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深入分析Kubernetes Critical Pod(三)

简介: 本文介绍了Kubelet在Predicate Admit准入检查时对CriticalPod的资源抢占的原理,以及Priority Admission Controller对CriticalPod的PriorityClassName特殊处理。

本文介绍了Kubelet在Predicate Admit准入检查时对CriticalPod的资源抢占的原理,以及Priority Admission Controller对CriticalPod的PriorityClassName特殊处理。

深入分析Kubernetes Critical Pod系列:
深入分析Kubernetes Critical Pod(一)
深入分析Kubernetes Critical Pod(二)
深入分析Kubernetes Critical Pod(三)
深入分析Kubernetes Critical Pod(四)

Kubelet Predicate Admit时对Critical的资源抢占处理

kubelet 在Predicate Admit流程中,会对Pods进行各种Predicate准入检查,包括GeneralPredicates检查本节点是否有足够的cpu,mem,gpu资源。如果GeneralPredicates准入检测失败,对于nonCriticalPod则直接Admit失败,但如果是CriticalPod则会触发kubelet preemption进行资源抢占,按照一定规则杀死一些Pods释放资源,抢占成功,则Admit成功。

流程的源头应该从kubelet初始化的流程开始。

pkg/kubelet/kubelet.go:315

// NewMainKubelet instantiates a new Kubelet object along with all the required internal modules.
// No initialization of Kubelet and its modules should happen here.
func NewMainKubelet(...) (*Kubelet, error) {
    ...
   criticalPodAdmissionHandler := preemption.NewCriticalPodAdmissionHandler(klet.GetActivePods, killPodNow(klet.podWorkers, kubeDeps.Recorder), kubeDeps.Recorder)
    klet.admitHandlers.AddPodAdmitHandler(lifecycle.NewPredicateAdmitHandler(klet.getNodeAnyWay, criticalPodAdmissionHandler, klet.containerManager.UpdatePluginResources))
    // apply functional Option's
    for _, opt := range kubeDeps.Options {
        opt(klet)
    }

    ...
    return klet, nil
}

在NewMainKubelet对kubelet进行初始化时,通过AddPodAdmitHandler注册了criticalPodAdmissionHandler,CriticalPod的Admit的特殊之处就体现在criticalPodAdmissionHandler。

然后,我们进入kubelet的predicateAdmitHandler流程中,看看GeneralPredicates失败后的处理逻辑。

pkg/kubelet/lifecycle/predicate.go:58

func (w *predicateAdmitHandler) Admit(attrs *PodAdmitAttributes) PodAdmitResult {
    ...

    fit, reasons, err := predicates.GeneralPredicates(podWithoutMissingExtendedResources, nil, nodeInfo)
    if err != nil {
        message := fmt.Sprintf("GeneralPredicates failed due to %v, which is unexpected.", err)
        glog.Warningf("Failed to admit pod %v - %s", format.Pod(pod), message)
        return PodAdmitResult{
            Admit:   fit,
            Reason:  "UnexpectedAdmissionError",
            Message: message,
        }
    }
    if !fit {
        fit, reasons, err = w.admissionFailureHandler.HandleAdmissionFailure(pod, reasons)
        if err != nil {
            message := fmt.Sprintf("Unexpected error while attempting to recover from admission failure: %v", err)
            glog.Warningf("Failed to admit pod %v - %s", format.Pod(pod), message)
            return PodAdmitResult{
                Admit:   fit,
                Reason:  "UnexpectedAdmissionError",
                Message: message,
            }
        }
    }
    ...
    return PodAdmitResult{
        Admit: true,
    }
}

在kubelet predicateAdmitHandler中对Pod进行GeneralPredicates检查cpu,mem,gpu资源时,如果发现资源不足导致Admit失败,则接着调用HandleAdmissionFailure进行额外处理。前提提到,kubelet初始化时注册了criticalPodAdmissionHandler为HandleAdmissionFailure。

CriticalPodAdmissionHandler struct定义如下:

pkg/kubelet/preemption/preemption.go:41

type CriticalPodAdmissionHandler struct {
    getPodsFunc eviction.ActivePodsFunc
    killPodFunc eviction.KillPodFunc
    recorder    record.EventRecorder
}

CriticalPodAdmissionHandler的HandleAdmissionFailure方法就是处理CriticalPod特殊的逻辑所在。

pkg/kubelet/preemption/preemption.go:66

// HandleAdmissionFailure gracefully handles admission rejection, and, in some cases,
// to allow admission of the pod despite its previous failure.
func (c *CriticalPodAdmissionHandler) HandleAdmissionFailure(pod *v1.Pod, failureReasons []algorithm.PredicateFailureReason) (bool, []algorithm.PredicateFailureReason, error) {
    if !kubetypes.IsCriticalPod(pod) || !utilfeature.DefaultFeatureGate.Enabled(features.ExperimentalCriticalPodAnnotation) {
        return false, failureReasons, nil
    }
    // InsufficientResourceError is not a reason to reject a critical pod.
    // Instead of rejecting, we free up resources to admit it, if no other reasons for rejection exist.
    nonResourceReasons := []algorithm.PredicateFailureReason{}
    resourceReasons := []*admissionRequirement{}
    for _, reason := range failureReasons {
        if r, ok := reason.(*predicates.InsufficientResourceError); ok {
            resourceReasons = append(resourceReasons, &admissionRequirement{
                resourceName: r.ResourceName,
                quantity:     r.GetInsufficientAmount(),
            })
        } else {
            nonResourceReasons = append(nonResourceReasons, reason)
        }
    }
    if len(nonResourceReasons) > 0 {
        // Return only reasons that are not resource related, since critical pods cannot fail admission for resource reasons.
        return false, nonResourceReasons, nil
    }
    err := c.evictPodsToFreeRequests(admissionRequirementList(resourceReasons))
    // if no error is returned, preemption succeeded and the pod is safe to admit.
    return err == nil, nil, err
}
  • 如果Pod不是CriticalPod,或者ExperimentalCriticalPodAnnotation Feature Gate是关闭的,则直接返回false,表示Admit失败。
  • 判断Admit的failureReasons是否包含predicate.InsufficientResourceError,如果包含,则调用evictPodsToFreeRequests触发kubelet preemption。注意这里的抢占不同于scheduler preemtion,不要混淆了。

evictPodsToFreeRequests就是kubelet preemption进行资源抢占的逻辑实现,其核心就是调用getPodsToPreempt挑选合适的待杀死的Pods(podsToPreempt)。

pkg/kubelet/preemption/preemption.go:121

// getPodsToPreempt returns a list of pods that could be preempted to free requests >= requirements
func getPodsToPreempt(pods []*v1.Pod, requirements admissionRequirementList) ([]*v1.Pod, error) {
    bestEffortPods, burstablePods, guaranteedPods := sortPodsByQOS(pods)

    // make sure that pods exist to reclaim the requirements
    unableToMeetRequirements := requirements.subtract(append(append(bestEffortPods, burstablePods...), guaranteedPods...)...)
    if len(unableToMeetRequirements) > 0 {
        return nil, fmt.Errorf("no set of running pods found to reclaim resources: %v", unableToMeetRequirements.toString())
    }
    // find the guaranteed pods we would need to evict if we already evicted ALL burstable and besteffort pods.
    guarateedToEvict, err := getPodsToPreemptByDistance(guaranteedPods, requirements.subtract(append(bestEffortPods, burstablePods...)...))
    if err != nil {
        return nil, err
    }
    // Find the burstable pods we would need to evict if we already evicted ALL besteffort pods, and the required guaranteed pods.
    burstableToEvict, err := getPodsToPreemptByDistance(burstablePods, requirements.subtract(append(bestEffortPods, guarateedToEvict...)...))
    if err != nil {
        return nil, err
    }
    // Find the besteffort pods we would need to evict if we already evicted the required guaranteed and burstable pods.
    bestEffortToEvict, err := getPodsToPreemptByDistance(bestEffortPods, requirements.subtract(append(burstableToEvict, guarateedToEvict...)...))
    if err != nil {
        return nil, err
    }
    return append(append(bestEffortToEvict, burstableToEvict...), guarateedToEvict...), nil
}

kubelet preemtion时候挑选待杀死Pods的逻辑如下:

  • 如果该Pod的某个Resource request quantity超过了现在的所有的bestEffortPods, burstablePods, guaranteedPods的该Resource request quantity,则podsToPreempt为nil,意味着无合适Pods以释放。
  • 如果释放所有bestEffortPods, burstablePods的资源都不足够,则再挑选guaranteedPods(guarateedToEvict)。挑选的规则是:

    • 规则一:越少的Pods被释放越好;
    • 规则二:释放的资源越少越好;
    • 规则一的优先级比规则二高;
  • 如果释放所有bestEffortPods及guarateedToEvict的资源都不足够,则再挑选burstablePods(burstableToEvict)。挑选的规则同上。
  • 如果释放所有burstableToEvict及guarateedToEvict的资源都不足够,则再挑选bestEffortPods(bestEffortToEvict)。挑选的规则同上。

也就是说:Pod Resource QoS优先级越低的越先被抢占,同一个QoS Level内挑选Pods按照如下规则:

  • 规则一:越少的Pods被释放越好;
  • 规则二:释放的资源越少越好;
  • 规则一的优先级比规则二高;

Priority Admission Controller对CriticalPod的特殊处理

我们先看看几类特殊的、系统预留的CriticalPod:

  • ClusterCriticalPod: PriorityClass Name是system-cluster-critical的Pod。
  • NodeCriticalPod:PriorityClass Name是system-node-critical的Pod。

如果AdmissionController中启动了Priority Admission Controller,那么在创建Pod时对Priority的检查也存在CriticalPod的特殊处理。

Priority Admission Controller主要作用是根据Pod中指定的PriorityClassName替换成对应的Spec.Pritory数值。

plugin/pkg/admission/priority/admission.go:138

// admitPod makes sure a new pod does not set spec.Priority field. It also makes sure that the PriorityClassName exists if it is provided and resolves the pod priority from the PriorityClassName.
func (p *priorityPlugin) admitPod(a admission.Attributes) error {
    operation := a.GetOperation()
    pod, ok := a.GetObject().(*api.Pod)
    if !ok {
        return errors.NewBadRequest("resource was marked with kind Pod but was unable to be converted")
    }

    // Make sure that the client has not set `priority` at the time of pod creation.
    if operation == admission.Create && pod.Spec.Priority != nil {
        return admission.NewForbidden(a, fmt.Errorf("the integer value of priority must not be provided in pod spec. Priority admission controller populates the value from the given PriorityClass name"))
    }
    if utilfeature.DefaultFeatureGate.Enabled(features.PodPriority) {
        var priority int32
        // TODO: @ravig - This is for backwards compatibility to ensure that critical pods with annotations just work fine.
        // Remove when no longer needed.
        if len(pod.Spec.PriorityClassName) == 0 &&
            utilfeature.DefaultFeatureGate.Enabled(features.ExperimentalCriticalPodAnnotation) &&
            kubelettypes.IsCritical(a.GetNamespace(), pod.Annotations) {
            pod.Spec.PriorityClassName = scheduling.SystemClusterCritical
        }
        if len(pod.Spec.PriorityClassName) == 0 {
            var err error
            priority, err = p.getDefaultPriority()
            if err != nil {
                return fmt.Errorf("failed to get default priority class: %v", err)
            }
        } else {
            // Try resolving the priority class name.
            pc, err := p.lister.Get(pod.Spec.PriorityClassName)
            if err != nil {
                if errors.IsNotFound(err) {
                    return admission.NewForbidden(a, fmt.Errorf("no PriorityClass with name %v was found", pod.Spec.PriorityClassName))
                }

                return fmt.Errorf("failed to get PriorityClass with name %s: %v", pod.Spec.PriorityClassName, err)
            }

            priority = pc.Value
        }
        pod.Spec.Priority = &priority
    }
    return nil
}

同时满足以下所有条件时,给Pod的Spec.PriorityClassName赋值为system-cluster-critical,即认为是ClusterCriticalPod。

  • 如果Enable了ExperimentalCriticalPodAnnotationPodPriority Feature Gate;
  • 该Pod没有指定PriorityClassName;
  • 该Pod属于kube-system namespace;
  • 该Pod打了scheduler.alpha.kubernetes.io/critical-pod="" Annotation;

总结

本文介绍了Kubelet在Predicate Admit准入检查时对CriticalPod的资源抢占的原理,以及Priority Admission Controller对CriticalPod的PriorityClassName特殊处理。下一篇是最后一处关于Kubernetes对CriticalPod进行特殊待遇的地方——DaemonSet Controller。

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