Kubernetes Scheduler的NominatedPods是什么

这篇文章主要介绍"Kubernetes Scheduler的NominatedPods是什么"，在日常操作中，相信很多人在Kubernetes Scheduler的NominatedPods是什么问题上存在疑惑，小编查阅了各式资料，整理出简单好用的操作方法，希望对大家解答"Kubernetes Scheduler的NominatedPods是什么"的疑惑有所帮助！接下来，请跟着小编一起来学习吧！

NominatedPods是什么？

当enable PodPriority feature gate后，scheduler会在集群资源资源不足时为preemptor抢占低优先级的Pods（成为victims）的资源，然后preemptor会再次入调度队列，等待下次victims的优雅终止并进行下一次调度。

为了尽量避免从preemptor抢占资源到真正再次执行调度这个时间段的scheduler能感知到那些资源已经被抢占，在scheduler调度其他更低优先级的Pods时考虑这些资源已经被抢占，因此在抢占阶段，为给preemptor设置pod.Status.NominatedNodeName，表示在NominatedNodeName上发生了抢占，preemptor期望调度在该node上。

PriorityQueue中缓存了每个node上的NominatedPods，这些NominatedPods表示已经被该node提名的，期望调度在该node上的，但是又还没最终成功调度过来的Pods。

抢占调度时发生了什么？

我们来重点关注下scheduler进行preempt时相关的流程。

func (sched *Scheduler) preempt(preemptor *v1.Pod, scheduleErr error) (string, error) {        ...        node, victims, nominatedPodsToClear, err := sched.config.Algorithm.Preempt(preemptor, sched.config.NodeLister, scheduleErr)        ...        var nodeName = ""        if node != nil {                nodeName = node.Name                err = sched.config.PodPreemptor.SetNominatedNodeName(preemptor, nodeName)                if err != nil {                        glog.Errorf("Error in preemption process. Cannot update pod %v annotations: %v", preemptor.Name, err)                        return "", err                }                ...        }        // Clearing nominated pods should happen outside of "if node != nil". Node could        // be nil when a pod with nominated node name is eligible to preempt again,        // but preemption logic does not find any node for it. In that case Preempt()        // function of generic_scheduler.go returns the pod itself for removal of the annotation.        for _, p := range nominatedPodsToClear {                rErr := sched.config.PodPreemptor.RemoveNominatedNodeName(p)                if rErr != nil {                        glog.Errorf("Cannot remove nominated node annotation of pod: %v", rErr)                        // We do not return as this error is not critical.                }        }        return nodeName, err}

• invoke ScheduleAlgorithm.Preempt进行资源抢占，返回抢占发生的node，victims，nominatedPodsToClear。

  func (g *genericScheduler) Preempt(pod *v1.Pod, nodeLister algorithm.NodeLister, scheduleErr error) (*v1.Node, []*v1.Pod, []*v1.Pod, error) {        ...        candidateNode := pickOneNodeForPreemption(nodeToVictims)        if candidateNode == nil {                return nil, nil, nil, err        }        nominatedPods := g.getLowerPriorityNominatedPods(pod, candidateNode.Name)        if nodeInfo, ok := g.cachedNodeInfoMap[candidateNode.Name]; ok {                return nodeInfo.Node(), nodeToVictims[candidateNode].Pods, nominatedPods, err        }        return nil, nil, nil, fmt.Errorf(                "preemption failed: the target node %s has been deleted from scheduler cache",                candidateNode.Name)}func (g *genericScheduler) getLowerPriorityNominatedPods(pod *v1.Pod, nodeName string) []*v1.Pod {        pods := g.schedulingQueue.WaitingPodsForNode(nodeName)        if len(pods) == 0 {                return nil        }        var lowerPriorityPods []*v1.Pod        podPriority := util.GetPodPriority(pod)        for _, p := range pods {                if util.GetPodPriority(p) < podPriority {                        lowerPriorityPods = append(lowerPriorityPods, p)                }        }        return lowerPriorityPods}

  
  

• node：抢占发生的最佳node；

• victims：待删除的pods，以释放资源给preemptor；

• nominatedPodsToClear：那些将要被删除.Status.NominatedNodeName的Pods列表，这些Pods是首先是属于PriorityQueue中的nominatedPods Cache中的Pods，并且他们的Pod Priority要低于preemptor Pod Priority，意味着这些nominatedPods已经不再适合调度到之前抢占时选择的这个node上了。

• 如果抢占成功（node非空），则调用podPreemptor.SetNominatedNodeName设置preemptor的.Status.NominatedNodeName为该node name，表示该preemptor期望抢占在该node上。

      func (p *podPreemptor) SetNominatedNodeName(pod *v1.Pod, nominatedNodeName string) error {                podCopy := pod.DeepCopy()                podCopy.Status.NominatedNodeName = nominatedNodeName                _, err := p.Client.CoreV1().Pods(pod.Namespace).UpdateStatus(podCopy)                return err        }

  
  

• 无论抢占是否成功（node是否为空），nominatedPodsToClear都可能不为空，都需要遍历nominatedPodsToClear内的所有Pods，调用podPreemptor.RemoveNominatedNodeName将其.Status.NominatedNodeName设置为空。

          func (p *podPreemptor) RemoveNominatedNodeName(pod *v1.Pod) error {                if len(pod.Status.NominatedNodeName) == 0 {                        return nil                }                return p.SetNominatedNodeName(pod, "")        }

  
  

Preemptor抢占成功后，发生了什么？

Premmptor抢占成功后，该Pod会被再次加入到PriorityQueue中的Unschedulable Sub-Queue队列中，等待条件再次出发调度。关于这部分内容更深入的解读，请参考我的博客深入分析Kubernetes Scheduler的优先级队列。preemptor再次会通过podFitsOnNode对node进行predicate逻辑处理。

func podFitsOnNode(        pod *v1.Pod,        meta algorithm.PredicateMetadata,        info *schedulercache.NodeInfo,        predicateFuncs map[string]algorithm.FitPredicate,        ecache *EquivalenceCache,        queue SchedulingQueue,        alwaysCheckAllPredicates bool,        equivCacheInfo *equivalenceClassInfo,) (bool, []algorithm.PredicateFailureReason, error) {        var (                eCacheAvailable  bool                failedPredicates []algorithm.PredicateFailureReason        )        predicateResults := make(map[string]HostPredicate)        podsAdded := false                for i := 0; i < 2; i++ {                metaToUse := meta                nodeInfoToUse := info                if i == 0 {                        podsAdded, metaToUse, nodeInfoToUse = addNominatedPods(util.GetPodPriority(pod), meta, info, queue)                } else if !podsAdded || len(failedPredicates) != 0 {  // 有问题吧？应该是podsAdded，而不是!podsAdded                        break                }                // Bypass eCache if node has any nominated pods.                // TODO(bsalamat): consider using eCache and adding proper eCache invalidations                // when pods are nominated or their nominations change.                eCacheAvailable = equivCacheInfo != nil && !podsAdded                for _, predicateKey := range predicates.Ordering() {                        var (                                fit     bool                                reasons []algorithm.PredicateFailureReason                                err     error                        )                                                        func() {                                        var invalid bool                                        if eCacheAvailable {                                                ...                                        }                                        if !eCacheAvailable || invalid {                                                // we need to execute predicate functions since equivalence cache does not work                                                fit, reasons, err = predicate(pod, metaToUse, nodeInfoToUse)                                                if err != nil {                                                        return                                                }                                                ...                                        }                                }()                                ...                        }                }        }        return len(failedPredicates) == 0, failedPredicates, nil}

一共会尝试进行两次predicate：

• 第一次predicate时，调用addNominatedPods，遍历PriorityQueue nominatedPods中所有Pods，将那些PodPriority大于等于该调度Pod的优先级的所有nominatedPods添加到SchedulerCache的NodeInfo中，意味着调度该pod时要考虑这些高优先级nominatedPods进行预选，比如要减去它们的resourceRequest等，并更新到PredicateMetadata中，接着执行正常的predicate逻辑。

• 第二次predicate时，如果前面的predicate逻辑有失败的情况，或者前面的podsAdded为false（如果在addNominatedPods时，发现该node对应nominatedPods cache是空的，那么返回值podAdded为false），那么第二次predicate立马结束，并不会触发真正的predicate逻辑。

• 第二次predicate时，如果前面的predicate逻辑都成功，并且podAdded为true的情况下，那么需要触发真正的第二次predicate逻辑，因为nominatedPods的添加成功，可能会Inter-Pod Affinity会影响predicate结果。

下面是addNominatedPods的代码，负责生成临时的schedulercache.NodeInfo和algorithm.PredicateMetadata，提供给具体的predicate Function进行预选处理。

// addNominatedPods adds pods with equal or greater priority which are nominated// to run on the node given in nodeInfo to meta and nodeInfo. It returns 1) whether// any pod was found, 2) augmented meta data, 3) augmented nodeInfo.func addNominatedPods(podPriority int32, meta algorithm.PredicateMetadata,        nodeInfo *schedulercache.NodeInfo, queue SchedulingQueue) (bool, algorithm.PredicateMetadata,        *schedulercache.NodeInfo) {        if queue == nil || nodeInfo == nil || nodeInfo.Node() == nil {                // This may happen only in tests.                return false, meta, nodeInfo        }        nominatedPods := queue.WaitingPodsForNode(nodeInfo.Node().Name)        if nominatedPods == nil || len(nominatedPods) == 0 {                return false, meta, nodeInfo        }        var metaOut algorithm.PredicateMetadata        if meta != nil {                metaOut = meta.ShallowCopy()        }        nodeInfoOut := nodeInfo.Clone()        for _, p := range nominatedPods {                if util.GetPodPriority(p) >= podPriority {                        nodeInfoOut.AddPod(p)                        if metaOut != nil {                                metaOut.AddPod(p, nodeInfoOut)                        }                }        }        return true, metaOut, nodeInfoOut}// WaitingPodsForNode returns pods that are nominated to run on the given node,// but they are waiting for other pods to be removed from the node before they// can be actually scheduled.func (p *PriorityQueue) WaitingPodsForNode(nodeName string) []*v1.Pod {        p.lock.RLock()        defer p.lock.RUnlock()        if list, ok := p.nominatedPods[nodeName]; ok {                return list        }        return nil}

addNominatedPods的逻辑如下：

• 调用WaitingPodsForNode获取PriorityQueue中的该node上的nominatedPods cache数据，如果nominatedPods为空，则返回podAdded为false，addNominatedPods流程结束。

• 克隆出PredicateMeta和NodeInfo对象，遍历nominatedPods，逐个将优先级不低于待调度pod的nominated pod加到克隆出来的NodeInfo对象中，并更新到克隆出来的PredicateMeta对象中。这些克隆出来的NodeInfo和PredicateMeta对象，最终会传入到predicate Functions中进行预选处理。遍历完成后，返回podAdded（true）和NodeInfo和PredicateMeta对象。

如何维护PriorityQueue NominatedPods Cache

深入分析Kubernetes Scheduler的优先级队列中分析了scheduler中podInformer、nodeInformer、serviceInformer、pvcInformer等注册的EventHandler中对PriorityQueue的操作，其中跟NominatedPods相关的EventHandler如下。

Add Pod to PriorityQueue

• 当往PriorityQueue中active queue添加Pod后，会调用addNominatedPodIfNeeded相应的将待添加的pod先从PriorityQueue nominatedPods Cache中删除，删除后再重新添加到nominatedPods cache中。

// Add adds a pod to the active queue. It should be called only when a new pod// is added so there is no chance the pod is already in either queue.func (p *PriorityQueue) Add(pod *v1.Pod) error {        p.lock.Lock()        defer p.lock.Unlock()        err := p.activeQ.Add(pod)        if err != nil {                glog.Errorf("Error adding pod %v to the scheduling queue: %v", pod.Name, err)        } else {                if p.unschedulableQ.get(pod) != nil {                        glog.Errorf("Error: pod %v is already in the unschedulable queue.", pod.Name)                        p.deleteNominatedPodIfExists(pod)                        p.unschedulableQ.delete(pod)                }                p.addNominatedPodIfNeeded(pod)                p.cond.Broadcast()        }        return err}func (p *PriorityQueue) addNominatedPodIfNeeded(pod *v1.Pod) {        nnn := NominatedNodeName(pod)        if len(nnn) > 0 {                for _, np := range p.nominatedPods[nnn] {                        if np.UID == pod.UID {                                glog.Errorf("Pod %v/%v already exists in the nominated map!", pod.Namespace, pod.Name)                                return                        }                }                p.nominatedPods[nnn] = append(p.nominatedPods[nnn], pod)        }}

• 当往PriorityQueue中unSchedulableQ queue添加Pod后，会调用addNominatedPodIfNeeded相应的将待添加的pod添加/更新到PriorityQueue nominatedPods Cache中。

func (p *PriorityQueue) AddUnschedulableIfNotPresent(pod *v1.Pod) error {        p.lock.Lock()        defer p.lock.Unlock()        if p.unschedulableQ.get(pod) != nil {                return fmt.Errorf("pod is already present in unschedulableQ")        }        if _, exists, _ := p.activeQ.Get(pod); exists {                return fmt.Errorf("pod is already present in the activeQ")        }        if !p.receivedMoveRequest && isPodUnschedulable(pod) {                p.unschedulableQ.addOrUpdate(pod)                p.addNominatedPodIfNeeded(pod)                return nil        }        err := p.activeQ.Add(pod)        if err == nil {                p.addNominatedPodIfNeeded(pod)                p.cond.Broadcast()        }        return err}

注意将pod添加到nominatedPods cache中的前提是该pod的.Status.NominatedNodeName不为空。

Update Pod in PriorityQueue

当更新PriorityQueue中Pod后，会接着调用updateNominatedPod更新PriorityQueue中nominatedPods Cache。

// Update updates a pod in the active queue if present. Otherwise, it removes// the item from the unschedulable queue and adds the updated one to the active// queue.func (p *PriorityQueue) Update(oldPod, newPod *v1.Pod) error {        p.lock.Lock()        defer p.lock.Unlock()        // If the pod is already in the active queue, just update it there.        if _, exists, _ := p.activeQ.Get(newPod); exists {                p.updateNominatedPod(oldPod, newPod)                err := p.activeQ.Update(newPod)                return err        }        // If the pod is in the unschedulable queue, updating it may make it schedulable.        if usPod := p.unschedulableQ.get(newPod); usPod != nil {                p.updateNominatedPod(oldPod, newPod)                if isPodUpdated(oldPod, newPod) {                        p.unschedulableQ.delete(usPod)                        err := p.activeQ.Add(newPod)                        if err == nil {                                p.cond.Broadcast()                        }                        return err                }                p.unschedulableQ.addOrUpdate(newPod)                return nil        }        // If pod is not in any of the two queue, we put it in the active queue.        err := p.activeQ.Add(newPod)        if err == nil {                p.addNominatedPodIfNeeded(newPod)                p.cond.Broadcast()        }        return err}

updateNominatedPod更新PriorityQueue nominatedPods Cache的逻辑是：先删除oldPod，再添加newPod进去。

// updateNominatedPod updates a pod in the nominatedPods.func (p *PriorityQueue) updateNominatedPod(oldPod, newPod *v1.Pod) {        // Even if the nominated node name of the Pod is not changed, we must delete and add it again        // to ensure that its pointer is updated.        p.deleteNominatedPodIfExists(oldPod)        p.addNominatedPodIfNeeded(newPod)}

Delete Pod from PriorityQueue

当从PriorityQueue中删除Pod前，会先调用deleteNominatedPodIfExists从PriorityQueue nominatedPods cache中删除该pod。

// Delete deletes the item from either of the two queues. It assumes the pod is// only in one queue.func (p *PriorityQueue) Delete(pod *v1.Pod) error {        p.lock.Lock()        defer p.lock.Unlock()        p.deleteNominatedPodIfExists(pod)        err := p.activeQ.Delete(pod)        if err != nil { // The item was probably not found in the activeQ.                p.unschedulableQ.delete(pod)        }        return nil}

deleteNominatedPodIfExists时，先检查该pod的.Status.NominatedNodeName是否为空：

• 如果为空，则不做任何操作，直接return结束流程。

• 如果不为空，则遍历nominatedPods cache，一旦找到UID匹配的pod，就说明nominatedPods中存在该pod，然后就从cache中删除该pod。如果删除后，发现该pod对应的NominatedNode上没有nominatePods了，则把整个node的nominatedPods从map cache中删除。

func (p *PriorityQueue) deleteNominatedPodIfExists(pod *v1.Pod) {        nnn := NominatedNodeName(pod)        if len(nnn) > 0 {                for i, np := range p.nominatedPods[nnn] {                        if np.UID == pod.UID {                                p.nominatedPods[nnn] = append(p.nominatedPods[nnn][:i], p.nominatedPods[nnn][i+1:]...)                                if len(p.nominatedPods[nnn]) == 0 {                                        delete(p.nominatedPods, nnn)                                }                                break                        }                }        }}

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