深入分析KubernetesCriticalPod（二）

深入分析Kubernetes Critical Pod（一）介紹了Scheduler對Critical Pod的處理邏輯，下面我們再看下Kubelet Eviction Manager對Critical Pod的處理邏輯是怎樣的，以便我們瞭解Kubelet Evict Pod時對Critical Pod是否有保護措施，如果有，又是如何保護的。

Kubelet Eviction Manager Admit

kubelet在syncLoop中每個1s會迴圈呼叫syncLoopIteration，從config change channel | pleg channel | sync channel | houseKeeping channel | liveness manager`s update channel中獲取event，然後分別呼叫對應的event handler進行處理。

• configCh: dispatch the pods for the config change to the appropriate handler callback for the event type
• plegCh: update the runtime cache; sync pod
• syncCh: sync all pods waiting for sync
• houseKeepingCh: trigger cleanup of pods
• liveness manager`s update channel: sync pods that have failed or in which one or more containers have failed liveness checks

特別提一下，houseKeeping channel是每隔houseKeeping（10s）時間就會有event，然後執行HandlePodCleanups，執行以下清理操作：

• Stop the workers for no-longer existing pods.（每個pod對應會有一個worker，也就是goruntine）
• killing unwanted pods
• removes the volumes of pods that should not be running and that have no containers running.
• Remove any orphaned mirror pods.
• Remove any cgroups in the hierarchy for pods that are no longer running.

pkg/kubelet/kubelet.go:1753

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:
        
        if !open {
            glog.Errorf("Update channel is closed. Exiting the sync loop.")
            return false
        }

        switch u.Op {
        case kubetypes.ADD:
            
            handler.HandlePodAdditions(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)
        ...
        }

        if u.Op != kubetypes.RESTORE {
            ...
        }
    case e := <-plegCh:
        ...
    case <-syncCh:
        ...
    case update := <-kl.livenessManager.Updates():
        ...
    case <-housekeepingCh:
        ...
    }
    return true
}

syncLoopIteration中定義了當kubelet配置變更重啟後的邏輯：kubelet會對正在running的Pods進行Admission處理，Admission的結果有可能會讓該Pod被本節點拒絕。

HandlePodAdditions就是用來處理Kubelet ConficCh中的event的Handler。

// 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 {
        ...

        if !kl.podIsTerminated(pod) {
            ...
            // 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
            }
        }
        ...
    }
}

如果該Pod Status不是屬於Terminated，就呼叫canAdmitPod對該Pod進行准入檢查。如果准入檢查結果表示該Pod被拒絕，那麼就會將該Pod Phase設定為Failed。

pkg/kubelet/kubelet.go:1643

func (kl *Kubelet) canAdmitPod(pods []*v1.Pod, pod *v1.Pod) (bool, string, string) {
    // the kubelet will invoke each pod admit handler in sequence
    // if any handler rejects, the pod is rejected.
    // TODO: move out of disk check into a pod admitter
    // TODO: out of resource eviction should have a pod admitter call-out
    attrs := &lifecycle.PodAdmitAttributes{Pod: pod, OtherPods: pods}
    for _, podAdmitHandler := range kl.admitHandlers {
        if result := podAdmitHandler.Admit(attrs); !result.Admit {
            return false, result.Reason, result.Message
        }
    }

    return true, "", ""
}

canAdmitPod就會呼叫kubelet啟動時註冊的一系列admitHandlers對該Pod進行准入檢查，其中就包括kubelet eviction manager對應的admitHandle。

pkg/kubelet/eviction/eviction_manager.go:123

// 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}
    }
    
    if utilfeature.DefaultFeatureGate.Enabled(features.ExperimentalCriticalPodAnnotation) && kubelettypes.IsCriticalPod(attrs.Pod) {
        return lifecycle.PodAdmitResult{Admit: true}
    }

    if hasNodeCondition(m.nodeConditions, v1.NodeMemoryPressure) {
        notBestEffort := v1.PodQOSBestEffort != v1qos.GetPodQOS(attrs.Pod)
        if notBestEffort {
            return lifecycle.PodAdmitResult{Admit: true}
        }
    }

        return lifecycle.PodAdmitResult{
        Admit:   false,
        Reason:  reason,
        Message: fmt.Sprintf(message, m.nodeConditions),
    }
}

eviction manager的Admit的邏輯如下：

• 如果該node的Conditions為空，則Admit成功；

• 如果enable了ExperimentalCriticalPodAnnotation Feature Gate，並且該Pod是Critical Pod（Pod有Critical的Annotation，或者Pod的優先順序不小於SystemCriticalPriority），則Admit成功；

  • SystemCriticalPriority的值為2 billion。
• 如果該node的Condition為Memory Pressure，並且Pod QoS為非best-effort，則Admit成功；
• 其他情況都表示Admit失敗，即不允許該Pod在該node上Running。

Kubelet Eviction Manager SyncLoop

另外，在kubelet eviction manager的syncLoop中，也會對Critical Pod有特殊處理，程式碼如下。

pkg/kubelet/eviction/eviction_manager.go:226

// synchronize is the main control loop that enforces eviction thresholds.
// Returns the pod that was killed, or nil if no pod was killed.
func (m *managerImpl) synchronize(diskInfoProvider DiskInfoProvider, podFunc ActivePodsFunc) []*v1.Pod {
    ...

    // we kill at most a single pod during each eviction interval
    for i := range activePods {
        pod := activePods[i]
        
        if utilfeature.DefaultFeatureGate.Enabled(features.ExperimentalCriticalPodAnnotation) &&
            kubelettypes.IsCriticalPod(pod) && kubepod.IsStaticPod(pod) {
            continue
        }
        ...
        return []*v1.Pod{pod}
    }
    glog.Infof("eviction manager: unable to evict any pods from the node")
    return nil
}

當觸發了kubelet evict pod時，如果該pod滿足以下所有條件時，將不會被kubelet eviction manager kill掉。

• 該Pod Status不是Terminated；
• Enable ExperimentalCriticalPodAnnotation Feature Gate；
• 該Pod是Critical Pod；
• 該Pod時Static Pod；

總結

經過上面的分析，我們得到以下Kubelet Eviction Manager對Critical Pod處理的關鍵點：

• kubelet重啟後，eviction manager的Admit流程中對Critical Pod做如下特殊處理：如果enable了ExperimentalCriticalPodAnnotation Feature Gate，則允許該Critical Pod准入該node，無視該node的Condition。

• 當觸發了kubelet evict pod時，如果該Critical Pod滿足以下所有條件時，將不會被kubelet eviction manager kill掉。

  • 該Pod Status不是Terminated；
  • Enable ExperimentalCriticalPodAnnotation Feature Gate；
  • 該Pod是Static Pod；