	kl.podWorkers.ForgetWorker(pod.UID)

	// Runtime cache may not have been updated to with the pod, but it's okay
	// because the periodic cleanup routine will attempt to delete again later.
	runningPods, err := kl.runtimeCache.GetPods()
	if err != nil {
		return fmt.Errorf("error listing containers: %v", err)
	}
	runningPod := kubecontainer.Pods(runningPods).FindPod("", pod.UID)
	if runningPod.IsEmpty() {
		return fmt.Errorf("pod not found")
	}
	podPair := kubecontainer.PodPair{APIPod: pod, RunningPod: &runningPod}

	kl.podKillingCh <- &podPair
	// TODO: delete the mirror pod here?

	// We leave the volume/directory cleanup to the periodic cleanup routine.
	return nil
}

kl.probeManager.RemovePod(pod)

// HandlePodReconcile is the callback in the SyncHandler interface for pods
// that should be reconciled.
func (kl *Kubelet) HandlePodReconcile(pods []*v1.Pod) {
	start := kl.clock.Now()
	for _, pod := range pods {
        ...
    }
}

for _, pod := range pods {
	// Update the pod in pod manager, status manager will do periodically reconcile according
	// to the pod manager.
	kl.podManager.UpdatePod(pod)
    ...
}

// Reconcile Pod "Ready" condition if necessary. Trigger sync pod for reconciliation.
if status.NeedToReconcilePodReadiness(pod) {
	mirrorPod, _ := kl.podManager.GetMirrorPodByPod(pod)
	kl.dispatchWork(pod, kubetypes.SyncPodSync, mirrorPod, start)
}

// After an evicted pod is synced, all dead containers in the pod can be removed.
if eviction.PodIsEvicted(pod.Status) {
	if podStatus, err := kl.podCache.Get(pod.UID); err == nil {
		kl.containerDeletor.deleteContainersInPod("", podStatus, true)
	}
}

// HandlePodSyncs is the callback in the syncHandler interface for pods
// that should be dispatched to pod workers for sync.
func (kl *Kubelet) HandlePodSyncs(pods []*v1.Pod) {
	start := kl.clock.Now()
	for _, pod := range pods {
		mirrorPod, _ := kl.podManager.GetMirrorPodByPod(pod)
		kl.dispatchWork(pod, kubetypes.SyncPodSync, mirrorPod, start)
	}
}

// HandlePodCleanups performs a series of cleanup work, including terminating
// pod workers, killing unwanted pods, and removing orphaned volumes/pod
// directories.
// NOTE: This function is executed by the main sync loop, so it
// should not contain any blocking calls.
func (kl *Kubelet) HandlePodCleanups() error {
	// The kubelet lacks checkpointing, so we need to introspect the set of pods
	// in the cgroup tree prior to inspecting the set of pods in our pod manager.
	// this ensures our view of the cgroup tree does not mistakenly observe pods
	// that are added after the fact...
	var (
		cgroupPods map[types.UID]cm.CgroupName
		err        error
	)
	if kl.cgroupsPerQOS {
		pcm := kl.containerManager.NewPodContainerManager()
		cgroupPods, err = pcm.GetAllPodsFromCgroups()
		if err != nil {
			return fmt.Errorf("failed to get list of pods that still exist on cgroup mounts: %v", err)
		}
	}
    ...
}

allPods, mirrorPods := kl.podManager.GetPodsAndMirrorPods()
// Pod phase progresses monotonically. Once a pod has reached a final state,
// it should never leave regardless of the restart policy. The statuses
// of such pods should not be changed, and there is no need to sync them.
// TODO: the logic here does not handle two cases:
//   1. If the containers were removed immediately after they died, kubelet
//      may fail to generate correct statuses, let alone filtering correctly.
//   2. If kubelet restarted before writing the terminated status for a pod
//      to the apiserver, it could still restart the terminated pod (even
//      though the pod was not considered terminated by the apiserver).
// These two conditions could be alleviated by checkpointing kubelet.
activePods := kl.filterOutTerminatedPods(allPods)

desiredPods := make(map[types.UID]empty)
for _, pod := range activePods {
	desiredPods[pod.UID] = empty{}
}

// Stop the workers for no-longer existing pods.
// TODO: is here the best place to forget pod workers?
kl.podWorkers.ForgetNonExistingPodWorkers(desiredPods)
kl.probeManager.CleanupPods(activePods)

runningPods, err := kl.runtimeCache.GetPods()
if err != nil {
	glog.Errorf("Error listing containers: %#v", err)
	return err
}
for _, pod := range runningPods {
	if _, found := desiredPods[pod.ID]; !found {
		kl.podKillingCh <- &kubecontainer.PodPair{APIPod: nil, RunningPod: pod}
	}
}

kl.removeOrphanedPodStatuses(allPods, mirrorPods)

// Remove any orphaned volumes.
// Note that we pass all pods (including terminated pods) to the function,
// so that we don't remove volumes associated with terminated but not yet
// deleted pods.
err = kl.cleanupOrphanedPodDirs(allPods, runningPods)
if err != nil {
	// We want all cleanup tasks to be run even if one of them failed. So
	// we just log an error here and continue other cleanup tasks.
	// This also applies to the other clean up tasks.
	glog.Errorf("Failed cleaning up orphaned pod directories: %v", err)
}

// Remove any orphaned mirror pods.
kl.podManager.DeleteOrphanedMirrorPods()

// Remove any cgroups in the hierarchy for pods that are no longer running.
if kl.cgroupsPerQOS {
	kl.cleanupOrphanedPodCgroups(cgroupPods, activePods)
}

kl.backOff.GC()

// dispatchWork starts the asynchronous sync of the pod in a pod worker.
// If the pod is terminated, dispatchWork
func (kl *Kubelet) dispatchWork(pod *v1.Pod, syncType kubetypes.SyncPodType, mirrorPod *v1.Pod, start time.Time) {
	if kl.podIsTerminated(pod) {
		if pod.DeletionTimestamp != nil {
			// If the pod is in a terminated state, there is no pod worker to
			// handle the work item. Check if the DeletionTimestamp has been
			// set, and force a status update to trigger a pod deletion request
			// to the apiserver.
			kl.statusManager.TerminatePod(pod)
		}
		return
	}
    ...
}

// Run the sync in an async worker.
kl.podWorkers.UpdatePod(&UpdatePodOptions{
	Pod:        pod,
	MirrorPod:  mirrorPod,
	UpdateType: syncType,
	OnCompleteFunc: func(err error) {
		if err != nil {
			metrics.PodWorkerLatency.WithLabelValues(syncType.String()).Observe(metrics.SinceInMicroseconds(start))
		}
	},
})

// Note the number of containers for new pods.
if syncType == kubetypes.SyncPodCreate {
	metrics.ContainersPerPodCount.Observe(float64(len(pod.Spec.Containers)))
}

// PodWorkers is an abstract interface for testability.
type PodWorkers interface {
	UpdatePod(options *UpdatePodOptions)
	ForgetNonExistingPodWorkers(desiredPods map[types.UID]empty)
	ForgetWorker(uid types.UID)
}

// Apply the new setting to the specified pod.
// If the options provide an OnCompleteFunc, the function is invoked if the update is accepted.
// Update requests are ignored if a kill pod request is pending.
func (p *podWorkers) UpdatePod(options *UpdatePodOptions) {
	pod := options.Pod
	uid := pod.UID
	var podUpdates chan UpdatePodOptions
	var exists bool

	p.podLock.Lock()
	defer p.podLock.Unlock()
	if podUpdates, exists = p.podUpdates[uid]; !exists {
		// We need to have a buffer here, because checkForUpdates() method that
		// puts an update into channel is called from the same goroutine where
		// the channel is consumed. However, it is guaranteed that in such case
		// the channel is empty, so buffer of size 1 is enough.
		podUpdates = make(chan UpdatePodOptions, 1)
		p.podUpdates[uid] = podUpdates

		// Creating a new pod worker either means this is a new pod, or that the
		// kubelet just restarted. In either case the kubelet is willing to believe
		// the status of the pod for the first pod worker sync. See corresponding
		// comment in syncPod.
		go func() {
			defer runtime.HandleCrash()
			p.managePodLoop(podUpdates)
		}()
	}
	if !p.isWorking[pod.UID] {
		p.isWorking[pod.UID] = true
		podUpdates <- *options
	} else {
		// if a request to kill a pod is pending, we do not let anything overwrite that request.
		update, found := p.lastUndeliveredWorkUpdate[pod.UID]
		if !found || update.UpdateType != kubetypes.SyncPodKill {
			p.lastUndeliveredWorkUpdate[pod.UID] = *options
		}
	}
}

klet.podWorkers = newPodWorkers(klet.syncPod, kubeDeps.Recorder, klet.workQueue, klet.resyncInterval, backOffPeriod, klet.podCache)

func newPodWorkers(syncPodFn syncPodFnType, recorder record.EventRecorder, workQueue queue.WorkQueue,
	resyncInterval, backOffPeriod time.Duration, podCache kubecontainer.Cache) *podWorkers {
	return &podWorkers{
		podUpdates:                map[types.UID]chan UpdatePodOptions{},
		isWorking:                 map[types.UID]bool{},
		lastUndeliveredWorkUpdate: map[types.UID]UpdatePodOptions{},
		syncPodFn:                 syncPodFn,
		recorder:                  recorder,
		workQueue:                 workQueue,
		resyncInterval:            resyncInterval,
		backOffPeriod:             backOffPeriod,
		podCache:                  podCache,
	}
}

func (p *podWorkers) managePodLoop(podUpdates <-chan UpdatePodOptions) {
	var lastSyncTime time.Time
	for update := range podUpdates {
		err := func() error {
			podUID := update.Pod.UID
			// This is a blocking call that would return only if the cache
			// has an entry for the pod that is newer than minRuntimeCache
			// Time. This ensures the worker doesn't start syncing until
			// after the cache is at least newer than the finished time of
			// the previous sync.
			status, err := p.podCache.GetNewerThan(podUID, lastSyncTime)
			if err != nil {
				// This is the legacy event thrown by manage pod loop
				// all other events are now dispatched from syncPodFn
				p.recorder.Eventf(update.Pod, v1.EventTypeWarning, events.FailedSync, "error determining status: %v", err)
				return err
			}
			err = p.syncPodFn(syncPodOptions{
				mirrorPod:      update.MirrorPod,
				pod:            update.Pod,
				podStatus:      status,
				killPodOptions: update.KillPodOptions,
				updateType:     update.UpdateType,
			})
			lastSyncTime = time.Now()
			return err
		}()
		// notify the call-back function if the operation succeeded or not
		if update.OnCompleteFunc != nil {
			update.OnCompleteFunc(err)
		}
		if err != nil {
			// IMPORTANT: we do not log errors here, the syncPodFn is responsible for logging errors
			glog.Errorf("Error syncing pod %s (%q), skipping: %v", update.Pod.UID, format.Pod(update.Pod), err)
		}
		p.wrapUp(update.Pod.UID, err)
	}
}

// syncPodOptions provides the arguments to a SyncPod operation.
type syncPodOptions struct {
	// the mirror pod for the pod to sync, if it is a static pod
	mirrorPod *v1.Pod
	// pod to sync
	pod *v1.Pod
	// the type of update (create, update, sync)
	updateType kubetypes.SyncPodType
	// the current status
	podStatus *kubecontainer.PodStatus
	// if update type is kill, use the specified options to kill the pod.
	killPodOptions *KillPodOptions
}

func (kl *Kubelet) syncPod(o syncPodOptions) error {
	// pull out the required options
	pod := o.pod
	mirrorPod := o.mirrorPod
	podStatus := o.podStatus
	updateType := o.updateType
    ...
}

// if we want to kill a pod, do it now!
if updateType == kubetypes.SyncPodKill {
	killPodOptions := o.killPodOptions
	if killPodOptions == nil || killPodOptions.PodStatusFunc == nil {
		return fmt.Errorf("kill pod options are required if update type is kill")
	}
	apiPodStatus := killPodOptions.PodStatusFunc(pod, podStatus)
	kl.statusManager.SetPodStatus(pod, apiPodStatus)
	// we kill the pod with the specified grace period since this is a termination
	if err := kl.killPod(pod, nil, podStatus, killPodOptions.PodTerminationGracePeriodSecondsOverride); err != nil {
		kl.recorder.Eventf(pod, v1.EventTypeWarning, events.FailedToKillPod, "error killing pod: %v", err)
		// there was an error killing the pod, so we return that error directly
		utilruntime.HandleError(err)
		return err
	}
	return nil
}

// Latency measurements for the main workflow are relative to the
// first time the pod was seen by the API server.
var firstSeenTime time.Time
if firstSeenTimeStr, ok := pod.Annotations[kubetypes.ConfigFirstSeenAnnotationKey]; ok {
	firstSeenTime = kubetypes.ConvertToTimestamp(firstSeenTimeStr).Get()
}

// Record pod worker start latency if being created
// TODO: make pod workers record their own latencies
if updateType == kubetypes.SyncPodCreate {
	if !firstSeenTime.IsZero() {
		// This is the first time we are syncing the pod. Record the latency
		// since kubelet first saw the pod if firstSeenTime is set.
		metrics.PodWorkerStartLatency.Observe(metrics.SinceInMicroseconds(firstSeenTime))
	} else {
		glog.V(3).Infof("First seen time not recorded for pod %q", pod.UID)
	}
}

// Generate final API pod status with pod and status manager status
apiPodStatus := kl.generateAPIPodStatus(pod, podStatus)
// The pod IP may be changed in generateAPIPodStatus if the pod is using host network. (See #24576)
// TODO(random-liu): After writing pod spec into container labels, check whether pod is using host network, and
// set pod IP to hostIP directly in runtime.GetPodStatus
podStatus.IP = apiPodStatus.PodIP

// Record the time it takes for the pod to become running.
existingStatus, ok := kl.statusManager.GetPodStatus(pod.UID)
if !ok || existingStatus.Phase == v1.PodPending && apiPodStatus.Phase == v1.PodRunning &&
	!firstSeenTime.IsZero() {
	metrics.PodStartLatency.Observe(metrics.SinceInMicroseconds(firstSeenTime))
}

runnable := kl.canRunPod(pod)
if !runnable.Admit {
	// Pod is not runnable; update the Pod and Container statuses to why.
	apiPodStatus.Reason = runnable.Reason
	apiPodStatus.Message = runnable.Message
	// Waiting containers are not creating.
	const waitingReason = "Blocked"
	for _, cs := range apiPodStatus.InitContainerStatuses {
		if cs.State.Waiting != nil {
			cs.State.Waiting.Reason = waitingReason
		}
	}
	for _, cs := range apiPodStatus.ContainerStatuses {
		if cs.State.Waiting != nil {
			cs.State.Waiting.Reason = waitingReason
		}
	}
}

// Update status in the status manager
kl.statusManager.SetPodStatus(pod, apiPodStatus)

// Kill pod if it should not be running
if !runnable.Admit || pod.DeletionTimestamp != nil || apiPodStatus.Phase == v1.PodFailed {
	var syncErr error
	if err := kl.killPod(pod, nil, podStatus, nil); err != nil {
		kl.recorder.Eventf(pod, v1.EventTypeWarning, events.FailedToKillPod, "error killing pod: %v", err)
		syncErr = fmt.Errorf("error killing pod: %v", err)
		utilruntime.HandleError(syncErr)
	} else {
		if !runnable.Admit {
			// There was no error killing the pod, but the pod cannot be run.
			// Return an error to signal that the sync loop should back off.
			syncErr = fmt.Errorf("pod cannot be run: %s", runnable.Message)
		}
	}
	return syncErr
}

// If the network plugin is not ready, only start the pod if it uses the host network
if rs := kl.runtimeState.networkErrors(); len(rs) != 0 && !kubecontainer.IsHostNetworkPod(pod) {
	kl.recorder.Eventf(pod, v1.EventTypeWarning, events.NetworkNotReady, "%s: %v", NetworkNotReadyErrorMsg, rs)
	return fmt.Errorf("%s: %v", NetworkNotReadyErrorMsg, rs)
}

// Create Cgroups for the pod and apply resource parameters
// to them if cgroups-per-qos flag is enabled.
pcm := kl.containerManager.NewPodContainerManager()
// If pod has already been terminated then we need not create
// or update the pod's cgroup
if !kl.podIsTerminated(pod) {
	// When the kubelet is restarted with the cgroups-per-qos
	// flag enabled, all the pod's running containers
	// should be killed intermittently and brought back up
	// under the qos cgroup hierarchy.
	// Check if this is the pod's first sync
	firstSync := true
	for _, containerStatus := range apiPodStatus.ContainerStatuses {
		if containerStatus.State.Running != nil {
			firstSync = false
			break
		}
	}
	// Don't kill containers in pod if pod's cgroups already
	// exists or the pod is running for the first time
	podKilled := false
	if !pcm.Exists(pod) && !firstSync {
		if err := kl.killPod(pod, nil, podStatus, nil); err == nil {
			podKilled = true
		}
	}
    ...

// Create and Update pod's Cgroups
// Don't create cgroups for run once pod if it was killed above
// The current policy is not to restart the run once pods when
// the kubelet is restarted with the new flag as run once pods are
// expected to run only once and if the kubelet is restarted then
// they are not expected to run again.
// We don't create and apply updates to cgroup if its a run once pod and was killed above
if !(podKilled && pod.Spec.RestartPolicy == v1.RestartPolicyNever) {
	if !pcm.Exists(pod) {
		if err := kl.containerManager.UpdateQOSCgroups(); err != nil {
			glog.V(2).Infof("Failed to update QoS cgroups while syncing pod: %v", err)
		}
		if err := pcm.EnsureExists(pod); err != nil {
			kl.recorder.Eventf(pod, v1.EventTypeWarning, events.FailedToCreatePodContainer, "unable to ensure pod container exists: %v", err)
			return fmt.Errorf("failed to ensure that the pod: %v cgroups exist and are correctly applied: %v", pod.UID, err)
		}
	}
}

if err := kl.containerManager.UpdateQOSCgroups(); err != nil {
	glog.V(2).Infof("Failed to update QoS cgroups while syncing pod: %v", err)
}

// Create Mirror Pod for Static Pod if it doesn't already exist
if kubepod.IsStaticPod(pod) {
	podFullName := kubecontainer.GetPodFullName(pod)
	deleted := false
	if mirrorPod != nil {
		if mirrorPod.DeletionTimestamp != nil || !kl.podManager.IsMirrorPodOf(mirrorPod, pod) {
			// The mirror pod is semantically different from the static pod. Remove
			// it. The mirror pod will get recreated later.
			glog.Warningf("Deleting mirror pod %q because it is outdated", format.Pod(mirrorPod))
			if err := kl.podManager.DeleteMirrorPod(podFullName); err != nil {
				glog.Errorf("Failed deleting mirror pod %q: %v", format.Pod(mirrorPod), err)
			} else {
				deleted = true
			}
		}
	}
	if mirrorPod == nil || deleted {
		node, err := kl.GetNode()
		if err != nil || node.DeletionTimestamp != nil {
			glog.V(4).Infof("No need to create a mirror pod, since node %q has been removed from the cluster", kl.nodeName)
		} else {
			glog.V(4).Infof("Creating a mirror pod for static pod %q", format.Pod(pod))
			if err := kl.podManager.CreateMirrorPod(pod); err != nil {
				glog.Errorf("Failed creating a mirror pod for %q: %v", format.Pod(pod), err)
			}
		}
	}
}

// Make data directories for the pod
if err := kl.makePodDataDirs(pod); err != nil {
	kl.recorder.Eventf(pod, v1.EventTypeWarning, events.FailedToMakePodDataDirectories, "error making pod data directories: %v", err)
	glog.Errorf("Unable to make pod data directories for pod %q: %v", format.Pod(pod), err)
	return err
}

// makePodDataDirs creates the dirs for the pod datas.
func (kl *Kubelet) makePodDataDirs(pod *v1.Pod) error {
	uid := pod.UID
	if err := os.MkdirAll(kl.getPodDir(uid), 0750); err != nil && !os.IsExist(err) {
		return err
	}
	if err := os.MkdirAll(kl.getPodVolumesDir(uid), 0750); err != nil && !os.IsExist(err) {
		return err
	}
	if err := os.MkdirAll(kl.getPodPluginsDir(uid), 0750); err != nil && !os.IsExist(err) {
		return err
	}
	return nil
}

// Volume manager will not mount volumes for terminated pods
if !kl.podIsTerminated(pod) {
	// Wait for volumes to attach/mount
	if err := kl.volumeManager.WaitForAttachAndMount(pod); err != nil {
		kl.recorder.Eventf(pod, v1.EventTypeWarning, events.FailedMountVolume, "Unable to mount volumes for pod %q: %v", format.Pod(pod), err)
		glog.Errorf("Unable to mount volumes for pod %q: %v; skipping pod", format.Pod(pod), err)
		return err
	}
}

// Fetch the pull secrets for the pod
pullSecrets := kl.getPullSecretsForPod(pod)

// getPullSecretsForPod inspects the Pod and retrieves the referenced pull
// secrets.
func (kl *Kubelet) getPullSecretsForPod(pod *v1.Pod) []v1.Secret {
	pullSecrets := []v1.Secret{}

	for _, secretRef := range pod.Spec.ImagePullSecrets {
		secret, err := kl.secretManager.GetSecret(pod.Namespace, secretRef.Name)
		if err != nil {
			glog.Warningf("Unable to retrieve pull secret %s/%s for %s/%s due to %v.  The image pull may not succeed.", pod.Namespace, secretRef.Name, pod.Namespace, pod.Name, err)
			continue
		}

		pullSecrets = append(pullSecrets, *secret)
	}

	return pullSecrets
}

// Call the container runtime's SyncPod callback
result := kl.containerRuntime.SyncPod(pod, apiPodStatus, podStatus, pullSecrets, kl.backOff)
kl.reasonCache.Update(pod.UID, result)
if err := result.Error(); err != nil {
	// Do not return error if the only failures were pods in backoff
	for _, r := range result.SyncResults {
		if r.Error != kubecontainer.ErrCrashLoopBackOff && r.Error != images.ErrImagePullBackOff {
			// Do not record an event here, as we keep all event logging for sync pod failures
			// local to container runtime so we get better errors
			return err
		}
	}

	return nil
}

// Step 1: Compute sandbox and container changes.
podContainerChanges := m.computePodActions(pod, podStatus)
glog.V(3).Infof("computePodActions got %+v for pod %q", podContainerChanges, format.Pod(pod))
if podContainerChanges.CreateSandbox {
	ref, err := ref.GetReference(legacyscheme.Scheme, pod)
	if err != nil {
		glog.Errorf("Couldn't make a ref to pod %q: '%v'", format.Pod(pod), err)
	}
	if podContainerChanges.SandboxID != "" {
		m.recorder.Eventf(ref, v1.EventTypeNormal, events.SandboxChanged, "Pod sandbox changed, it will be killed and re-created.")
	} else {
		glog.V(4).Infof("SyncPod received new pod %q, will create a sandbox for it", format.Pod(pod))
	}
}

// Step 2: Kill the pod if the sandbox has changed.
if podContainerChanges.KillPod {
	if !podContainerChanges.CreateSandbox {
		glog.V(4).Infof("Stopping PodSandbox for %q because all other containers are dead.", format.Pod(pod))
	} else {
		glog.V(4).Infof("Stopping PodSandbox for %q, will start new one", format.Pod(pod))
	}

	killResult := m.killPodWithSyncResult(pod, kubecontainer.ConvertPodStatusToRunningPod(m.runtimeName, podStatus), nil)
	result.AddPodSyncResult(killResult)
	if killResult.Error() != nil {
		glog.Errorf("killPodWithSyncResult failed: %v", killResult.Error())
		return
	}

	if podContainerChanges.CreateSandbox {
		m.purgeInitContainers(pod, podStatus)
	}
}

// Step 3: kill any running containers in this pod which are not to keep.
for containerID, containerInfo := range podContainerChanges.ContainersToKill {
	glog.V(3).Infof("Killing unwanted container %q(id=%q) for pod %q", containerInfo.name, containerID, format.Pod(pod))
	killContainerResult := kubecontainer.NewSyncResult(kubecontainer.KillContainer, containerInfo.name)
	result.AddSyncResult(killContainerResult)
	if err := m.killContainer(pod, containerID, containerInfo.name, containerInfo.message, nil); err != nil {
		killContainerResult.Fail(kubecontainer.ErrKillContainer, err.Error())
		glog.Errorf("killContainer %q(id=%q) for pod %q failed: %v", containerInfo.name, containerID, format.Pod(pod), err)
		return
	}
}

// Step 4: Create a sandbox for the pod if necessary.
...
glog.V(4).Infof("Creating sandbox for pod %q", format.Pod(pod))
createSandboxResult := kubecontainer.NewSyncResult(kubecontainer.CreatePodSandbox, format.Pod(pod))
result.AddSyncResult(createSandboxResult)
podSandboxID, msg, err = m.createPodSandbox(pod, podContainerChanges.Attempt)
if err != nil {
	createSandboxResult.Fail(kubecontainer.ErrCreatePodSandbox, msg)
	glog.Errorf("createPodSandbox for pod %q failed: %v", format.Pod(pod), err)
	ref, referr := ref.GetReference(legacyscheme.Scheme, pod)
	if referr != nil {
		glog.Errorf("Couldn't make a ref to pod %q: '%v'", format.Pod(pod), referr)
	}
	m.recorder.Eventf(ref, v1.EventTypeWarning, events.FailedCreatePodSandBox, "Failed create pod sandbox: %v", err)
	return
}
glog.V(4).Infof("Created PodSandbox %q for pod %q", podSandboxID, format.Pod(pod))

// Step 5: start the init container.
if container := podContainerChanges.NextInitContainerToStart; container != nil {
	// Start the next init container.
	startContainerResult := kubecontainer.NewSyncResult(kubecontainer.StartContainer, container.Name)
	result.AddSyncResult(startContainerResult)
	isInBackOff, msg, err := m.doBackOff(pod, container, podStatus, backOff)
	if isInBackOff {
		startContainerResult.Fail(err, msg)
		glog.V(4).Infof("Backing Off restarting init container %+v in pod %v", container, format.Pod(pod))
		return
	}

	glog.V(4).Infof("Creating init container %+v in pod %v", container, format.Pod(pod))
	if msg, err := m.startContainer(podSandboxID, podSandboxConfig, container, pod, podStatus, pullSecrets, podIP, kubecontainer.ContainerTypeInit); err != nil {
		startContainerResult.Fail(err, msg)
		utilruntime.HandleError(fmt.Errorf("init container start failed: %v: %s", err, msg))
		return
	}

	// Successfully started the container; clear the entry in the failure
	glog.V(4).Infof("Completed init container %q for pod %q", container.Name, format.Pod(pod))
}

// Step 6: start containers in podContainerChanges.ContainersToStart.
for _, idx := range podContainerChanges.ContainersToStart {
	container := &pod.Spec.Containers[idx]
	startContainerResult := kubecontainer.NewSyncResult(kubecontainer.StartContainer, container.Name)
	result.AddSyncResult(startContainerResult)

	isInBackOff, msg, err := m.doBackOff(pod, container, podStatus, backOff)
	if isInBackOff {
		startContainerResult.Fail(err, msg)
		glog.V(4).Infof("Backing Off restarting container %+v in pod %v", container, format.Pod(pod))
		continue
	}

	glog.V(4).Infof("Creating container %+v in pod %v", container, format.Pod(pod))
	if msg, err := m.startContainer(podSandboxID, podSandboxConfig, container, pod, podStatus, pullSecrets, podIP, kubecontainer.ContainerTypeRegular); err != nil {
		startContainerResult.Fail(err, msg)
		// known errors that are logged in other places are logged at higher levels here to avoid
		// repetitive log spam
		switch {
		case err == images.ErrImagePullBackOff:
			glog.V(3).Infof("container start failed: %v: %s", err, msg)
		default:
			utilruntime.HandleError(fmt.Errorf("container start failed: %v: %s", err, msg))
		}
		continue
	}
}

// startContainer starts a container and returns a message indicates why it is failed on error.
// It starts the container through the following steps:
// * pull the image
// * create the container
// * start the container
// * run the post start lifecycle hooks (if applicable)
func (m *kubeGenericRuntimeManager) startContainer(podSandboxID string, podSandboxConfig *runtimeapi.PodSandboxConfig, container *v1.Container, pod *v1.Pod, podStatus *kubecontainer.PodStatus, pullSecrets []v1.Secret, podIP string, containerType kubecontainer.ContainerType) (string, error) {
...
}

// Step 1: pull the image.
imageRef, msg, err := m.imagePuller.EnsureImageExists(pod, container, pullSecrets)
if err != nil {
	m.recordContainerEvent(pod, container, "", v1.EventTypeWarning, events.FailedToCreateContainer, "Error: %v", grpc.ErrorDesc(err))
	return msg, err
}

// Step 2: create the container.
ref, err := kubecontainer.GenerateContainerRef(pod, container)
if err != nil {
	glog.Errorf("Can't make a ref to pod %q, container %v: %v", format.Pod(pod), container.Name, err)
}
glog.V(4).Infof("Generating ref for container %s: %#v", container.Name, ref)

// For a new container, the RestartCount should be 0
restartCount := 0
containerStatus := podStatus.FindContainerStatusByName(container.Name)
if containerStatus != nil {
	restartCount = containerStatus.RestartCount + 1
}

containerConfig, cleanupAction, err := m.generateContainerConfig(container, pod, restartCount, podIP, imageRef, containerType)
if cleanupAction != nil {
	defer cleanupAction()
}
if err != nil {
	m.recordContainerEvent(pod, container, "", v1.EventTypeWarning, events.FailedToCreateContainer, "Error: %v", grpc.ErrorDesc(err))
	return grpc.ErrorDesc(err), ErrCreateContainerConfig
}

containerID, err := m.runtimeService.CreateContainer(podSandboxID, containerConfig, podSandboxConfig)
if err != nil {
	m.recordContainerEvent(pod, container, containerID, v1.EventTypeWarning, events.FailedToCreateContainer, "Error: %v", grpc.ErrorDesc(err))
	return grpc.ErrorDesc(err), ErrCreateContainer
}
err = m.internalLifecycle.PreStartContainer(pod, container, containerID)
if err != nil {
	m.recordContainerEvent(pod, container, containerID, v1.EventTypeWarning, events.FailedToStartContainer, "Internal PreStartContainer hook failed: %v", grpc.ErrorDesc(err))
	return grpc.ErrorDesc(err), ErrPreStartHook
}
m.recordContainerEvent(pod, container, containerID, v1.EventTypeNormal, events.CreatedContainer, "Created container")

if ref != nil {
	m.containerRefManager.SetRef(kubecontainer.ContainerID{
		Type: m.runtimeName,
		ID:   containerID,
	}, ref)
}

// Step 3: start the container.
err = m.runtimeService.StartContainer(containerID)
if err != nil {
	m.recordContainerEvent(pod, container, containerID, v1.EventTypeWarning, events.FailedToStartContainer, "Error: %v", grpc.ErrorDesc(err))
	return grpc.ErrorDesc(err), kubecontainer.ErrRunContainer
}
m.recordContainerEvent(pod, container, containerID, v1.EventTypeNormal, events.StartedContainer, "Started container")

// Symlink container logs to the legacy container log location for cluster logging
// support.
// TODO(random-liu): Remove this after cluster logging supports CRI container log path.
containerMeta := containerConfig.GetMetadata()
sandboxMeta := podSandboxConfig.GetMetadata()
legacySymlink := legacyLogSymlink(containerID, containerMeta.Name, sandboxMeta.Name,
	sandboxMeta.Namespace)
containerLog := filepath.Join(podSandboxConfig.LogDirectory, containerConfig.LogPath)
// only create legacy symlink if containerLog path exists (or the error is not IsNotExist).
// Because if containerLog path does not exist, only dandling legacySymlink is created.
// This dangling legacySymlink is later removed by container gc, so it does not make sense
// to create it in the first place. it happens when journald logging driver is used with docker.
if _, err := m.osInterface.Stat(containerLog); !os.IsNotExist(err) {
	if err := m.osInterface.Symlink(containerLog, legacySymlink); err != nil {
		glog.Errorf("Failed to create legacy symbolic link %q to container %q log %q: %v",
			legacySymlink, containerID, containerLog, err)
	}
}

// Step 4: execute the post start hook.
if container.Lifecycle != nil && container.Lifecycle.PostStart != nil {
	kubeContainerID := kubecontainer.ContainerID{
		Type: m.runtimeName,
		ID:   containerID,
	}
	msg, handlerErr := m.runner.Run(kubeContainerID, pod, container, container.Lifecycle.PostStart)
	if handlerErr != nil {
		m.recordContainerEvent(pod, container, kubeContainerID.ID, v1.EventTypeWarning, events.FailedPostStartHook, msg)
		if err := m.killContainer(pod, kubeContainerID, container.Name, "FailedPostStartHook", nil); err != nil {
			glog.Errorf("Failed to kill container %q(id=%q) in pod %q: %v, %v",
				container.Name, kubeContainerID.String(), format.Pod(pod), ErrPostStartHook, err)
		}
		return msg, fmt.Errorf("%s: %v", ErrPostStartHook, handlerErr)
	}
}