Golang 面试驱动学习

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	// Check the global runnable queue once in a while to ensure fairness.
	// Otherwise two goroutines can completely occupy the local runqueue
	// by constantly respawning each other.
	if _p_.schedtick%61 == 0 && sched.runqsize > 0 {
		lock(&sched.lock)
		gp = globrunqget(_p_, 1)
		unlock(&sched.lock)
		if gp != nil {
			return gp, false, false
		}
	}

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	// local runq
	if gp, inheritTime := runqget(_p_); gp != nil {
		return gp, inheritTime, false
	}

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	// global runq
	if sched.runqsize != 0 {
		lock(&sched.lock)
		gp := globrunqget(_p_, 0)
		unlock(&sched.lock)
		if gp != nil {
			return gp, false, false
		}
	}

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	// Poll network.
	// This netpoll is only an optimization before we resort to stealing.
	// We can safely skip it if there are no waiters or a thread is blocked
	// in netpoll already. If there is any kind of logical race with that
	// blocked thread (e.g. it has already returned from netpoll, but does
	// not set lastpoll yet), this thread will do blocking netpoll below
	// anyway.
	if netpollinited() && atomic.Load(&netpollWaiters) > 0 && atomic.Load64(&sched.lastpoll) != 0 {
		if list := netpoll(0); !list.empty() { // non-blocking
			gp := list.pop()
			injectglist(&list)
			casgstatus(gp, _Gwaiting, _Grunnable)
			if trace.enabled {
				traceGoUnpark(gp, 0)
			}
			return gp, false, false
		}
	}

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	// Spinning Ms: steal work from other Ps.
	//
	// Limit the number of spinning Ms to half the number of busy Ps.
	// This is necessary to prevent excessive CPU consumption when
	// GOMAXPROCS>>1 but the program parallelism is low.
	procs := uint32(gomaxprocs)
	if _g_.m.spinning || 2*atomic.Load(&sched.nmspinning) < procs-atomic.Load(&sched.npidle) {
		if !_g_.m.spinning {
			_g_.m.spinning = true
			atomic.Xadd(&sched.nmspinning, 1)
		}

		gp, inheritTime, tnow, w, newWork := stealWork(now)
		now = tnow
		if gp != nil {
			// Successfully stole.
			return gp, inheritTime, false
		}
		if newWork {
			// There may be new timer or GC work; restart to
			// discover.
			goto top
		}
		if w != 0 && (pollUntil == 0 || w < pollUntil) {
			// Earlier timer to wait for.
			pollUntil = w
		}
	}

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	// If we're in the GC mark phase, can safely scan and blacken objects,
	// and have work to do, run idle-time marking rather than give up the P.
	if gcBlackenEnabled != 0 && gcMarkWorkAvailable(_p_) && gcController.addIdleMarkWorker() {
		node := (*gcBgMarkWorkerNode)(gcBgMarkWorkerPool.pop())
		if node != nil {
			_p_.gcMarkWorkerMode = gcMarkWorkerIdleMode
			gp := node.gp.ptr()
			casgstatus(gp, _Gwaiting, _Grunnable)
			if trace.enabled {
				traceGoUnpark(gp, 0)
			}
			return gp, false, false
		}
		gcController.removeIdleMarkWorker()
	}

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	// return P and block
	lock(&sched.lock)
	if sched.gcwaiting != 0 || _p_.runSafePointFn != 0 {
		unlock(&sched.lock)
		goto top
	}
	if sched.runqsize != 0 {
		gp := globrunqget(_p_, 0)
		unlock(&sched.lock)
		return gp, false, false
	}
	if releasep() != _p_ {
		throw("findrunnable: wrong p")
	}
	pidleput(_p_)
	unlock(&sched.lock)

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		// Check all runqueues once again.
		_p_ = checkRunqsNoP(allpSnapshot, idlepMaskSnapshot)
		if _p_ != nil {
			acquirep(_p_)
			_g_.m.spinning = true
			atomic.Xadd(&sched.nmspinning, 1)
			goto top
		}

		// Check for idle-priority GC work again.
		_p_, gp = checkIdleGCNoP()
		if _p_ != nil {
			acquirep(_p_)
			_g_.m.spinning = true
			atomic.Xadd(&sched.nmspinning, 1)

			// Run the idle worker.
			_p_.gcMarkWorkerMode = gcMarkWorkerIdleMode
			casgstatus(gp, _Gwaiting, _Grunnable)
			if trace.enabled {
				traceGoUnpark(gp, 0)
			}
			return gp, false, false
		}

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		list := netpoll(delay) // block until new work is available
		atomic.Store64(&sched.pollUntil, 0)
		atomic.Store64(&sched.lastpoll, uint64(nanotime()))
		if faketime != 0 && list.empty() {
			// Using fake time and nothing is ready; stop M.
			// When all M's stop, checkdead will call timejump.
			stopm()
			goto top
		}
		lock(&sched.lock)
		_p_ = pidleget()
		unlock(&sched.lock)
		if _p_ == nil {
			injectglist(&list)
		} else {
			acquirep(_p_)
			if !list.empty() {
				gp := list.pop()
				injectglist(&list)
				casgstatus(gp, _Gwaiting, _Grunnable)
				if trace.enabled {
					traceGoUnpark(gp, 0)
				}
				return gp, false, false
			}

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// 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()

	// Assign gp.m before entering _Grunning so running Gs have an
	// M.
	_g_.m.curg = gp
	gp.m = _g_.m
	casgstatus(gp, _Grunnable, _Grunning)
	gp.waitsince = 0
	gp.preempt = false
	gp.stackguard0 = gp.stack.lo + _StackGuard
	if !inheritTime {
		_g_.m.p.ptr().schedtick++
	}

	// 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)
}

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// func gogo(buf *gobuf)
// restore state from Gobuf; longjmp
TEXT runtime·gogo(SB), NOSPLIT, $0-8
	MOVQ	buf+0(FP), BX		// gobuf
	MOVQ	gobuf_g(BX), DX
	MOVQ	0(DX), CX		// make sure g != nil
	JMP	gogo<>(SB)

TEXT gogo<>(SB), NOSPLIT, $0
	get_tls(CX)
	MOVQ	DX, g(CX)
	MOVQ	DX, R14		// set the g register
	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 // 获取待执行任务函数的程序计数器
	JMP	BX // 执行传入 goroutine 的任务代码

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// The top-most function running on a goroutine
// returns to goexit+PCQuantum.
TEXT runtime·goexit(SB),NOSPLIT|TOPFRAME,$0-0
	BYTE	$0x90	// NOP
	CALL	runtime·goexit1(SB)	// does not return
	// traceback from goexit1 must hit code range of goexit
	BYTE	$0x90	// NOP

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// Finishes execution of the current goroutine.
func goexit1() {
	if raceenabled {
		racegoend()
	}
	if trace.enabled {
		traceGoEnd()
	}
	mcall(goexit0)
}

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func goexit0(gp *g) {
	_g_ := getg()
	_p_ := _g_.m.p.ptr()

	casgstatus(gp, _Grunning, _Gdead) // Goroutine 状态转移
	gcController.addScannableStack(_p_, -int64(gp.stack.hi-gp.stack.lo))
	if isSystemGoroutine(gp, false) {
		atomic.Xadd(&sched.ngsys, -1)
	}
	gp.m = nil
	locked := gp.lockedm != 0
	gp.lockedm = 0
	_g_.m.lockedg = 0
	gp.preemptStop = false
	gp.paniconfault = false
	gp._defer = nil // should be true already but just in case.
	gp._panic = nil // non-nil for Goexit during panic. points at stack-allocated data.
	gp.writebuf = nil
	gp.waitreason = 0
	gp.param = nil
	gp.labels = nil
	gp.timer = nil

	if gcBlackenEnabled != 0 && gp.gcAssistBytes > 0 {
		// Flush assist credit to the global pool. This gives
		// better information to pacing if the application is
		// rapidly creating an exiting goroutines.
		assistWorkPerByte := gcController.assistWorkPerByte.Load()
		scanCredit := int64(assistWorkPerByte * float64(gp.gcAssistBytes))
		atomic.Xaddint64(&gcController.bgScanCredit, scanCredit)
		gp.gcAssistBytes = 0
	}

	dropg() // 移除当前 Goroutine 与线程的关联

	if GOARCH == "wasm" { // no threads yet on wasm
		gfput(_p_, gp)
		schedule() // never returns
	}

	if _g_.m.lockedInt != 0 {
		print("invalid m->lockedInt = ", _g_.m.lockedInt, "\n")
		throw("internal lockOSThread error")
	}
	gfput(_p_, gp) // 将 Goroutine 键入处理器的 Goroutine 空闲列表 gFree
	if locked {
		// The goroutine may have locked this thread because
		// it put it in an unusual kernel state. Kill it
		// rather than returning it to the thread pool.

		// Return to mstart, which will release the P and exit
		// the thread.
		if GOOS != "plan9" { // See golang.org/issue/22227.
			gogo(&_g_.m.g0.sched)
		} else {
			// Clear lockedExt on plan9 since we may end up re-using
			// this thread.
			_g_.m.lockedExt = 0
		}
	}
	schedule() // 重新触发新一轮的调度循环
}