本文介紹了Kubelet在Predicate Admit準入檢查時對CriticalPod的資源搶占的原理，以及Priority Admission Controller對CriticalPod的PriorityClassName特殊處理。
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時對Critical的資源搶占處理
kubelet 在Predicate Admit流程中，會對Pods進行各種Predicate準入檢查，包括GeneralPredicates檢查本節點是否有足夠的cpu,mem,gpu資源。如果GeneralPredicates準入檢測失敗，對于nonCriticalPod則直接Admit失敗，但如果是CriticalPod則會觸發kubelet preemption進行資源搶占，按照一定規則殺死一些Pods釋放資源，搶占成功，則Admit成功。
流程的源頭應該從kubelet初始化的流程開始。
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對kubelet進行初始化時，通過AddPodAdmitHandler注冊了criticalPodAdmissionHandler，CriticalPod的Admit的特殊之處就展現在criticalPodAdmissionHandler。
然後，我們進入kubelet的predicateAdmitHandler流程中，看看GeneralPredicates失敗後的處理邏輯。
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中對Pod進行GeneralPredicates檢查cpu,mem,gpu資源時，如果發現資源不足導緻Admit失敗，則接着調用HandleAdmissionFailure進行額外處理。前提提到，kubelet初始化時注冊了criticalPodAdmissionHandler為HandleAdmissionFailure。
CriticalPodAdmissionHandler struct定義如下：
pkg/kubelet/preemption/preemption.go:41
type CriticalPodAdmissionHandler struct {
getPodsFunc eviction.ActivePodsFunc
killPodFunc eviction.KillPodFunc
recorder record.EventRecorder
} CriticalPodAdmissionHandler的HandleAdmissionFailure方法就是處理CriticalPod特殊的邏輯所在。
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是關閉的，則直接傳回false，表示Admit失敗。
- 判斷Admit的failureReasons是否包含
predicate.InsufficientResourceError
evictPodsToFreeRequests就是kubelet preemption進行資源搶占的邏輯實作，其核心就是調用getPodsToPreempt挑選合适的待殺死的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時候挑選待殺死Pods的邏輯如下：
- 如果該Pod的某個Resource request quantity超過了現在的所有的bestEffortPods, burstablePods, guaranteedPods的該Resource request quantity，則podsToPreempt為nil，意味着無合适Pods以釋放。
- 如果釋放所有bestEffortPods, burstablePods的資源都不足夠，則再挑選guaranteedPods（guarateedToEvict）。挑選的規則是：
- 規則一：越少的Pods被釋放越好；
- 規則二：釋放的資源越少越好；
- 規則一的優先級比規則二高；
- 如果釋放所有bestEffortPods及guarateedToEvict的資源都不足夠，則再挑選burstablePods(burstableToEvict)。挑選的規則同上。
- 如果釋放所有burstableToEvict及guarateedToEvict的資源都不足夠，則再挑選bestEffortPods(bestEffortToEvict)。挑選的規則同上。
也就是說：Pod Resource QoS優先級越低的越先被搶占，同一個QoS Level内挑選Pods按照如下規則：
Priority Admission Controller對CriticalPod的特殊處理
我們先看看幾類特殊的、系統預留的CriticalPod：
- ClusterCriticalPod: PriorityClass Name是
system-cluster-critical
- NodeCriticalPod:PriorityClass Name是
system-node-critical
如果AdmissionController中啟動了Priority Admission Controller，那麼在建立Pod時對Priority的檢查也存在CriticalPod的特殊處理。
Priority Admission Controller主要作用是根據Pod中指定的PriorityClassName替換成對應的Spec.Pritory數值。
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
} 同時滿足以下所有條件時，給Pod的
Spec.PriorityClassName
指派為
system-cluster-critical
,即認為是ClusterCriticalPod。
- 如果Enable了ExperimentalCriticalPodAnnotation和PodPriority Feature Gate；
- 該Pod沒有指定PriorityClassName；
- 該Pod屬于kube-system namespace；
- 該Pod打了
scheduler.alpha.kubernetes.io/critical-pod=""
總結
本文介紹了Kubelet在Predicate Admit準入檢查時對CriticalPod的資源搶占的原理，以及Priority Admission Controller對CriticalPod的PriorityClassName特殊處理。下一篇是最後一處關于Kubernetes對CriticalPod進行特殊待遇的地方——DaemonSet Controller。
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