Goroutine怎么主动让出权力？

https://golang.org/pkg/runtime/#Gosched

Goroutine上下文切换的时候会发生什么？

跟进去，看 Gosched 的源码：

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

mcall:

// mcall switches from the g to the g0 stack and invokes fn(g),
// where g is the goroutine that made the call.
// mcall saves g's current PC/SP in g->sched so that it can be restored later.
// It is up to fn to arrange for that later execution, typically by recording
// g in a data structure, causing something to call ready(g) later.
// mcall returns to the original goroutine g later, when g has been rescheduled.
// fn must not return at all; typically it ends by calling schedule, to let the m
// run other goroutines.
//
// mcall can only be called from g stacks (not g0, not gsignal).
//
// This must NOT be go:noescape: if fn is a stack-allocated closure,
// fn puts g on a run queue, and g executes before fn returns, the
// closure will be invalidated while it is still executing.
func mcall(fn func(*g))

发现mcall的作用是从g切到g0，然后执行fn(g)。这篇文章 里说过，g0是绑定在m上的一个g，使用系统栈。

我们接下来跟 gosched_m:

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

然后是 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，让出权力的过程是：

• 读取当前g的状态，将状态从 _Grunning 切换成 _Grunnable
• 解除当前g和m的关系
• 锁定全局调度器
• 将这个g丢到全局g队列去
• 解锁全局调度器
• 调用 schedule 去寻找可执行的g

关于 schedule 的分析，看 这篇文章

g切换的时候，要做哪些事情？

如果你跟进了 schedule，会发现，找到了g之后，会执行 execute 函数：

// Schedules gp to run on the current M.
// If inheritTime is true, gp inherits the remaining time in the
// current time slice. Otherwise, it starts a new time slice.
// Never returns.
//
// Write barriers are allowed because this is called immediately after
// acquiring a P in several places.
//
//go:yeswritebarrierrec
func execute(gp *g, inheritTime bool) {
	_g_ := getg()

	casgstatus(gp, _Grunnable, _Grunning)
	gp.waitsince = 0
	gp.preempt = false
	gp.stackguard0 = gp.stack.lo + _StackGuard
	if !inheritTime {
		_g_.m.p.ptr().schedtick++
	}
	_g_.m.curg = gp
	gp.m = _g_.m

	// Check whether the profiler needs to be turned on or off.
	hz := sched.profilehz
	if _g_.m.profilehz != hz {
		setThreadCPUProfiler(hz)
	}

	if trace.enabled {
		// GoSysExit has to happen when we have a P, but before GoStart.
		// So we emit it here.
		if gp.syscallsp != 0 && gp.sysblocktraced {
			traceGoSysExit(gp.sysexitticks)
		}
		traceGoStart()
	}

	gogo(&gp.sched)
}

然后继续跟进 gogo:

func gogo(buf *gobuf)

发现是汇编写的，那我们搜索一下，然后跳到 amd64 版本的：

// void gogo(Gobuf*)
// restore state from Gobuf; longjmp
TEXT runtime·gogo(SB), NOSPLIT, $16-8
	MOVQ	buf+0(FP), BX		// gobuf
	MOVQ	gobuf_g(BX), DX
	MOVQ	0(DX), CX		// make sure g != nil
	get_tls(CX)
	MOVQ	DX, g(CX)
	MOVQ	gobuf_sp(BX), SP	// restore SP
	MOVQ	gobuf_ret(BX), AX
	MOVQ	gobuf_ctxt(BX), DX
	MOVQ	gobuf_bp(BX), BP
	MOVQ	$0, gobuf_sp(BX)	// clear to help garbage collector
	MOVQ	$0, gobuf_ret(BX)
	MOVQ	$0, gobuf_ctxt(BX)
	MOVQ	$0, gobuf_bp(BX)
	MOVQ	gobuf_pc(BX), BX
	JMP	BX

即把对应的寄存器的值刷成要执行的g的值，如SP，PC等。可以看看 gobuf 是啥：

type gobuf struct {
	// The offsets of sp, pc, and g are known to (hard-coded in) libmach.
	//
	// ctxt is unusual with respect to GC: it may be a
	// heap-allocated funcval, so GC needs to track it, but it
	// needs to be set and cleared from assembly, where it's
	// difficult to have write barriers. However, ctxt is really a
	// saved, live register, and we only ever exchange it between
	// the real register and the gobuf. Hence, we treat it as a
	// root during stack scanning, which means assembly that saves
	// and restores it doesn't need write barriers. It's still
	// typed as a pointer so that any other writes from Go get
	// write barriers.
	sp   uintptr
	pc   uintptr
	g    guintptr
	ctxt unsafe.Pointer
	ret  sys.Uintreg
	lr   uintptr
	bp   uintptr // for GOEXPERIMENT=framepointer
}

其实就是goroutine切换的时候要换的东西。

参考资料：

• Go的源码: https://github.com/golang/go
• Go的自定义汇编: https://golang.org/doc/asm