Kubernetes Eviction Manager源码分析

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2020-6-16

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Kubernetes Eviction Manager介绍及工作原理

这部分内容,请看我的前一篇博文:Kubernetes Eviction Manager工作机制分析

Kubernetes Eviction Manager源码分析

Kubernetes Eviction Manager在何处启动

Kubelet在实例化一个kubelet对象的时候,调用eviction.NewManager新建了一个evictionManager对象。

pkg/kubelet/kubelet.go:273
func NewMainKubelet(kubeCfg *componentconfig.KubeletConfiguration, kubeDeps *KubeletDeps, standaloneMode bool) (*Kubelet, error) {

	...

	thresholds, err := eviction.ParseThresholdConfig(kubeCfg.EvictionHard, kubeCfg.EvictionSoft, kubeCfg.EvictionSoftGracePeriod, kubeCfg.EvictionMinimumReclaim)
	if err != nil {
		return nil, err
	}
	evictionConfig := eviction.Config{
		PressureTransitionPeriod: kubeCfg.EvictionPressureTransitionPeriod.Duration,
		MaxPodGracePeriodSeconds: int64(kubeCfg.EvictionMaxPodGracePeriod),
		Thresholds:               thresholds,
		KernelMemcgNotification:  kubeCfg.ExperimentalKernelMemcgNotification,
	}
	...

	// setup eviction manager
	evictionManager, evictionAdmitHandler, err := eviction.NewManager(klet.resourceAnalyzer, evictionConfig, killPodNow(klet.podWorkers, kubeDeps.Recorder), klet.imageManager, kubeDeps.Recorder, nodeRef, klet.clock)
	
	if err != nil {
		return nil, fmt.Errorf("failed to initialize eviction manager: %v", err)
	}
	klet.evictionManager = evictionManager
	klet.admitHandlers.AddPodAdmitHandler(evictionAdmitHandler)
	...
}


kubelet执行Run方法开始工作时,启动了一个goroutine,每5s执行一次updateRuntimeUp。在updateRuntimeUp中,待确认runtime启动成功后,会调用initializeRuntimeDependentModules完成runtime依赖模块的初始化工作。

pkg/kubelet/kubelet.go:1219
func (kl *Kubelet) Run(updates <-chan kubetypes.PodUpdate) {
	go wait.Until(kl.updateRuntimeUp, 5*time.Second, wait.NeverStop)
}


pkg/kubelet/kubelet.go:2040
func (kl *Kubelet) updateRuntimeUp() {
	...
	
	kl.oneTimeInitializer.Do(kl.initializeRuntimeDependentModules)
	
	...
}

再跟踪到initializeRuntimeDependentModules的代码可见,runtime的依赖模块包括cadvisor和evictionManager,初始化的工作其实就是分别调用它们的Start方法进行启动。

pkg/kubelet/kubelet.go:1206
func (kl *Kubelet) initializeRuntimeDependentModules() {
	if err := kl.cadvisor.Start(); err != nil {
		// Fail kubelet and rely on the babysitter to retry starting kubelet.
		// TODO(random-liu): Add backoff logic in the babysitter
		glog.Fatalf("Failed to start cAdvisor %v", err)
	}
	// eviction manager must start after cadvisor because it needs to know if the container runtime has a dedicated imagefs
	if err := kl.evictionManager.Start(kl, kl.getActivePods, evictionMonitoringPeriod); err != nil {
		kl.runtimeState.setInternalError(fmt.Errorf("failed to start eviction manager %v", err))
	}
}

因此,从这里开始就进入到evictionManager的分析了。

Kubernetes Eviction Manager的定义

从上面的分析可见,kubelet在启动过程中进行runtime依赖模块的初始化过程中,将evictionManager启动了。先别急,我们必须先来看看Eviction Manager是如何定义的。

pkg/kubelet/eviction/eviction_manager.go:40
// managerImpl implements Manager
type managerImpl struct {
	//  used to track time
	clock clock.Clock
	// config is how the manager is configured
	config Config
	// the function to invoke to kill a pod
	killPodFunc KillPodFunc
	// the interface that knows how to do image gc
	imageGC ImageGC
	// protects access to internal state
	sync.RWMutex
	// node conditions are the set of conditions present
	nodeConditions []v1.NodeConditionType
	// captures when a node condition was last observed based on a threshold being met
	nodeConditionsLastObservedAt nodeConditionsObservedAt
	// nodeRef is a reference to the node
	nodeRef *v1.ObjectReference
	// used to record events about the node
	recorder record.EventRecorder
	// used to measure usage stats on system
	summaryProvider stats.SummaryProvider
	// records when a threshold was first observed
	thresholdsFirstObservedAt thresholdsObservedAt
	// records the set of thresholds that have been met (including graceperiod) but not yet resolved
	thresholdsMet []Threshold
	// resourceToRankFunc maps a resource to ranking function for that resource.
	resourceToRankFunc map[v1.ResourceName]rankFunc
	// resourceToNodeReclaimFuncs maps a resource to an ordered list of functions that know how to reclaim that resource.
	resourceToNodeReclaimFuncs map[v1.ResourceName]nodeReclaimFuncs
	// last observations from synchronize
	lastObservations signalObservations
	// notifiersInitialized indicates if the threshold notifiers have been initialized (i.e. synchronize() has been called once)
	notifiersInitialized bool
}

managerImpl就是evictionManager的具体定义,重点关注:

  • config – evictionManager的配置,包括:

    • PressureTransitionPeriod( –eviction-pressure-transition-period)
    • MaxPodGracePeriodSeconds(–eviction-max-pod-grace-period)
    • Thresholds(–eviction-hard, –eviction-soft)
    • KernelMemcgNotification(–experimental-kernel-memcg-notification)
  • killPodFunc – evict pod时kill pod的接口,kubelet NewManager的时候,赋值为killPodNow方法(pkg/kubelet/pod_workers.go:285)

  • imageGC – 当node出现diskPressure condition时,imageGC进行unused images删除操作以回收disk space。

  • summaryProvider – 提供node和node上所有pods的最新status数据汇总,既NodeStats and []PodStats。

  • thresholdsFirstObservedAt – 记录threshold第一次观察到的时间。

  • thresholdsMet – 保存已经触发但还没解决的Thresholds,包括那些处于grace period等待阶段的Thresholds。

  • resourceToRankFunc – 定义各种Resource进行evict 挑选时的排名方法。

  • resourceToNodeReclaimFuncs – 定义各种Resource进行回收时调用的方法。

  • lastObservations – 上一次获取的eviction signal的记录,确保每次更新thresholds时都是按照正确的时间序列进行。

  • notifierInitialized – bool值,表示threshold notifier是否已经初始化,以确定是否可以利用kernel memcg notification功能来提高evict的响应速度。目前创建manager时该值为false,是否要利用kernel memcg notification,完全取决于kubelet的--experimental-kernel-memcg-notification参数。

kubelet在NewMainKubelet时调用eviction.NewManager进行evictionManager的创建,eviction.NewManager的代码很简单,就是赋值。

pkg/kubelet/eviction/eviction_manager.go:79
// NewManager returns a configured Manager and an associated admission handler to enforce eviction configuration.
func NewManager(
	summaryProvider stats.SummaryProvider,
	config Config,
	killPodFunc KillPodFunc,
	imageGC ImageGC,
	recorder record.EventRecorder,
	nodeRef *v1.ObjectReference,
	clock clock.Clock) (Manager, lifecycle.PodAdmitHandler, error) {
	manager := &managerImpl{
		clock:           clock,
		killPodFunc:     killPodFunc,
		imageGC:         imageGC,
		config:          config,
		recorder:        recorder,
		summaryProvider: summaryProvider,
		nodeRef:         nodeRef,
		nodeConditionsLastObservedAt: nodeConditionsObservedAt{},
		thresholdsFirstObservedAt:    thresholdsObservedAt{},
	}
	return manager, manager, nil
}

但是,有一点很重要,NewManager不但返回evictionManager对象,还返回了一个lifecycle.PodAdmitHandler实例evictionAdmitHandler,它其实和evictionManager的内容相同,但是不同的两个实例。evictionAdmitHandler用来kubelet创建Pod前进行准入检查,满足条件后才会继续创建Pod,通过Admit(attrs *lifecycle.PodAdmitAttributes)方法来检查,代码如下:

pkg/kubelet/eviction/eviction_manager.go:102
// Admit rejects a pod if its not safe to admit for node stability.
func (m *managerImpl) Admit(attrs *lifecycle.PodAdmitAttributes) lifecycle.PodAdmitResult {
	m.RLock()
	defer m.RUnlock()
	if len(m.nodeConditions) == 0 {
		return lifecycle.PodAdmitResult{Admit: true}
	}

	// the node has memory pressure, admit if not best-effort
	if hasNodeCondition(m.nodeConditions, v1.NodeMemoryPressure) {
		notBestEffort := qos.BestEffort != qos.GetPodQOS(attrs.Pod)
		if notBestEffort || kubepod.IsCriticalPod(attrs.Pod) {
			return lifecycle.PodAdmitResult{Admit: true}
		}
	}

	// reject pods when under memory pressure (if pod is best effort), or if under disk pressure.
	glog.Warningf("Failed to admit pod %v - %s", format.Pod(attrs.Pod), "node has conditions: %v", m.nodeConditions)
	return lifecycle.PodAdmitResult{
		Admit:   false,
		Reason:  reason,
		Message: fmt.Sprintf(message, m.nodeConditions),
	}
}

上述Pod Admit逻辑,正是Kubernetes Eviction Manager工作机制分析中Scheduler一节提到的EvictionManager对Pod调度的逻辑影响:

Kubelet会定期的将Node Condition传给kube-apiserver并存于etcd。kube-scheduler watch到Node Condition Pressure之后,会根据以下策略,阻止更多Pods Bind到该Node。

Node Condition Scheduler Behavior
MemoryPressure No new BestEffort pods are scheduled to the node.
DiskPressure No new pods are scheduled to the node.

killPodNow的代码,后面再分析。

基本上,这一小节我们把evictionManager是什么以及怎么来的问题搞清楚了。下面我们来看看evictionManager的启动过程。

Kubernetes Eviction Manager的启动

上面分析过,kubelet在启动过程中进行runtime依赖模块的初始化过程中,将evictionManager启动了(kl.evictionManager.Start(kl, kl.getActivePods, evictionMonitoringPeriod)),那我们先来看看Start方法:

pkg/kubelet/eviction/eviction_manager.go:126
// Start starts the control loop to observe and response to low compute resources.
func (m *managerImpl) Start(diskInfoProvider DiskInfoProvider, podFunc ActivePodsFunc, monitoringInterval time.Duration) error {
	// start the eviction manager monitoring
	go wait.Until(func() { m.synchronize(diskInfoProvider, podFunc) }, monitoringInterval, wait.NeverStop)
	return nil
}

很简单,启动一个goroutine,每执行完一次m.synchronize就间隔monitoringInterval(10s)的时间再次执行m.synchronize,如此反复。

接下来,就是evictionManager的关键工作流程了:

pkg/kubelet/eviction/eviction_manager.go:181
// synchronize is the main control loop that enforces eviction thresholds.
func (m *managerImpl) synchronize(diskInfoProvider DiskInfoProvider, podFunc ActivePodsFunc) {
	// if we have nothing to do, just return
	thresholds := m.config.Thresholds
	if len(thresholds) == 0 {
		return
	}

	// build the ranking functions (if not yet known)
	if len(m.resourceToRankFunc) == 0 || len(m.resourceToNodeReclaimFuncs) == 0 {
		// this may error if cadvisor has yet to complete housekeeping, so we will just try again in next pass.
		hasDedicatedImageFs, err := diskInfoProvider.HasDedicatedImageFs()
		if err != nil {
			return
		}
		m.resourceToRankFunc = buildResourceToRankFunc(hasDedicatedImageFs)
		m.resourceToNodeReclaimFuncs = buildResourceToNodeReclaimFuncs(m.imageGC, hasDedicatedImageFs)
	}

	// make observations and get a function to derive pod usage stats relative to those observations.
	observations, statsFunc, err := makeSignalObservations(m.summaryProvider)
	if err != nil {
		glog.Errorf("eviction manager: unexpected err: %v", err)
		return
	}

	// attempt to create a threshold notifier to improve eviction response time
	if m.config.KernelMemcgNotification && !m.notifiersInitialized {
		glog.Infof("eviction manager attempting to integrate with kernel memcg notification api")
		m.notifiersInitialized = true
		// start soft memory notification
		err = startMemoryThresholdNotifier(m.config.Thresholds, observations, false, func(desc string) {
			glog.Infof("soft memory eviction threshold crossed at %s", desc)
			// TODO wait grace period for soft memory limit
			m.synchronize(diskInfoProvider, podFunc)
		})
		if err != nil {
			glog.Warningf("eviction manager: failed to create hard memory threshold notifier: %v", err)
		}
		// start hard memory notification
		err = startMemoryThresholdNotifier(m.config.Thresholds, observations, true, func(desc string) {
			glog.Infof("hard memory eviction threshold crossed at %s", desc)
			m.synchronize(diskInfoProvider, podFunc)
		})
		if err != nil {
			glog.Warningf("eviction manager: failed to create soft memory threshold notifier: %v", err)
		}
	}

	// determine the set of thresholds met independent of grace period
	thresholds = thresholdsMet(thresholds, observations, false)

	// determine the set of thresholds previously met that have not yet satisfied the associated min-reclaim
	if len(m.thresholdsMet) > 0 {
		thresholdsNotYetResolved := thresholdsMet(m.thresholdsMet, observations, true)
		thresholds = mergeThresholds(thresholds, thresholdsNotYetResolved)
	}

	// determine the set of thresholds whose stats have been updated since the last sync
	thresholds = thresholdsUpdatedStats(thresholds, observations, m.lastObservations)

	// track when a threshold was first observed
	now := m.clock.Now()
	thresholdsFirstObservedAt := thresholdsFirstObservedAt(thresholds, m.thresholdsFirstObservedAt, now)

	// the set of node conditions that are triggered by currently observed thresholds
	nodeConditions := nodeConditions(thresholds)

	// track when a node condition was last observed
	nodeConditionsLastObservedAt := nodeConditionsLastObservedAt(nodeConditions, m.nodeConditionsLastObservedAt, now)

	// node conditions report true if it has been observed within the transition period window
	nodeConditions = nodeConditionsObservedSince(nodeConditionsLastObservedAt, m.config.PressureTransitionPeriod, now)

	// determine the set of thresholds we need to drive eviction behavior (i.e. all grace periods are met)
	thresholds = thresholdsMetGracePeriod(thresholdsFirstObservedAt, now)

	// update internal state
	m.Lock()
	m.nodeConditions = nodeConditions
	m.thresholdsFirstObservedAt = thresholdsFirstObservedAt
	m.nodeConditionsLastObservedAt = nodeConditionsLastObservedAt
	m.thresholdsMet = thresholds
	m.lastObservations = observations
	m.Unlock()

	// determine the set of resources under starvation
	starvedResources := getStarvedResources(thresholds)
	if len(starvedResources) == 0 {
		glog.V(3).Infof("eviction manager: no resources are starved")
		return
	}

	// rank the resources to reclaim by eviction priority
	sort.Sort(byEvictionPriority(starvedResources))
	resourceToReclaim := starvedResources[0]
	glog.Warningf("eviction manager: attempting to reclaim %v", resourceToReclaim)

	// determine if this is a soft or hard eviction associated with the resource
	softEviction := isSoftEvictionThresholds(thresholds, resourceToReclaim)

	// record an event about the resources we are now attempting to reclaim via eviction
	m.recorder.Eventf(m.nodeRef, v1.EventTypeWarning, "EvictionThresholdMet", "Attempting to reclaim %s", resourceToReclaim)

	// check if there are node-level resources we can reclaim to reduce pressure before evicting end-user pods.
	if m.reclaimNodeLevelResources(resourceToReclaim, observations) {
		glog.Infof("eviction manager: able to reduce %v pressure without evicting pods.", resourceToReclaim)
		return
	}

	glog.Infof("eviction manager: must evict pod(s) to reclaim %v", resourceToReclaim)

	// rank the pods for eviction
	rank, ok := m.resourceToRankFunc[resourceToReclaim]
	if !ok {
		glog.Errorf("eviction manager: no ranking function for resource %s", resourceToReclaim)
		return
	}

	// the only candidates viable for eviction are those pods that had anything running.
	activePods := podFunc()
	if len(activePods) == 0 {
		glog.Errorf("eviction manager: eviction thresholds have been met, but no pods are active to evict")
		return
	}

	// rank the running pods for eviction for the specified resource
	rank(activePods, statsFunc)

	glog.Infof("eviction manager: pods ranked for eviction: %s", format.Pods(activePods))

	// we kill at most a single pod during each eviction interval
	for i := range activePods {
		pod := activePods[i]
		status := v1.PodStatus{
			Phase:   v1.PodFailed,
			Message: fmt.Sprintf(message, resourceToReclaim),
			Reason:  reason,
		}
		// record that we are evicting the pod
		m.recorder.Eventf(pod, v1.EventTypeWarning, reason, fmt.Sprintf(message, resourceToReclaim))
		gracePeriodOverride := int64(0)
		if softEviction {
			gracePeriodOverride = m.config.MaxPodGracePeriodSeconds
		}
		// this is a blocking call and should only return when the pod and its containers are killed.
		err := m.killPodFunc(pod, status, &gracePeriodOverride)
		if err != nil {
			glog.Infof("eviction manager: pod %s failed to evict %v", format.Pod(pod), err)
			continue
		}
		// success, so we return until the next housekeeping interval
		glog.Infof("eviction manager: pod %s evicted successfully", format.Pod(pod))
		return
	}
	glog.Infof("eviction manager: unable to evict any pods from the node")
}

代码写的非常工整,注释也很到位,很棒。关键流程如下:

  • 通过buildResourceToRankFuncbuildResourceToNodeReclaimFuncs分别注册Evict Pod时各种Resource的排名函数和回收Node Resource的Reclaim函数。
  • 通过makeSignalObservations从cAdvisor中获取Eviction Signal Observation和Pod的StatsFunc(后续对Pods进行Rank时需要用)。
  • 如果kubelet配置了--experimental-kernel-memcg-notification且为true,则通过startMemoryThresholdNotifier启动soft & hard memory notification,当system usage第一时间达到soft & hard memory thresholds时,会立刻通知kubelet,并触发evictionManager.synchronize进行资源回收的流程。这样提高了eviction的实时性。
  • 根据从cAdvisor数据计算得到的Observation(observasions)和配置的thresholds通过thresholdsMet计算得到此次Met的thresholds。
  • 再根据从cAdvisor数据计算得到的Observation(observasions)和thresholdsMet通过thresholdsMet计算得到已记录但还没解决的thresholds,然后与上一步中的thresholds进行合并。
  • 根据lastObservations中Signal的时间,对比observasions的中Signal中的时间,过滤thresholds。
  • 更新thresholdsFirstObservedAt, nodeConditions
  • 过滤出那些从observed time到now,已经历过grace period时间的thresholds。
  • 更新evictionManager对象的内部数据: nodeConditions,thresholdsFirstObservedAt,nodeConditionsLastObservedAt,thresholds,observations。
  • 根据thresholds得到starvedResources,并进行排序,如果memory属于starvedResources,则memory排序第一。
  • 取starvedResources排第一的Resource,调用reclaimNodeLevelResources对Node上这种Resource进行资源回收。如果回收完后,available满足thresholdValue+evictionMinimumReclaim,则流程结束,不再evict user-pods。
  • 如果reclaimNodeLevelResources后,还不足以达到要求,则会继续evict user-pods,首先根据前面buildResourceToRankFunc注册的方法对所有active Pods进行排序。
  • 按照前面的排序,顺序的调用killPodNow将选出的pod干掉。如果kill某个pod失败,则会跳过这个pod,再按顺序挑下一个pod进行kill。只要某个pod kill成功,就返回结束,也就是说这个流程中,最多只会kill最多一个Pod。

上面流程中,有两个最关键的步骤,回收节点资源(reclaimNodeLevelResources)和evict user-pods(killPodNow)。

pkg/kubelet/eviction/eviction_manager.go:340
// reclaimNodeLevelResources attempts to reclaim node level resources.  returns true if thresholds were satisfied and no pod eviction is required.
func (m *managerImpl) reclaimNodeLevelResources(resourceToReclaim v1.ResourceName, observations signalObservations) bool {
	nodeReclaimFuncs := m.resourceToNodeReclaimFuncs[resourceToReclaim]
	for _, nodeReclaimFunc := range nodeReclaimFuncs {
		// attempt to reclaim the pressured resource.
		reclaimed, err := nodeReclaimFunc()
		if err == nil {
			// update our local observations based on the amount reported to have been reclaimed.
			// note: this is optimistic, other things could have been still consuming the pressured resource in the interim.
			signal := resourceToSignal[resourceToReclaim]
			value, ok := observations[signal]
			if !ok {
				glog.Errorf("eviction manager: unable to find value associated with signal %v", signal)
				continue
			}
			value.available.Add(*reclaimed)

			// evaluate all current thresholds to see if with adjusted observations, we think we have met min reclaim goals
			if len(thresholdsMet(m.thresholdsMet, observations, true)) == 0 {
				return true
			}
		} else {
			glog.Errorf("eviction manager: unexpected error when attempting to reduce %v pressure: %v", resourceToReclaim, err)
		}
	}
	return false
}


pkg/kubelet/pod_workers.go:283
// killPodNow returns a KillPodFunc that can be used to kill a pod.
// It is intended to be injected into other modules that need to kill a pod.
func killPodNow(podWorkers PodWorkers, recorder record.EventRecorder) eviction.KillPodFunc {
	return func(pod *v1.Pod, status v1.PodStatus, gracePeriodOverride *int64) error {
		// determine the grace period to use when killing the pod
		gracePeriod := int64(0)
		if gracePeriodOverride != nil {
			gracePeriod = *gracePeriodOverride
		} else if pod.Spec.TerminationGracePeriodSeconds != nil {
			gracePeriod = *pod.Spec.TerminationGracePeriodSeconds
		}

		// we timeout and return an error if we don't get a callback within a reasonable time.
		// the default timeout is relative to the grace period (we settle on 2s to wait for kubelet->runtime traffic to complete in sigkill)
		timeout := int64(gracePeriod + (gracePeriod / 2))
		minTimeout := int64(2)
		if timeout < minTimeout {
			timeout = minTimeout
		}
		timeoutDuration := time.Duration(timeout) * time.Second

		// open a channel we block against until we get a result
		type response struct {
			err error
		}
		ch := make(chan response)
		podWorkers.UpdatePod(&UpdatePodOptions{
			Pod:        pod,
			UpdateType: kubetypes.SyncPodKill,
			OnCompleteFunc: func(err error) {
				ch <- response{err: err}
			},
			KillPodOptions: &KillPodOptions{
				PodStatusFunc: func(p *v1.Pod, podStatus *kubecontainer.PodStatus) v1.PodStatus {
					return status
				},
				PodTerminationGracePeriodSecondsOverride: gracePeriodOverride,
			},
		})

		// wait for either a response, or a timeout
		select {
		case r := <-ch:
			return r.err
		case <-time.After(timeoutDuration):
			recorder.Eventf(pod, v1.EventTypeWarning, events.ExceededGracePeriod, "Container runtime did not kill the pod within specified grace period.")
			return fmt.Errorf("timeout waiting to kill pod")
		}
	}
}

讲到这里,整个evictionManager的主要流程都分析完了。

总结

  • kubelet在NewMainKubelet时创建了evictionManager。
  • kubelet在启动过程中进行runtime依赖模块的初始化过程中,将evictionManager启动了。
  • 整个EvictionManager工作流程中两个最关键的步骤是:回收节点资源(reclaimNodeLevelResources)和evict user-pods(killPodNow)。
  • 每次evict pods的流程中,最多只能成功kill一个pod,如果kill某个pod时候,会从排序好的pods中选择下一个进行kill,直到kill成功或者遍历完本节点所有的Pods为止。
  • 每次synchronize操作完成一次eviction流程,10s后都会再次循环这个流程。
  • 如果配置了--experimental-kernel-memcg-notification为true,那么会利用kernel memcg notification,当system usage第一时间达到soft & hard memory thresholds时,会立刻通知kubelet,并触发evictionManager.synchronize进行资源回收的流程,这样提高了eviction的实时性。

作者:WaltonWang