kube-scheduler源码分析（一）之 NewSchedulerCommand

以下代码分析基于 kubernetes v1.12.0 版本。

scheduler的cmd代码目录结构如下：

kube-scheduler
├── BUILD
├── OWNERS
├── app            # app的目录下主要为运行scheduler相关的对象
│   ├── BUILD
│   ├── config      
│   │   ├── BUILD
│   │   └── config.go    # Scheduler的配置对象config
│   ├── options      # options主要记录 Scheduler 使用到的参数
│   │   ├── BUILD
│   │   ├── configfile.go
│   │   ├── deprecated.go
│   │   ├── deprecated_test.go
│   │   ├── insecure_serving.go
│   │   ├── insecure_serving_test.go
│   │   ├── options.go    # 主要包括Options、NewOptions、AddFlags、Config等函数
│   │   └── options_test.go
│   └── server.go    # 主要包括 NewSchedulerCommand、NewSchedulerConfig、Run等函数
└── scheduler.go     # main入口函数

1. Main函数

此部分的代码为/cmd/kube-scheduler/scheduler.go

kube-scheduler的入口函数Main函数，仍然是采用统一的代码风格，使用Cobra命令行框架。

func main() {
    rand.Seed(time.Now().UTC().UnixNano())
    command := app.NewSchedulerCommand()
    // TODO: once we switch everything over to Cobra commands, we can go back to calling
    // utilflag.InitFlags() (by removing its pflag.Parse() call). For now, we have to set the
    // normalize func and add the go flag set by hand.
    pflag.CommandLine.SetNormalizeFunc(utilflag.WordSepNormalizeFunc)
    pflag.CommandLine.AddGoFlagSet(goflag.CommandLine)
    // utilflag.InitFlags()
    logs.InitLogs()
    defer logs.FlushLogs()
    if err := command.Execute(); err != nil {
        fmt.Fprintf(os.Stderr, "%v\n", err)
        os.Exit(1)
    }
}

核心代码：

// 初始化scheduler命令结构体
command := app.NewSchedulerCommand()
// 执行Execute
err := command.Execute()

2. NewSchedulerCommand

此部分的代码为/cmd/kube-scheduler/app/server.go

NewSchedulerCommand主要用来构造和初始化SchedulerCommand结构体，

// NewSchedulerCommand creates a *cobra.Command object with default parameters
func NewSchedulerCommand() *cobra.Command {
    opts, err := options.NewOptions()
    if err != nil {
        glog.Fatalf("unable to initialize command options: %v", err)
    }
    cmd := &cobra.Command{
        Use: "kube-scheduler",
        Long: `The Kubernetes scheduler is a policy-rich, topology-aware,
workload-specific function that significantly impacts availability, performance,
and capacity. The scheduler needs to take into account individual and collective
resource requirements, quality of service requirements, hardware/software/policy
constraints, affinity and anti-affinity specifications, data locality, inter-workload
interference, deadlines, and so on. Workload-specific requirements will be exposed
through the API as necessary.`,
        Run: func(cmd *cobra.Command, args []string) {
            verflag.PrintAndExitIfRequested()
            utilflag.PrintFlags(cmd.Flags())
            if len(args) != 0 {
                fmt.Fprint(os.Stderr, "arguments are not supported\n")
            }
            if errs := opts.Validate(); len(errs) > 0 {
                fmt.Fprintf(os.Stderr, "%v\n", utilerrors.NewAggregate(errs))
                os.Exit(1)
            }
            if len(opts.WriteConfigTo) > 0 {
                if err := options.WriteConfigFile(opts.WriteConfigTo, &opts.ComponentConfig); err != nil {
                    fmt.Fprintf(os.Stderr, "%v\n", err)
                    os.Exit(1)
                }
                glog.Infof("Wrote configuration to: %s\n", opts.WriteConfigTo)
                return
            }
            c, err := opts.Config()
            if err != nil {
                fmt.Fprintf(os.Stderr, "%v\n", err)
                os.Exit(1)
            }
            stopCh := make(chan struct{})
            if err := Run(c.Complete(), stopCh); err != nil {
                fmt.Fprintf(os.Stderr, "%v\n", err)
                os.Exit(1)
            }
        },
    }
    opts.AddFlags(cmd.Flags())
    cmd.MarkFlagFilename("config", "yaml", "yml", "json")
    return cmd
}

核心代码：

// 构造option
opts, err := options.NewOptions()
// 初始化config对象
c, err := opts.Config()
// 执行run函数
err := Run(c.Complete(), stopCh)
// 添加参数
opts.AddFlags(cmd.Flags())

2.1. NewOptions

NewOptions主要用来构造SchedulerServer使用的参数和上下文，其中核心参数是KubeSchedulerConfiguration。

opts, err := options.NewOptions()

NewOptions:

// NewOptions returns default scheduler app options.
func NewOptions() (*Options, error) {
    cfg, err := newDefaultComponentConfig()
    if err != nil {
        return nil, err
    }
    hhost, hport, err := splitHostIntPort(cfg.HealthzBindAddress)
    if err != nil {
        return nil, err
    }
    o := &Options{
        ComponentConfig: *cfg,
        SecureServing:   nil, // TODO: enable with apiserveroptions.NewSecureServingOptions()
        CombinedInsecureServing: &CombinedInsecureServingOptions{
            Healthz: &apiserveroptions.DeprecatedInsecureServingOptions{
                BindNetwork: "tcp",
            },
            Metrics: &apiserveroptions.DeprecatedInsecureServingOptions{
                BindNetwork: "tcp",
            },
            BindPort:    hport,
            BindAddress: hhost,
        },
        Authentication: nil, // TODO: enable with apiserveroptions.NewDelegatingAuthenticationOptions()
        Authorization:  nil, // TODO: enable with apiserveroptions.NewDelegatingAuthorizationOptions()
        Deprecated: &DeprecatedOptions{
            UseLegacyPolicyConfig:    false,
            PolicyConfigMapNamespace: metav1.NamespaceSystem,
        },
    }
    return o, nil
}

2.2. Options.Config

Config初始化调度器的配置对象。

c, err := opts.Config()

Config函数主要执行以下操作：

• 构建scheduler client、leaderElectionClient、eventClient。
• 创建event recorder
• 设置leader选举
• 创建informer对象，主要函数有NewSharedInformerFactory和NewPodInformer。

Config具体代码如下：

// Config return a scheduler config object
func (o *Options) Config() (*schedulerappconfig.Config, error) {
    c := &schedulerappconfig.Config{}
    if err := o.ApplyTo(c); err != nil {
        return nil, err
    }
    // prepare kube clients.
    client, leaderElectionClient, eventClient, err := createClients(c.ComponentConfig.ClientConnection, o.Master, c.ComponentConfig.LeaderElection.RenewDeadline.Duration)
    if err != nil {
        return nil, err
    }
    // Prepare event clients.
    eventBroadcaster := record.NewBroadcaster()
    recorder := eventBroadcaster.NewRecorder(legacyscheme.Scheme, corev1.EventSource{Component: c.ComponentConfig.SchedulerName})
    // Set up leader election if enabled.
    var leaderElectionConfig *leaderelection.LeaderElectionConfig
    if c.ComponentConfig.LeaderElection.LeaderElect {
        leaderElectionConfig, err = makeLeaderElectionConfig(c.ComponentConfig.LeaderElection, leaderElectionClient, recorder)
        if err != nil {
            return nil, err
        }
    }
    c.Client = client
    c.InformerFactory = informers.NewSharedInformerFactory(client, 0)
    c.PodInformer = factory.NewPodInformer(client, 0)
    c.EventClient = eventClient
    c.Recorder = recorder
    c.Broadcaster = eventBroadcaster
    c.LeaderElection = leaderElectionConfig
    return c, nil
}

2.3. AddFlags

AddFlags为SchedulerServer添加指定的参数。

opts.AddFlags(cmd.Flags())

AddFlags函数的具体代码如下：

// AddFlags adds flags for the scheduler options.
func (o *Options) AddFlags(fs *pflag.FlagSet) {
    fs.StringVar(&o.ConfigFile, "config", o.ConfigFile, "The path to the configuration file. Flags override values in this file.")
    fs.StringVar(&o.WriteConfigTo, "write-config-to", o.WriteConfigTo, "If set, write the configuration values to this file and exit.")
    fs.StringVar(&o.Master, "master", o.Master, "The address of the Kubernetes API server (overrides any value in kubeconfig)")
    o.SecureServing.AddFlags(fs)
    o.CombinedInsecureServing.AddFlags(fs)
    o.Authentication.AddFlags(fs)
    o.Authorization.AddFlags(fs)
    o.Deprecated.AddFlags(fs, &o.ComponentConfig)
    leaderelectionconfig.BindFlags(&o.ComponentConfig.LeaderElection.LeaderElectionConfiguration, fs)
    utilfeature.DefaultFeatureGate.AddFlag(fs)
}

3. Run

此部分的代码为/cmd/kube-scheduler/app/server.go

err := Run(c.Complete(), stopCh)

Run运行一个不退出的常驻进程，来执行scheduler的相关操作。

Run函数的主要内容如下：

• 通过scheduler config来创建scheduler的结构体。
• 运行event broadcaster、healthz server、metrics server。
• 运行所有的informer并在调度前等待cache的同步（重点）。
• 执行sched.Run()来运行scheduler的调度逻辑。
• 如果多个scheduler并开启了LeaderElect，则执行leader选举。

以下对重点代码分开分析：

3.1. NewSchedulerConfig

NewSchedulerConfig初始化SchedulerConfig（此部分具体逻辑待后续专门分析），最后初始化生成scheduler结构体。

// Build a scheduler config from the provided algorithm source.
schedulerConfig, err := NewSchedulerConfig(c)
if err != nil {
    return err
}
// Create the scheduler.
sched := scheduler.NewFromConfig(schedulerConfig)

3.2. InformerFactory.Start

运行PodInformer，并运行InformerFactory。此部分的逻辑为client-go的informer机制，在Informer机制中有详细分析。

// Start all informers.
go c.PodInformer.Informer().Run(stopCh)
c.InformerFactory.Start(stopCh)

3.3. WaitForCacheSync

在调度前等待cache同步。

// Wait for all caches to sync before scheduling.
c.InformerFactory.WaitForCacheSync(stopCh)
controller.WaitForCacheSync("scheduler", stopCh, c.PodInformer.Informer().HasSynced)

3.3.1. InformerFactory.WaitForCacheSync

InformerFactory.WaitForCacheSync等待所有启动的informer的cache进行同步，保持本地的store信息与etcd的信息是最新一致的。

// WaitForCacheSync waits for all started informers' cache were synced.
func (f *sharedInformerFactory) WaitForCacheSync(stopCh <-chan struct{}) map[reflect.Type]bool {
    informers := func() map[reflect.Type]cache.SharedIndexInformer {
        f.lock.Lock()
        defer f.lock.Unlock()
        informers := map[reflect.Type]cache.SharedIndexInformer{}
        for informerType, informer := range f.informers {
            if f.startedInformers[informerType] {
                informers[informerType] = informer
            }
        }
        return informers
    }()
    res := map[reflect.Type]bool{}
    for informType, informer := range informers {
        res[informType] = cache.WaitForCacheSync(stopCh, informer.HasSynced)
    }
    return res
}

接着调用cache.WaitForCacheSync。

// WaitForCacheSync waits for caches to populate.  It returns true if it was successful, false
// if the controller should shutdown
func WaitForCacheSync(stopCh <-chan struct{}, cacheSyncs ...InformerSynced) bool {
    err := wait.PollUntil(syncedPollPeriod,
        func() (bool, error) {
            for _, syncFunc := range cacheSyncs {
                if !syncFunc() {
                    return false, nil
                }
            }
            return true, nil
        },
        stopCh)
    if err != nil {
        glog.V(2).Infof("stop requested")
        return false
    }
    glog.V(4).Infof("caches populated")
    return true
}

3.3.2. controller.WaitForCacheSync

controller.WaitForCacheSync是对cache.WaitForCacheSync的一层封装，通过不同的controller的名字来记录不同controller等待cache同步。

controller.WaitForCacheSync("scheduler", stop, s.PodInformer.Informer().HasSynced)

controller.WaitForCacheSync具体代码如下：

// WaitForCacheSync is a wrapper around cache.WaitForCacheSync that generates log messages
// indicating that the controller identified by controllerName is waiting for syncs, followed by
// either a successful or failed sync.
func WaitForCacheSync(controllerName string, stopCh <-chan struct{}, cacheSyncs ...cache.InformerSynced) bool {
    glog.Infof("Waiting for caches to sync for %s controller", controllerName)
    if !cache.WaitForCacheSync(stopCh, cacheSyncs...) {
        utilruntime.HandleError(fmt.Errorf("Unable to sync caches for %s controller", controllerName))
        return false
    }
    glog.Infof("Caches are synced for %s controller", controllerName)
    return true
}

3.4. LeaderElection

如果有多个scheduler，并开启leader选举，则运行LeaderElector直到选举结束或退出。

// If leader election is enabled, run via LeaderElector until done and exit.
if c.LeaderElection != nil {
    c.LeaderElection.Callbacks = leaderelection.LeaderCallbacks{
        OnStartedLeading: run,
        OnStoppedLeading: func() {
            utilruntime.HandleError(fmt.Errorf("lost master"))
        },
    }
    leaderElector, err := leaderelection.NewLeaderElector(*c.LeaderElection)
    if err != nil {
        return fmt.Errorf("couldn't create leader elector: %v", err)
    }
    leaderElector.Run(ctx)
    return fmt.Errorf("lost lease")
}

3.5. Scheduler.Run

// Prepare a reusable run function.
run := func(ctx context.Context) {
    sched.Run()
    <-ctx.Done()
}
ctx, cancel := context.WithCancel(context.TODO()) // TODO once Run() accepts a context, it should be used here
defer cancel()
go func() {
    select {
    case <-stopCh:
        cancel()
    case <-ctx.Done():
    }
}()
...
run(ctx)

Scheduler.Run先等待cache同步，然后开启调度逻辑的goroutine。

Scheduler.Run的具体代码如下：

// Run begins watching and scheduling. It waits for cache to be synced, then starts a goroutine and returns immediately.
func (sched *Scheduler) Run() {
    if !sched.config.WaitForCacheSync() {
        return
    }
    go wait.Until(sched.scheduleOne, 0, sched.config.StopEverything)
}

以上是对/cmd/kube-scheduler/scheduler.go部分代码的分析，Scheduler.Run后续的具体代码位于pkg/scheduler/scheduler.go待后续文章分析。

参考：

• https://github.com/kubernetes/kubernetes/tree/v1.12.0/cmd/kube-scheduler
• https://github.com/kubernetes/kubernetes/blob/v1.12.0/cmd/kube-scheduler/scheduler.go
• https://github.com/kubernetes/kubernetes/blob/v1.12.0/cmd/kube-scheduler/app/server.go