Question

如果 MP 没找到可执行任务，那么就会进入闲置状态。

// proc.go

func schedule() {
    ...
    
top:
    
    // 尝试 n 种任务查找方式...
    
	if gp == nil {
		gp, inheritTime = findrunnable() // blocks until work is available
	}

	execute(gp, inheritTime)
}

// Finds a runnable goroutine to execute.
// Tries to steal from other P's, get g from local or global queue, poll network.

func findrunnable() (gp *g, inheritTime bool) {

top:

	// local runq ...
	// global runq ...
	// poll network ...
	// steal work from other Ps ...


	// 解除关联 P，放入空闲列表。
	if releasep() != _p_ {
		throw("findrunnable: wrong p")
	}
	pidleput(_p_)

    // 当前 M 休眠。放入闲置列表，直到再次启用。
	stopm()
    
	goto top
}

// Stops execution of the current m until new work is available.
func stopm() {
	_g_ := getg()
    
    // 放入闲置列表。
	mput(_g_.m)
	
    // 休眠。
	mPark()
    
    // 关联由唤醒者提供的 P。
	acquirep(_g_.m.nextp.ptr())
	_g_.m.nextp = 0
}

调度器使用专门的链表管理闲置 M。

// runtime2.go

type schedt struct {
	midle        muintptr // idle m's waiting for work
	nmidle       int32    // number of idle m's waiting for work    
}

// proc.go

// Put mp on midle list.
func mput(mp *m) {
	mp.schedlink = sched.midle
	sched.midle.set(mp)
	sched.nmidle++
}

// Try to get an m from midle list.
func mget() *m {
	mp := sched.midle.ptr()
	if mp != nil {
		sched.midle = mp.schedlink
		sched.nmidle--
	}
	return mp
}