浅析go语言如何实现协程的抢占式调度的

func foo1() {
 fmt.Println("foo1调用foo2")
 foo2()
}

func foo2() {
 fmt.Println("foo2调用foo3")
 foo3()
}

func foo3() {
 fmt.Println("foo3")
}

func main() {
 //设置WaitGroup等待协程运行结束
 var wg sync.WaitGroup
 wg.Add(1)
 //通过协程调用foo1
 go func() {
  defer wg.Done()
  foo1()
 }()
 //等待协程运行结束
 wg.Wait()
}

go build -gcflags -S main.go

TEXT runtime·morestack(SB),NOSPLIT,$0-0
 // Cannot grow scheduler stack (m->g0).
 get_tls(CX)
 MOVQ g(CX), BX
 MOVQ g_m(BX), BX
 MOVQ m_g0(BX), SI
 CMPQ g(CX), SI
 JNE 3(PC)
 CALL runtime·badmorestackg0(SB)
 CALL runtime·abort(SB)

    //......
    //函数调用前会调用newstack进行抢占式的检查
 CALL runtime·newstack(SB)
 CALL runtime·abort(SB) // crash if newstack returns
 RET

func newstack() {
 preempt := stackguard0 == stackPreempt


 //如果preempt 为true，则直接当前协程被标记为抢占直接调用gopreempt_m让出线程执行权
 if preempt {
  if gp == thisg.m.g0 {
   throw("runtime: preempt g0")
  }
  //......

  // Act like goroutine called runtime.Gosched.
  gopreempt_m(gp) // never return
 }
}

func sighandler(sig uint32, info *siginfo, ctxt unsafe.Pointer, gp *g) {
 
 //如果传入的信号为_SIGURG则调用doSigPreempt回到schedule实现抢占式调度
 if sig == sigPreempt && debug.asyncpreemptoff == 0 && !delayedSignal {
  // Might be a preemption signal.
  doSigPreempt(gp, c)
  
 }
 //......
}

func doSigPreempt(gp *g, ctxt *sigctxt) {
 //......
 if wantAsyncPreempt(gp) {
  if ok, newpc := isAsyncSafePoint(gp, ctxt.sigpc(), ctxt.sigsp(), ctxt.siglr()); ok {
   // 调用asyncPreempt内部会得到一个和上文函数调用时抢占式调度的方法gopreempt_m的调用从而回到schedule方法
   ctxt.pushCall(abi.FuncPCABI0(asyncPreempt), newpc)
  }
 }

 //......
}