你应该知道的 Go 调度器知识：Go 核心原理 — 协程调度时机

golang 调度模型

模型总揽

核心实体

Goroutines (G)

golang 调度单元，golang 可以开启成千上万个 g，每个 g 可以理解为一个任务，等待被调度。其存储了 goroutine 的执行 stack 信息、goroutine 状态以及 goroutine 的任务函数等。g 只能感知到 p，下文说的 m 对其透明的。

OSThread (M)

系统线程，实际执行 g 的狠角色，但 m 并不维护 g 的状态，一切都是由幕后黑手 p 来控制。

Processor (P)

维护 m 执行时所需要的上下文，p 的个数通常和 cpu 核数一致（可以设置），代表 gorotine 的并发度。其维护了 g 的队列。

实体间的关系

一图胜千言，直接看这个经典的图

调度本质

即 schedule 函数，通过调度，放弃目前执行的 g，选择一个 g 来执行。选择算法不是本文重点，这里不做过多讲述。

切换时机

• 会阻塞的系统调用，比如文件 io，网络 io；
• time 系列定时操作；
• go func 的时候, func 执行完的时候；
• 管道读写阻塞的情况；
• 垃圾回收之后。
• 主动调用 runtime.Gosched()

调度时机分析

阻塞性系统调用

系统调用，如 read，golang 重写了所有系统调用，在系统调用加入了调度逻辑。

拿 read 举例

/usr/local/go/src/os/file.go:97

// Read reads up to len(b) bytes from the File.
// It returns the number of bytes read and an error, if any.
// EOF is signaled by a zero count with err set to io.EOF.
func (f *File) Read(b []byte) (n int, err error) {
  if f == nil {
    return 0, ErrInvalid
  }
  n, e := f.read(b)
  if n == 0 && len(b) > 0 && e == nil {
    return 0, io.EOF
  }
  if e != nil {
    err = &PathError{"read", f.name, e}
  }
  return n, err
}

嵌套到几层，就不全部贴出来，跟到底是如下函数：

func read(fd int, p []byte) (n int, err error) {
  var _p0 unsafe.Pointer
  if len(p) > 0 {
    _p0 = unsafe.Pointer(&p[0])
  } else {
    _p0 = unsafe.Pointer(&_zero)
  }
  r0, _, e1 := Syscall(SYS_READ, uintptr(fd), uintptr(_p0), uintptr(len(p)))
  n = int(r0)
  if e1 != 0 {
    err = errnoErr(e1)
  }
  return
}

func Syscall(trap, a1, a2, a3 uintptr) (r1, r2 uintptr, err Errno)

Syscall 是汇编实现

TEXT    ·Syscall(SB),NOSPLIT,$0-56
    BL  runtime·entersyscall(SB)
    MOVD    a1+8(FP), R3
    MOVD    a2+16(FP), R4
    MOVD    a3+24(FP), R5
    MOVD    R0, R6
    MOVD    R0, R7
    MOVD    R0, R8
    MOVD    trap+0(FP), R9  // syscall entry
    SYSCALL R9
    BVC ok
    MOVD    $-1, R4
    MOVD    R4, r1+32(FP)   // r1
    MOVD    R0, r2+40(FP)   // r2
    MOVD    R3, err+48(FP)  // errno
    BL  runtime·exitsyscall(SB)
    RET
ok:
    MOVD    R3, r1+32(FP)   // r1
    MOVD    R4, r2+40(FP)   // r2
    MOVD    R0, err+48(FP)  // errno
    BL  runtime·exitsyscall(SB)
    RET

可以看到，进入系统调用时，是调用 entersyscall，当离开系统调用，会运行 exitsyscall

// Standard syscall entry used by the go syscall library and normal cgo calls.
//go:nosplit
func entersyscall(dummy int32) {
    reentersyscall(getcallerpc(unsafe.Pointer(&dummy)), getcallersp(unsafe.Pointer(&dummy)))
}

func reentersyscall(pc, sp uintptr) {
  _g_ := getg()

  // Disable preemption because during this function g is in Gsyscall status,
  // but can have inconsistent g->sched, do not let GC observe it.
  _g_.m.locks++

  // Entersyscall must not call any function that might split/grow the stack.
  // (See details in comment above.)
  // Catch calls that might, by replacing the stack guard with something that
  // will trip any stack check and leaving a flag to tell newstack to die.
  _g_.stackguard0 = stackPreempt
  _g_.throwsplit = true

  // Leave SP around for GC and traceback.
  save(pc, sp)
  _g_.syscallsp = sp
  _g_.syscallpc = pc
  casgstatus(_g_, _Grunning, _Gsyscall)
  if _g_.syscallsp < _g_.stack.lo || _g_.stack.hi < _g_.syscallsp {
    systemstack(func() {
      print("entersyscall inconsistent ", hex(_g_.syscallsp), " [", hex(_g_.stack.lo), ",", hex(_g_.stack.hi), "]\n")
      throw("entersyscall")
    })
  }

  if trace.enabled {
    systemstack(traceGoSysCall)
    // systemstack itself clobbers g.sched.{pc,sp} and we might
    // need them later when the G is genuinely blocked in a
    // syscall
    save(pc, sp)
  }

  if atomic.Load(&sched.sysmonwait) != 0 { // TODO: fast atomic
    systemstack(entersyscall_sysmon)
    save(pc, sp)
  }

  if _g_.m.p.ptr().runSafePointFn != 0 {
    // runSafePointFn may stack split if run on this stack
    systemstack(runSafePointFn)
    save(pc, sp)
  }

  _g_.m.syscalltick = _g_.m.p.ptr().syscalltick
  _g_.sysblocktraced = true
  _g_.m.mcache = nil
  _g_.m.p.ptr().m = 0
  atomic.Store(&_g_.m.p.ptr().status, _Psyscall)
  if sched.gcwaiting != 0 {
    systemstack(entersyscall_gcwait)
    save(pc, sp)
  }

  // Goroutines must not split stacks in Gsyscall status (it would corrupt g->sched).
  // We set _StackGuard to StackPreempt so that first split stack check calls morestack.
  // Morestack detects this case and throws.
  _g_.stackguard0 = stackPreempt
  _g_.m.locks--
}

进入系统调用时，p 和 m 分离，当前运行的 g 状态变为 _Gsyscall。

_Gsyscall 恢复时机：

1. 当 m 执行完，调用 exitsyscall 重新和之前的 p 绑定，其中调度的还是 schedule 函数；
2. sysmon 线程，发现该 p 一定时间没有执行，会其分配一个新的 m。此时进入调度。

time 定时类操作

都拿 time.Sleep 举例

// Sleep pauses the current goroutine for at least the duration d.
// A negative or zero duration causes Sleep to return immediately.
func Sleep(d Duration)

实际定义在runtime
// timeSleep puts the current goroutine to sleep for at least ns nanoseconds.
//go:linkname timeSleep time.Sleep
func timeSleep(ns int64) {
    if ns <= 0 {
        return
    }

    t := getg().timer
    if t == nil {
        t = new(timer)
        getg().timer = t
    }
    *t = timer{}
    t.when = nanotime() + ns
    t.f = goroutineReady
    t.arg = getg()
    lock(&timers.lock)
    addtimerLocked(t)
    goparkunlock(&timers.lock, "sleep", traceEvGoSleep, 2)
}

goparkunlock 最终调用 gopark

func gopark(unlockf func(*g, unsafe.Pointer) bool, lock unsafe.Pointer, reason string, traceEv byte, traceskip int) {
  mp := acquirem()
  gp := mp.curg
  status := readgstatus(gp)
  if status != _Grunning && status != _Gscanrunning {
    throw("gopark: bad g status")
  }
  mp.waitlock = lock
  mp.waitunlockf = *(*unsafe.Pointer)(unsafe.Pointer(&unlockf))
  gp.waitreason = reason
  mp.waittraceev = traceEv
  mp.waittraceskip = traceskip
  releasem(mp)
  // can't do anything that might move the G between Ms here.
  mcall(park_m)
}

mcall(fn) 是切换到 g0，让 g0 来调用 fn，这里我们看下 park_m 定义 park_m

func park_m(gp *g) {mcall(park_m)
  _g_ := getg()

  if trace.enabled {
    traceGoPark(_g_.m.waittraceev, _g_.m.waittraceskip)
  }

  casgstatus(gp, _Grunning, _Gwaiting)
  dropg()

  if _g_.m.waitunlockf != nil {
    fn := *(*func(*g, unsafe.Pointer) bool)(unsafe.Pointer(&_g_.m.waitunlockf))
    ok := fn(gp, _g_.m.waitlock)
    _g_.m.waitunlockf = nil
    _g_.m.waitlock = nil
    if !ok {
      if trace.enabled {
        traceGoUnpark(gp, 2)
      }
      casgstatus(gp, _Gwaiting, _Grunnable)
      execute(gp, true) // Schedule it back, never returns.
    }
  }
  schedule()
}

可以看到，先把状态转化为 _Gwaiting, 再进行了一次 schedule 针对 _Gwaiting 的 g，需要调用 goready,才能恢复。

新起一个协程和退出

新开一个协程，g 状态会变为 _GIdle，触发调度。当协程执行完，会调用 goexit1 此时状态变为 _GDead，_Gdead 可以被复用，或者被 gc 清除。

管道阻塞

chansend 即 c <-channel 的实现

func chansend(c *hchan, ep unsafe.Pointer, block bool, callerpc uintptr) bool {
  if c == nil {
    if !block {
      returnfalse
    }
    gopark(nil, nil, "chan send (nil chan)", traceEvGoStop, 2)
    throw("unreachable")
  }

  if debugChan {
    print("chansend: chan=", c, "\n")
  }

  if raceenabled {
    racereadpc(unsafe.Pointer(c), callerpc, funcPC(chansend))
  }

    ........
    // 省略无关代码
    ........

  // Block on the channel. Some receiver will complete our operation for us.
  gp := getg()
  mysg := acquireSudog()
  mysg.releasetime = 0
  if t0 != 0 {
    mysg.releasetime = -1
  }
  // No stack splits between assigning elem and enqueuing mysg
  // on gp.waiting where copystack can find it.
  mysg.elem = ep
  mysg.waitlink = nil
  mysg.g = gp
  mysg.selectdone = nil
  mysg.c = c
  gp.waiting = mysg
  gp.param = nil
  c.sendq.enqueue(mysg)
  goparkunlock(&c.lock, "chan send", traceEvGoBlockSend, 3)

  // someone woke us up.
  if mysg != gp.waiting {
    throw("G waiting list is corrupted")
  }
  gp.waiting = nil
  if gp.param == nil {
    if c.closed == 0 {
      throw("chansend: spurious wakeup")
    }
    panic(plainError("send on closed channel"))
  }
  gp.param = nil
  if mysg.releasetime > 0 {
    blockevent(mysg.releasetime-t0, 2)
  }
  mysg.c = nil
  releaseSudog(mysg)
  returntrue
}

可以看到，实际还是调用 goparkunlock->gopark，来进行调度。

gc 之后

stw 之后，会重新选择 g 开始执行。此处不对垃圾回收做过多扩展。

主动调用 runtime.Gosched()

没有找到非要调用 runtime.Gosched 的场景，主要作用还是为了调试，学习 runtime 吧

// Gosched yields the processor, allowing other goroutines to run. It does not
// suspend the current goroutine, so execution resumes automatically.
//go:nosplit
func Gosched() {
  mcall(gosched_m)
}

第一步就将环境切换到 g0，然后执行一个叫 gosched_m 的函数

// Gosched continuation on g0.
func gosched_m(gp *g) {
  if trace.enabled {
    traceGoSched()
  }
  goschedImpl(gp)
}

func goschedImpl(gp *g) {
  status := readgstatus(gp)
  if status&^_Gscan != _Grunning {
    dumpgstatus(gp)
    throw("bad g status")
  }
  casgstatus(gp, _Grunning, _Grunnable)
  dropg()
  lock(&sched.lock)
  globrunqput(gp)
  unlock(&sched.lock)

  schedule()
}

可以看到，当前 g 被设置为 _Grunnable，放入执行队列。然后调用 schedule，选择一个合适的 g 进行执行。

总结

golang 协程调度时机主要是阻塞性操作开始，结束。研究每个场景相关代码，即可对 golang 有更深的理解。这里也分享一个阅读源码的小经验，每次带着一个特定问题去寻找答案，比如本文的调度时机，后面再看调度算法，垃圾回收，这样每次能忽略无关因素，通过多个不同的主题，整个框架会越来越完善。

参考文章

1. A complete journey with Goroutines: https://medium.com/@riteeksrivastava/a-complete-journey-with-goroutines-8472630c7f5c
2. Go's work-stealing scheduler: https://rakyll.org/scheduler/