本节开始主要分析kubernetes源码部分，版本基于当前最新的1.13.4。
启动分析
Kubernetes基础组件的入口均在cmd目录下，kube-schduler入口在scheduler.go下，如图
command的形式，引用的是 spf13类库 通过将配置文件转化成
command的形式，调用
Execute方法执行定义的
Run方法 进入
runCommand方法，通过完成配置的初始化，调用
Run方法，进一步启动。
Run方法分析
Run方法主要做了以下工作：
1、判断是否需要添加VolumeScheduling新特性；
2、初始化调度参数的相关结构体；
3、配置准备事件广播；
4、健康检查相关配置；
5、Metrics相关配置；
6、启动所有的Informer(kubernetes主要就是通过Informer和Workqueue机制监听事件的变化)；
7、判断是否需要LeaderElection，决定最终的启动。
调度接口
最终的调度接口进入的是pkg下的scheduler.go文件，通过启动单独的协程处理调度工作。
scheduleOne方法分析
scheduleOne，顾名思义，每次调度一个Pod，整体文件如下
// scheduleOne does the entire scheduling workflow for a single pod. It is serialized on the scheduling algorithm's host fitting.
func (sched *Scheduler) scheduleOne() {
// 1.从队列中取出待调度的Pod
pod := sched.config.NextPod()
// pod could be nil when schedulerQueue is closed
if pod == nil {
return
}
if pod.DeletionTimestamp != nil {
sched.config.Recorder.Eventf(pod, v1.EventTypeWarning, "FailedScheduling", "skip schedule deleting pod: %v/%v", pod.Namespace, pod.Name)
klog.V(3).Infof("Skip schedule deleting pod: %v/%v", pod.Namespace, pod.Name)
return
}
klog.V(3).Infof("Attempting to schedule pod: %v/%v", pod.Namespace, pod.Name)
// Synchronously attempt to find a fit for the pod.
start := time.Now()
// 2.获取待调度Pod匹配的主机名
suggestedHost, err := sched.schedule(pod)
if err != nil {
// schedule() may have failed because the pod would not fit on any host, so we try to
// preempt, with the expectation that the next time the pod is tried for scheduling it
// will fit due to the preemption. It is also possible that a different pod will schedule
// into the resources that were preempted, but this is harmless.
if fitError, ok := err.(*core.FitError); ok {
preemptionStartTime := time.Now()
sched.preempt(pod, fitError)
metrics.PreemptionAttempts.Inc()
metrics.SchedulingAlgorithmPremptionEvaluationDuration.Observe(metrics.SinceInMicroseconds(preemptionStartTime))
metrics.SchedulingLatency.WithLabelValues(metrics.PreemptionEvaluation).Observe(metrics.SinceInSeconds(preemptionStartTime))
// Pod did not fit anywhere, so it is counted as a failure. If preemption
// succeeds, the pod should get counted as a success the next time we try to
// schedule it. (hopefully)
metrics.PodScheduleFailures.Inc()
} else {
klog.Errorf("error selecting node for pod: %v", err)
metrics.PodScheduleErrors.Inc()
}
return
}
metrics.SchedulingAlgorithmLatency.Observe(metrics.SinceInMicroseconds(start))
// Tell the cache to assume that a pod now is running on a given node, even though it hasn't been bound yet.
// This allows us to keep scheduling without waiting on binding to occur.
// 3.Pod与Node缓存，保证调度一直进行，不用等待每次绑定完成（绑定是一个耗时的过程）
assumedPod := pod.DeepCopy()
// Assume volumes first before assuming the pod.
//
// If all volumes are completely bound, then allBound is true and binding will be skipped.
//
// Otherwise, binding of volumes is started after the pod is assumed, but before pod binding.
//
// This function modifies 'assumedPod' if volume binding is required.
// 4.判断是否需要VolumeScheduling特性
allBound, err := sched.assumeVolumes(assumedPod, suggestedHost)
if err != nil {
klog.Errorf("error assuming volumes: %v", err)
metrics.PodScheduleErrors.Inc()
return
}
// assume modifies `assumedPod` by setting NodeName=suggestedHost
// 5.Pod对应的NodeName写上主机名，存入缓存
err = sched.assume(assumedPod, suggestedHost)
if err != nil {
klog.Errorf("error assuming pod: %v", err)
metrics.PodScheduleErrors.Inc()
return
}
// bind the pod to its host asynchronously (we can do this b/c of the assumption step above).
// 6.请求apiserver，异步处理最终的绑定，写入到etcd
go func() {
// Bind volumes first before Pod
if !allBound {
err := sched.bindVolumes(assumedPod)
if err != nil {
klog.Errorf("error binding volumes: %v", err)
metrics.PodScheduleErrors.Inc()
return
}
}
err := sched.bind(assumedPod, &v1.Binding{
ObjectMeta: metav1.ObjectMeta{Namespace: assumedPod.Namespace, Name: assumedPod.Name, UID: assumedPod.UID},
Target: v1.ObjectReference{
Kind: "Node",
Name: suggestedHost,
},
})
metrics.E2eSchedulingLatency.Observe(metrics.SinceInMicroseconds(start))
if err != nil {
klog.Errorf("error binding pod: %v", err)
metrics.PodScheduleErrors.Inc()
} else {
metrics.PodScheduleSuccesses.Inc()
}
}()
}
复制代码主要做了以下工作：
1、从队列中取出待调度的Pod
2、根据调度算法（预选+优选）获取待调度Pod匹配的主机，如果未获取到合适的主机，判断是否需要preempt，即Pod的抢占策略，为Pod分配节点
3、将当前Pod缓存起来，假定已经绑定成功（主要是为了将scheduling与binding过程分开）
4、判断是否需要VolumeScheduling特性继续添加Pod信息
5、Pod对应的NodeName写上主机名（调度的本质就是将为空的NodeName写上相应的Node的值）
6、启动新的binding协程，请求apiserver，异步处理最终的绑定，将结果写入到etcd中
调度算法
最终的调度在generic_scheduler.go的Schedule方法。调度主要分两步，预选和优选。
预选
预选算法调用的接口是findNodesThatFit，主要代码如下：
// Filters the nodes to find the ones that fit based on the given predicate functions
// Each node is passed through the predicate functions to determine if it is a fit
func (g *genericScheduler) findNodesThatFit(pod *v1.Pod, nodes []*v1.Node) ([]*v1.Node, FailedPredicateMap, error) {
var filtered []*v1.Node
failedPredicateMap := FailedPredicateMap{}
// 该if表示，如果没有配置预选的算法，则直接将所有的Node写入匹配数组
if len(g.predicates) == 0 {
filtered = nodes
} else {
allNodes := int32(g.cache.NodeTree().NumNodes)
// numFeasibleNodesToFind保证一次性不用返回过多的Node数量，避免数组过大
numNodesToFind := g.numFeasibleNodesToFind(allNodes)
// Create filtered list with enough space to avoid growing it
// and allow assigning.
filtered = make([]*v1.Node, numNodesToFind)
errs := errors.MessageCountMap{}
var (
predicateResultLock sync.Mutex
filteredLen int32
equivClass *equivalence.Class
)
ctx, cancel := context.WithCancel(context.Background())
// We can use the same metadata producer for all nodes.
meta := g.predicateMetaProducer(pod, g.cachedNodeInfoMap)
if g.equivalenceCache != nil {
// getEquivalenceClassInfo will return immediately if no equivalence pod found
equivClass = equivalence.NewClass(pod)
}
// checkNode处理预选策略
checkNode := func(i int) {
var nodeCache *equivalence.NodeCache
// 每次获取Node信息
nodeName := g.cache.NodeTree().Next()
if g.equivalenceCache != nil {
nodeCache = g.equivalenceCache.LoadNodeCache(nodeName)
}
fits, failedPredicates, err := podFitsOnNode(
pod,
meta,
g.cachedNodeInfoMap[nodeName],
g.predicates,
nodeCache,
g.schedulingQueue,
g.alwaysCheckAllPredicates,
equivClass,
)
if err != nil {
predicateResultLock.Lock()
errs[err.Error()]++
predicateResultLock.Unlock()
return
}
if fits {
// 保证获取的Node数量在numNodesToFind内
length := atomic.AddInt32(&filteredLen, 1)
if length > numNodesToFind {
// 通知ParallelizeUntil任务结束
cancel()
atomic.AddInt32(&filteredLen, -1)
} else {
filtered[length-1] = g.cachedNodeInfoMap[nodeName].Node()
}
} else {
predicateResultLock.Lock()
failedPredicateMap[nodeName] = failedPredicates
predicateResultLock.Unlock()
}
}
// Stops searching for more nodes once the configured number of feasible nodes
// are found.
// 并行处理多个Node的checkNode工作
workqueue.ParallelizeUntil(ctx, 16, int(allNodes), checkNode)
filtered = filtered[:filteredLen]
if len(errs) > 0 {
return []*v1.Node{}, FailedPredicateMap{}, errors.CreateAggregateFromMessageCountMap(errs)
}
}
if len(filtered) > 0 && len(g.extenders) != 0 {
for _, extender := range g.extenders {
if !extender.IsInterested(pod) {
continue
}
filteredList, failedMap, err := extender.Filter(pod, filtered, g.cachedNodeInfoMap)
if err != nil {
if extender.IsIgnorable() {
klog.Warningf("Skipping extender %v as it returned error %v and has ignorable flag set",
extender, err)
continue
} else {
return []*v1.Node{}, FailedPredicateMap{}, err
}
}
for failedNodeName, failedMsg := range failedMap {
if _, found := failedPredicateMap[failedNodeName]; !found {
failedPredicateMap[failedNodeName] = []algorithm.PredicateFailureReason{}
}
failedPredicateMap[failedNodeName] = append(failedPredicateMap[failedNodeName], predicates.NewFailureReason(failedMsg))
}
filtered = filteredList
if len(filtered) == 0 {
break
}
}
}
return filtered, failedPredicateMap, nil
}
复制代码findNodesThatFit主要做了几个操作
1、判断是否配置了预选算法，如果没有，直接返回Node列表信息；
2、如果配置了预选算法，则同时对多个Node（最多一次16个）调用checkNode方法，判断Pod是否可以调度在该Node上；
3、预选筛选之后，如果配置了调度的扩展算法，需要继续对筛选后的Pod与Node进行再一次的筛选，获取最终匹配的Node列表。
这里有一个注意的地方，获取匹配的Node节点数量时，通过numFeasibleNodesToFind函数限制了每次获取的节点数，最大值为100。这样当匹配到相应的Node数时，checkNode方法不再调用。
这里个人觉着有些问题，当Node数量足够多的时候（大于100），由于numFeasibleNodesToFind限制了Node数量，导致并不能扫描到所有的Node，这样可能导致最合适的Node没有被扫描到，匹配到的只是较优先的Node，则最终调度到的Node也不是最合适的Node，只是相较于比较合适。
最终实现调度判断的接口是podFitsOnNode。podFitsOnNode最难理解的就是for循环了两次，根据注释，大致意思如下：
1、第一次循环，将所有的优先级比较高或者相等的nominatedPods加入到Node中，更新meta和nodeInfo。nominatedPods是指已经分配到Node内但是还没有真正运行起来的Pods。这样做可以保证优先级高的Pods不会因为现在的Pod的加入而导致调度失败；
2、第二次调度，不将nominatedPods加入到Node内。这样的原因是因为考虑到像Pod affinity策略的话，如果当前的Pod依赖的是nominatedPods，这样就会有问题。因为，nominatedPods不能保证一定可以调度到相应的Node上。
// podFitsOnNode checks whether a node given by NodeInfo satisfies the given predicate functions.
// For given pod, podFitsOnNode will check if any equivalent pod exists and try to reuse its cached
// predicate results as possible.
// This function is called from two different places: Schedule and Preempt.
// When it is called from Schedule, we want to test whether the pod is schedulable
// on the node with all the existing pods on the node plus higher and equal priority
// pods nominated to run on the node.
// When it is called from Preempt, we should remove the victims of preemption and
// add the nominated pods. Removal of the victims is done by SelectVictimsOnNode().
// It removes victims from meta and NodeInfo before calling this function.
// ---
// podFitsOnNode根据给定的NodeInfo判断是否匹配相应的预选函数
// 对于一个给定的Pod，podFitsOnNode会检查之前是否有等价的Pod，这样就可以直接复用等价Pod的预选结果
// 该函数会有两个地方调用：Schedule和Preempt
// 当Schedule（正常调度）的时候，判断Node上所有已经存在的Pod和将被指定将要调度到这个Node上的其他所有高优先级Pod外，当前的Pod是否可以调度
// 当Preempt（抢占式）的时候，待定。。。
func podFitsOnNode(
pod *v1.Pod,
meta algorithm.PredicateMetadata,
info *schedulercache.NodeInfo,
predicateFuncs map[string]algorithm.FitPredicate,
nodeCache *equivalence.NodeCache,
queue internalqueue.SchedulingQueue,
alwaysCheckAllPredicates bool,
equivClass *equivalence.Class,
) (bool, []algorithm.PredicateFailureReason, error) {
var (
eCacheAvailable bool
failedPredicates []algorithm.PredicateFailureReason
)
podsAdded := false
// We run predicates twice in some cases. If the node has greater or equal priority
// nominated pods, we run them when those pods are added to meta and nodeInfo.
// If all predicates succeed in this pass, we run them again when these
// nominated pods are not added. This second pass is necessary because some
// predicates such as inter-pod affinity may not pass without the nominated pods.
// If there are no nominated pods for the node or if the first run of the
// predicates fail, we don't run the second pass.
// We consider only equal or higher priority pods in the first pass, because
// those are the current "pod" must yield to them and not take a space opened
// for running them. It is ok if the current "pod" take resources freed for
// lower priority pods.
// Requiring that the new pod is schedulable in both circumstances ensures that
// we are making a conservative decision: predicates like resources and inter-pod
// anti-affinity are more likely to fail when the nominated pods are treated
// as running, while predicates like pod affinity are more likely to fail when
// the nominated pods are treated as not running. We can't just assume the
// nominated pods are running because they are not running right now and in fact,
// they may end up getting scheduled to a different node.
// 两次循环的原因主要就是因为NominatedPods调度的不一定就是此Node，还有Pod的亲和性等问题
for i := 0; i < 2; i++ {
metaToUse := meta
nodeInfoToUse := info
if i == 0 {
// 第一次调度，根据NominatedPods更新meta和nodeInfo信息，pod根据更新后的信息去预选
// 第二次调度，meta和nodeInfo信息不变，保证pod不完全依赖于NominatedPods（主要考虑到pod亲和性之类的）
podsAdded, metaToUse, nodeInfoToUse = addNominatedPods(pod, meta, info, queue)
} else if !podsAdded || len(failedPredicates) != 0 {
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 = equivClass != nil && nodeCache != nil && !podsAdded
for predicateID, predicateKey := range predicates.Ordering() {
var (
fit bool
reasons []algorithm.PredicateFailureReason
err error
)
//TODO (yastij) : compute average predicate restrictiveness to export it as Prometheus metric
if predicate, exist := predicateFuncs[predicateKey]; exist {
if eCacheAvailable {
fit, reasons, err = nodeCache.RunPredicate(predicate, predicateKey, predicateID, pod, metaToUse, nodeInfoToUse, equivClass)
} else {
fit, reasons, err = predicate(pod, metaToUse, nodeInfoToUse)
}
if err != nil {
return false, []algorithm.PredicateFailureReason{}, err
}
if !fit {
// eCache is available and valid, and predicates result is unfit, record the fail reasons
failedPredicates = append(failedPredicates, reasons...)
// if alwaysCheckAllPredicates is false, short circuit all predicates when one predicate fails.
if !alwaysCheckAllPredicates {
klog.V(5).Infoln("since alwaysCheckAllPredicates has not been set, the predicate " +
"evaluation is short circuited and there are chances " +
"of other predicates failing as well.")
break
}
}
}
}
}
return len(failedPredicates) == 0, failedPredicates, nil
}
复制代码之后就是根据预选的调度算法，一个个判断是否都满足。这里有个小优化，如果当前的Pod在之前有一个等价的Pod，则直接从缓存返回相应上一次的结果。如果成功则不用继续调用预选算法。但是，对于缓存部分，我个人有些疑问，可能对于上一个Pod缓存的结果是成功的，但是本次调度，Node信息发生变化了，缓存结果是成功的，但是实际上可能并不一定会成功。
预选调度算法
本节主要说的是默认的调度算法。默认的代码在pkg/scheduler/algorithmprovider/defaults/defaults.go下，defaultPredicates方法返回的是默认的一系列预选算法。与预选相关的代码都在pkg/scheduler/algorithm/predicates/predicates.go下
调度方法基本一致，参数为
(pod *v1.Pod, meta algorithm.PredicateMetadata, nodeInfo *schedulercache.NodeInfo)，返回值为
(bool, []algorithm.PredicateFailureReason, error)。
优选
预选完成之后会得到一个Node的数组。如果预选合适的节点数大于1，则需要调用优选算法根据评分获取最优的节点。
优选算法调用的接口是PrioritizeNodes，使用与预选类似的多任务同步调用方式，采用MapReduce的思想，Map根据不同的优选算法获取对某一Node的值，根据Reduce统计最终的结果。
优选调度算法
优选调度算法默认代码在pkg/scheduler/algorithmprovider/defaults/defaults.go下，defaultPriorities方法返回的是默认的一系列优选算法，通过工厂模式处理相应的优选算法，代码如下
func defaultPriorities() sets.String {
return sets.NewString(
// spreads pods by minimizing the number of pods (belonging to the same service or replication controller) on the same node.
factory.RegisterPriorityConfigFactory(
"SelectorSpreadPriority",
factory.PriorityConfigFactory{
MapReduceFunction: func(args factory.PluginFactoryArgs) (algorithm.PriorityMapFunction, algorithm.PriorityReduceFunction) {
return priorities.NewSelectorSpreadPriority(args.ServiceLister, args.ControllerLister, args.ReplicaSetLister, args.StatefulSetLister)
},
Weight: 1,
},
),
// pods should be placed in the same topological domain (e.g. same node, same rack, same zone, same power domain, etc.)
// as some other pods, or, conversely, should not be placed in the same topological domain as some other pods.
factory.RegisterPriorityConfigFactory(
"InterPodAffinityPriority",
factory.PriorityConfigFactory{
Function: func(args factory.PluginFactoryArgs) algorithm.PriorityFunction {
return priorities.NewInterPodAffinityPriority(args.NodeInfo, args.NodeLister, args.PodLister, args.HardPodAffinitySymmetricWeight)
},
Weight: 1,
},
),
// Prioritize nodes by least requested utilization.
factory.RegisterPriorityFunction2("LeastRequestedPriority", priorities.LeastRequestedPriorityMap, nil, 1),
// Prioritizes nodes to help achieve balanced resource usage
factory.RegisterPriorityFunction2("BalancedResourceAllocation", priorities.BalancedResourceAllocationMap, nil, 1),
// Set this weight large enough to override all other priority functions.
// TODO: Figure out a better way to do this, maybe at same time as fixing #24720.
factory.RegisterPriorityFunction2("NodePreferAvoidPodsPriority", priorities.CalculateNodePreferAvoidPodsPriorityMap, nil, 10000),
// Prioritizes nodes that have labels matching NodeAffinity
factory.RegisterPriorityFunction2("NodeAffinityPriority", priorities.CalculateNodeAffinityPriorityMap, priorities.CalculateNodeAffinityPriorityReduce, 1),
// Prioritizes nodes that marked with taint which pod can tolerate.
factory.RegisterPriorityFunction2("TaintTolerationPriority", priorities.ComputeTaintTolerationPriorityMap, priorities.ComputeTaintTolerationPriorityReduce, 1),
// ImageLocalityPriority prioritizes nodes that have images requested by the pod present.
factory.RegisterPriorityFunction2("ImageLocalityPriority", priorities.ImageLocalityPriorityMap, nil, 1),
)
}
复制代码用到的优选算法通过代码结构基本可以看出
每一个不同的优选策略独立成一个单独的文件。通过优选之后，调用
selectHost方法获取分数最高的Node。如果多个Node分数相同，则使用轮询的方式得到最终的Node。
抢占调度
当通过正常的调度流程如果没有找到合适的节点（主要是预选没有合适的节点），会判断需不需要进行抢占调度，具体的代码在pkg/scheduler/scheduler.go文件下，用到的方法preempt，具体如下：
// preempt tries to create room for a pod that has failed to schedule, by preempting lower priority pods if possible.
// If it succeeds, it adds the name of the node where preemption has happened to the pod annotations.
// It returns the node name and an error if any.
// ---
// preempt尽可能的通过去抢占低优先级的Pod的空间，为调度失败的Pod创造空间
// 如果成功了，就会去添加在Pod注解中声明的Node名称
// 返回Node名称和错误（如果有错误的话）
func (sched *Scheduler) preempt(preemptor *v1.Pod, scheduleErr error) (string, error) {
// 1.判断是否开启Pod优先级，调度器是否配置了DisablePreemption，两者中任一满足即停止抢占
if !util.PodPriorityEnabled() || sched.config.DisablePreemption {
klog.V(3).Infof("Pod priority feature is not enabled or preemption is disabled by scheduler configuration." +
" No preemption is performed.")
return "", nil
}
// 2.获取待抢占Pod的信息
preemptor, err := sched.config.PodPreemptor.GetUpdatedPod(preemptor)
if err != nil {
klog.Errorf("Error getting the updated preemptor pod object: %v", err)
return "", err
}
// 3.根据配置的算法获取抢占的节点
// 获取到的四个参数
// 1.抢占获取到的Node
// 2.需要被删除掉的低优先级的Pod列表
// 3.需要删除掉的nominatedPods列表
// 4.错误信息
node, victims, nominatedPodsToClear, err := sched.config.Algorithm.Preempt(preemptor, sched.config.NodeLister, scheduleErr)
metrics.PreemptionVictims.Set(float64(len(victims)))
if err != nil {
klog.Errorf("Error preempting victims to make room for %v/%v.", preemptor.Namespace, preemptor.Name)
return "", err
}
var nodeName = ""
if node != nil {
// 1.将Pod和Node结合，更新相应的信息(Pod的nodeName有值)，并且构造apiserver的调用
// 2.所有的将要被删除的Pod一一被删除
// 只有两者都满足了，才能保证抢占成功
nodeName = node.Name
// Update the scheduling queue with the nominated pod information. Without
// this, there would be a race condition between the next scheduling cycle
// and the time the scheduler receives a Pod Update for the nominated pod.
sched.config.SchedulingQueue.UpdateNominatedPodForNode(preemptor, nodeName)
// Make a call to update nominated node name of the pod on the API server.
err = sched.config.PodPreemptor.SetNominatedNodeName(preemptor, nodeName)
if err != nil {
klog.Errorf("Error in preemption process. Cannot update pod %v/%v annotations: %v", preemptor.Namespace, preemptor.Name, err)
sched.config.SchedulingQueue.DeleteNominatedPodIfExists(preemptor)
return "", err
}
for _, victim := range victims {
if err := sched.config.PodPreemptor.DeletePod(victim); err != nil {
klog.Errorf("Error preempting pod %v/%v: %v", victim.Namespace, victim.Name, err)
return "", err
}
sched.config.Recorder.Eventf(victim, v1.EventTypeNormal, "Preempted", "by %v/%v on node %v", preemptor.Namespace, preemptor.Name, nodeName)
}
}
// 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.
// 4.删除nominatedPods，不要求一定成功，对整体结果不影响
for _, p := range nominatedPodsToClear {
rErr := sched.config.PodPreemptor.RemoveNominatedNodeName(p)
if rErr != nil {
klog.Errorf("Cannot remove nominated node annotation of pod: %v", rErr)
// We do not return as this error is not critical.
}
}
return nodeName, err
}
复制代码整体代码结构比较清晰，有如下几个步骤：
1、判断是否需要进行抢占调度，主要有两个判断项（PodPriority是否开启、调度器是否配置DisablePreemption），两者缺一不可；
2、获取待抢占调度Pod配置的信息；
3、通过配置算法的抢占策略获取抢占调度的结果（最核心的步骤）；
4、收尾工作（更新Pod的信息、删除低优先级的Pod、删除一些资源如nominatedPods）
整个过程最核心的是调度算法获取调度结果的接口，同预选优选一样，默认的调度实现均在generic_scheduler.go文件，方法是Preempt。Preempt方法返回四个参数，分别是
1)Preempt得到的Node；
2)被抢占的Pod的列表（待删除）；
3)将要被清除的nominatedPods（待清除）；
4)可能返回的error消息Preempt方法主要执行以下几个步骤：
1、从预选失败的节点中获取可以用来做抢占调度的节点，通过一个switch语句排除不可以用来做抢占调度的节点
2、获取PDB（Pod中断预算）列表，用来做后续的判断标准；
3、通过调用
selectNodesForPreemption方法，判断哪些Node可以进行抢占调度。通过
ParallelizeUntil方法同步对所有的Node进行判断，判断路径为
checkNode-->selectVictimsOnNode-->podFitsOnNode，最终同预选方法类似，使用了
podFitsOnNode方法。不同于普通预选，抢占调度会先对Pod优先级判断，然后在移除掉优先级较低的Pod之后再调用
podFitsOnNode方法，以此达到抢占的效果。
selectNodesForPreemption方法返回的参数是一个map类型的值，key为Node信息，value为该Node如果作为调度节点，将要清除的一些信息，包括Pods和PDB信息
4、获取到抢占调度可以实现的Nodes资源后，继续通过扩展的算法进行过滤；
5、选中最终的抢占调度的Node，调用
pickOneNodeForPreemption方法，主要基于5个原则：
a)PDB violations（违规）值最小的Node；
b)挑选具有最低优先级受害者的节点，即被清除的Node上的Pods，它的优先级是最低的；
c)通过所有受害者Pods（将被删除的低优先级Pods）的优先级总和做区分；
d)如果多个Node优先级总和仍然相等，则选择具有最小受害者数量的Node；
e)如果多个Node优先级总和仍然相等，则选择第一个这样的Node（随机排序）;
6、选中最终的Node之后，记录该Node上优先级较低的NominatedPods，这些Pod还未调度，需要将其调度关系进行删除，重新应用。代码如下：
// preempt finds nodes with pods that can be preempted to make room for "pod" to
// schedule. It chooses one of the nodes and preempts the pods on the node and
// returns 1) the node, 2) the list of preempted pods if such a node is found,
// 3) A list of pods whose nominated node name should be cleared, and 4) any
// possible error.
// Preempt does not update its snapshot. It uses the same snapshot used in the
// scheduling cycle. This is to avoid a scenario where preempt finds feasible
// nodes without preempting any pod. When there are many pending pods in the
// scheduling queue a nominated pod will go back to the queue and behind
// other pods with the same priority. The nominated pod prevents other pods from
// using the nominated resources and the nominated pod could take a long time
// before it is retried after many other pending pods.
func (g *genericScheduler) Preempt(pod *v1.Pod, nodeLister algorithm.NodeLister, scheduleErr error) (*v1.Node, []*v1.Pod, []*v1.Pod, error) {
// Scheduler may return various types of errors. Consider preemption only if
// the error is of type FitError.
fitError, ok := scheduleErr.(*FitError)
if !ok || fitError == nil {
return nil, nil, nil, nil
}
if !podEligibleToPreemptOthers(pod, g.cachedNodeInfoMap) {
klog.V(5).Infof("Pod %v/%v is not eligible for more preemption.", pod.Namespace, pod.Name)
return nil, nil, nil, nil
}
allNodes, err := nodeLister.List()
if err != nil {
return nil, nil, nil, err
}
if len(allNodes) == 0 {
return nil, nil, nil, ErrNoNodesAvailable
}
// 1.获取预选调度失败的节点，但是可能是潜在的抢占可能成功的节点(所有的抢占节点都是在潜在节点内部选择)
potentialNodes := nodesWherePreemptionMightHelp(allNodes, fitError.FailedPredicates)
if len(potentialNodes) == 0 {
klog.V(3).Infof("Preemption will not help schedule pod %v/%v on any node.", pod.Namespace, pod.Name)
// In this case, we should clean-up any existing nominated node name of the pod.
return nil, nil, []*v1.Pod{pod}, nil
}
// 2.获取PDB（Pod中断预算）列表
pdbs, err := g.pdbLister.List(labels.Everything())
if err != nil {
return nil, nil, nil, err
}
// 3.获取所有可以进行Preempt的Node节点的信息，主要包含该节点哪些Pod需要被抢占掉
nodeToVictims, err := selectNodesForPreemption(pod, g.cachedNodeInfoMap, potentialNodes, g.predicates,
g.predicateMetaProducer, g.schedulingQueue, pdbs)
if err != nil {
return nil, nil, nil, err
}
// We will only check nodeToVictims with extenders that support preemption.
// Extenders which do not support preemption may later prevent preemptor from being scheduled on the nominated
// node. In that case, scheduler will find a different host for the preemptor in subsequent scheduling cycles.
// 4.扩展的Preempt调度判断
nodeToVictims, err = g.processPreemptionWithExtenders(pod, nodeToVictims)
if err != nil {
return nil, nil, nil, err
}
// 5.选中某一个Node
candidateNode := pickOneNodeForPreemption(nodeToVictims)
if candidateNode == nil {
return nil, nil, nil, err
}
// Lower priority pods nominated to run on this node, may no longer fit on
// this node. So, we should remove their nomination. Removing their
// nomination updates these pods and moves them to the active queue. It
// lets scheduler find another place for them.
// 6.判断哪些Pod优先级较低，后续需要被清除掉，不作为NominatedPods存在
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)
}
复制代码综上，抢占调度主要强调的一点是Pod的优先级。与普通调度不同的是，抢占调度对Pod做了明确的优先级区分，以此来达到抢占的目的。
选举
在Scheduler启动的时候，需要判断是否需要做选主操作。配置选举操作很简单，只需要在配置文件中添加--leader-elect=true即可。代码中，如果检测到了配置选举，则首先会参加选举，只有拿到主节点的scheduler才能执行调度相关工作。
选举代码结构比较简单，如图，代码位于client-go包中，路径为client-go/tools/leaderelection/leaderelection.go
le.acquire(ctx)、
le.renew(ctx)以及
le.config.Callbacks.OnStartedLeading(ctx)。
acquire表示是否选主成功，只有成功了之后，才能执行
OnStartedLeading和
renew。
OnStartedLeading是一个回调方法，执行的就是scheduler的
run方法。
renew主要做选主的更新操作。当节点上的scheduler被选主时，还需要不断的更新信息，判断是否主节点功能正常。
进入
acquire或者
renew方法，有一个共同的调用方法是
tryAcquireOrRenew，该方法就是整个选举的核心实现。
tryAcquireOrRenew顾名思义，如果没有获取到租约，就去获取leader的租约，否则就去更新租约。主要有三部分操作：
1、调用
Get操作获取是否存在ElectionRecord。如果不存在，则调用
Create方法新建一个新的Endpoint，当前节点为scheduler的主节点，选举成功；否则，执行更新操作；
2、获取到记录，表明执行的是更新租约操作，需要验证当前节点的身份和时间，判断是否可以执行更新租约操作；
3、更新信息，执行
Update操作，更新选主信息。
// tryAcquireOrRenew tries to acquire a leader lease if it is not already acquired,
// else it tries to renew the lease if it has already been acquired. Returns true
// on success else returns false.
// ---
// tryAcquireOrRenew，如果没有获取到租约，就去获取leader的租约，否则去更新租约。
func (le *LeaderElector) tryAcquireOrRenew() bool {
now := metav1.Now()
leaderElectionRecord := rl.LeaderElectionRecord{
HolderIdentity: le.config.Lock.Identity(),
LeaseDurationSeconds: int(le.config.LeaseDuration / time.Second),
RenewTime: now,
AcquireTime: now,
}
// 1. obtain or create the ElectionRecord
// 1. 调用Endpoint的Get操作，获取oldLeaderElectionRecord
oldLeaderElectionRecord, err := le.config.Lock.Get()
if err != nil {
if !errors.IsNotFound(err) {
klog.Errorf("error retrieving resource lock %v: %v", le.config.Lock.Describe(), err)
return false
}
// 创建新的Endpoint
if err = le.config.Lock.Create(leaderElectionRecord); err != nil {
klog.Errorf("error initially creating leader election record: %v", err)
return false
}
le.observedRecord = leaderElectionRecord
le.observedTime = le.clock.Now()
return true
}
// 2. Record obtained, check the Identity & Time
// 2. 获取到了记录，检查下身份和时间信息，判断是否合法
if !reflect.DeepEqual(le.observedRecord, *oldLeaderElectionRecord) {
le.observedRecord = *oldLeaderElectionRecord
le.observedTime = le.clock.Now()
}
if le.observedTime.Add(le.config.LeaseDuration).After(now.Time) &&
!le.IsLeader() {
klog.V(4).Infof("lock is held by %v and has not yet expired", oldLeaderElectionRecord.HolderIdentity)
return false
}
// 3. We're going to try to update. The leaderElectionRecord is set to it's default
// here. Let's correct it before updating.
if le.IsLeader() {
leaderElectionRecord.AcquireTime = oldLeaderElectionRecord.AcquireTime
leaderElectionRecord.LeaderTransitions = oldLeaderElectionRecord.LeaderTransitions
} else {
leaderElectionRecord.LeaderTransitions = oldLeaderElectionRecord.LeaderTransitions + 1
}
// update the lock itself
if err = le.config.Lock.Update(leaderElectionRecord); err != nil {
klog.Errorf("Failed to update lock: %v", err)
return false
}
le.observedRecord = leaderElectionRecord
le.observedTime = le.clock.Now()
return true
}
复制代码Scheduler的选举操作比较简单，主要就是通过判断记录在Etcd中的Endpoints是否可以更新来判断是否可以进行选举。整个选举操作依赖于Etcd的特点来保证分布式操作的成功和唯一。在kube-system的namespace下可以查看相应的endpoint：kube-scheduler。