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今天小编给大家分享一下Golang并发编程之main goroutine的创建与调度的方法是什么的相关知识点，内容详细，逻辑清晰，相信大部分人都还太了解这方面的知识，所以分享这篇文章给大家参考一下，希望大家阅读完这篇文章后有所收获，下面我们一起来了解一下吧。
接上文，在runtime/asm_amd64.s文件的runtime·rt0_go中，在执行完runtime.schedinit函数进行调度器的初始化后，就开始创建main goroutine了。
// create a new goroutine to start program
MOVQ $runtime·mainPC(SB), AX // entry // mainPC是runtime.main
PUSHQ AX // 将runtime.main函数地址入栈，作为参数
CALL runtime·newproc(SB) // 创建main goroutine，入参就是runtime.main
POPQ AX
以上代码创建了一个新的协程（在Go中，go func()之类的相当于调用runtime.newproc），这个协程就是main goroutine，那我们就看看runtime·newproc函数做了什么。
// Create a new g running fn.
// Put it on the queue of g's waiting to run.
// The compiler turns a go statement into a call to this.
func newproc(fn *funcval) {
gp := getg() // 获取正在运行的g，初始化时是m0.g0
pc := getcallerpc() // 返回的是调用newproc函数时由call指令压栈的函数的返回地址，即上面汇编语言的第5行`POPQ AX`这条指令的地址
systemstack(func() { // systemstack函数的作用是切换到系统栈来执行其参数函数，也就是`g0`栈，这里当然就是m0.g0，所以基本不需要做什么
newg := newproc1(fn, gp, pc)
_p_ := getg().m.p.ptr()
runqput(_p_, newg, true)
if mainStarted {
wakep()
}
})
}所以以上代码的重点就是调用newproc1函数进行协程的创建。
// Create a new g in state _Grunnable, starting at fn. callerpc is the
// address of the go statement that created this. The caller is responsible
// for adding the new g to the scheduler.
func newproc1(fn *funcval, callergp *g, callerpc uintptr) *g {
_g_ := getg() // _g_ = g0，即m0.g0
if fn == nil {
_g_.m.throwing = -1 // do not dump full stacks
throw("go of nil func value")
}
acquirem() // disable preemption because it can be holding p in a local var
_p_ := _g_.m.p.ptr()
newg := gfget(_p_) // 从本地的已经废弃的g列表中获取一个g先，此时才刚初始化，所以肯定返回nil
if newg == nil {
newg = malg(_StackMin) // new一个g的结构体对象，然后在堆上分配2k的栈大小，并设置stack和stackguard0/1
casgstatus(newg, _Gidle, _Gdead)
allgadd(newg) // publishes with a g->status of Gdead so GC scanner doesn't look at uninitialized stack.
}
if newg.stack.hi == 0 {
throw("newproc1: newg missing stack")
}
if readgstatus(newg) != _Gdead {
throw("newproc1: new g is not Gdead")
}
// 调整栈顶指针
totalSize := uintptr(4*goarch.PtrSize + sys.MinFrameSize) // extra space in case of reads slightly beyond frame
totalSize = alignUp(totalSize, sys.StackAlign)
sp := newg.stack.hi - totalSize
spArg := sp
if usesLR {
// caller's LR
*(*uintptr)(unsafe.Pointer(sp)) = 0
prepGoExitFrame(sp)
spArg += sys.MinFrameSize
}
...
}上述代码从堆上分配了一个g的结构体，并且在堆上为其分配了一个2k大小的栈，并设置了好了newg的stack等相关参数。此时，newg的状态如图所示：
接着我们继续分析newproc1函数：
memclrNoHeapPointers(unsafe.Pointer(&newg.sched), unsafe.Sizeof(newg.sched))
newg.sched.sp = sp // 设置newg的栈顶
newg.stktopsp = sp
// newg.sched.pc表示当newg运行起来时的运行起始位置，下面一段是类似于代码注入，就好像每个go func()
// 函数都是由goexit函数引起的一样，以便后面当newg结束后，
// 完成newg的回收（当然这里main goroutine结束后进程就结束了，不会被回收）。
newg.sched.pc = abi.FuncPCABI0(goexit) + sys.PCQuantum // +PCQuantum so that previous instruction is in same function
newg.sched.g = guintptr(unsafe.Pointer(newg))
gostartcallfn(&newg.sched, fn) // 调整sched成员和newg的栈
newg.gopc = callerpc
newg.ancestors = saveAncestors(callergp)
newg.startpc = fn.fn
if isSystemGoroutine(newg, false) {
atomic.Xadd(&sched.ngsys, +1)
} else {
// Only user goroutines inherit pprof labels.
if _g_.m.curg != nil {
newg.labels = _g_.m.curg.labels
}
}以上代码对newg的sched成员进行初始化，其中newg.sched.sp表示其被调度起来后应该使用的栈顶，newg.sched.pc表示其被调度起来从这个地址开始运行，但是这个值被设置成了goexit函数的下一条指令，所以我们看看，在gostartcallfn函数中，到底做了什么才能实现此功能：
// adjust Gobuf as if it executed a call to fn
// and then stopped before the first instruction in fn.
func gostartcallfn(gobuf *gobuf, fv *funcval) {
var fn unsafe.Pointer
if fv != nil {
fn = unsafe.Pointer(fv.fn)
} else {
fn = unsafe.Pointer(abi.FuncPCABIInternal(nilfunc))
}
gostartcall(gobuf, fn, unsafe.Pointer(fv))
}
// sys_x86.go
// adjust Gobuf as if it executed a call to fn with context ctxt
// and then stopped before the first instruction in fn.
func gostartcall(buf *gobuf, fn, ctxt unsafe.Pointer) {
sp := buf.sp
sp -= goarch.PtrSize
*(*uintptr)(unsafe.Pointer(sp)) = buf.pc // 插入goexit的第二条指令，返回时可以调用
buf.sp = sp
buf.pc = uintptr(fn) // 此时才是真正地设置pc
buf.ctxt = ctxt
}以上操作的目的就是：
调整newg的栈空间，把goexit函数的第二条指令的地址入栈，伪造成goexit函数调用了fn，从而使fn执行完成后执行ret指令时返回到goexit继续执行完成最后的清理工作；
重新设置newg.buf.pc 为需要执行的函数的地址，即fn，此场景为runtime.main函数的地址。
接下来会设置newg的状态为runnable；最后别忘了newproc函数中还有几行：
newg := newproc1(fn, gp, pc)
_p_ := getg().m.p.ptr()
runqput(_p_, newg, true)
if mainStarted {
wakep()
}在创建完newg后，将其放到此线程的g0（这里是m0.g0）所在的runq队列，并且优先插入到队列的前端（runqput第三个参数为true），做完这些后，我们可以得出以下的关系：
上一节我们分析了main goroutine的创建过程，这一节我们讨论一下，调度器如何把main goroutine调度到CPU上去运行。让我们继续回到runtime/asm_amd64.s中，在完成runtime.newproc创建完main goroutine之后，正式执行runtime·mstart来执行，而runtime·mstart最终会调用go写的runtime·mstart0函数。
// start this M
CALL runtime·mstart(SB)
CALL runtime·abort(SB) // mstart should never return
RET
TEXT runtime·mstart(SB),NOSPLIT|TOPFRAME,$0
CALL runtime·mstart0(SB)
RET // not reached
runtime·mstart0函数如下：
func mstart0() {
_g_ := getg() // _g_ = &g0
osStack := _g_.stack.lo == 0
if osStack { // g0的stack.lo已经初始化，所以不会走以下逻辑
// Initialize stack bounds from system stack.
// Cgo may have left stack size in stack.hi.
// minit may update the stack bounds.
//
// Note: these bounds may not be very accurate.
// We set hi to &size, but there are things above
// it. The 1024 is supposed to compensate this,
// but is somewhat arbitrary.
size := _g_.stack.hi
if size == 0 {
size = 8192 * sys.StackGuardMultiplier
}
_g_.stack.hi = uintptr(noescape(unsafe.Pointer(&size)))
_g_.stack.lo = _g_.stack.hi - size + 1024
}
// Initialize stack guard so that we can start calling regular
// Go code.
_g_.stackguard0 = _g_.stack.lo + _StackGuard
// This is the g0, so we can also call go:systemstack
// functions, which check stackguard1.
_g_.stackguard1 = _g_.stackguard0
mstart1()
// Exit this thread.
if mStackIsSystemAllocated() {
// Windows, Solaris, illumos, Darwin, AIX and Plan 9 always system-allocate
// the stack, but put it in _g_.stack before mstart,
// so the logic above hasn't set osStack yet.
osStack = true
}
mexit(osStack)
}以上代码设置了一些栈信息之后，调用runtime.mstart1函数：
func mstart1() {
_g_ := getg() // _g_ = &g0
if _g_ != _g_.m.g0 { // _g_ = &g0
throw("bad runtime·mstart")
}
// Set up m.g0.sched as a label returning to just
// after the mstart1 call in mstart0 above, for use by goexit0 and mcall.
// We're never coming back to mstart1 after we call schedule,
// so other calls can reuse the current frame.
// And goexit0 does a gogo that needs to return from mstart1
// and let mstart0 exit the thread.
_g_.sched.g = guintptr(unsafe.Pointer(_g_))
_g_.sched.pc = getcallerpc() // getcallerpc()获取mstart1执行完的返回地址
_g_.sched.sp = getcallersp() // getcallersp()获取调用mstart1时的栈顶地址
asminit()
minit() // 信号相关初始化
// Install signal handlers; after minit so that minit can
// prepare the thread to be able to handle the signals.
if _g_.m == &m0 {
mstartm0()
}
if fn := _g_.m.mstartfn; fn != nil {
fn()
}
if _g_.m != &m0 {
acquirep(_g_.m.nextp.ptr())
_g_.m.nextp = 0
}
schedule()
}可以看到mstart1函数保存额调度相关的信息，特别是保存了正在运行的g0的下一条指令和栈顶地址， 这些调度信息对于goroutine而言是很重要的。
接下来就是golang调度系统的核心函数runtime.schedule了：
func schedule() {
_g_ := getg() // _g_ 是每个工作线程的m的m0，在初始化的场景就是m0.g0
...
var gp *g
var inheritTime bool
...
if gp == nil {
// 为了保证调度的公平性，每进行61次调度就需要优先从全局队列中获取goroutine
// 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 _g_.m.p.ptr().schedtick%61 == 0 && sched.runqsize > 0 {
lock(&sched.lock)
gp = globrunqget(_g_.m.p.ptr(), 1)
unlock(&sched.lock)
}
}
if gp == nil { // 从p本地的队列中获取goroutine
gp, inheritTime = runqget(_g_.m.p.ptr())
// We can see gp != nil here even if the M is spinning,
// if checkTimers added a local goroutine via goready.
}
if gp == nil { // 如果以上两者都没有，那么就需要从其他p哪里窃取goroutine
gp, inheritTime = findrunnable() // blocks until work is available
}
...
execute(gp, inheritTime)
}以上我们节选了一些和调度相关的代码，意图简化我们的理解，调度中获取goroutine的规则是：
每调度61次就需要从全局队列中获取goroutine；
其次优先从本P所在队列中获取goroutine；
如果还没有获取到，则从其他P的运行队列中窃取goroutine；
最后调用runtime.excute函数运行代码：
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) // 设置gp的状态
gp.waitsince = 0
gp.preempt = false
gp.stackguard0 = gp.stack.lo + _StackGuard
...
gogo(&gp.sched)
}在完成gp运行前的准备工作后，excute函数调用gogo函数完成从g0到gp的转换：
让出CPU的执行权；
栈的切换；
gogo函数是用汇编语言编写的精悍的一段代码，这里就不详细分析了，其主要做了两件事：
把gp.sched的成员恢复到CPU的寄存器完成状态以及栈的切换；
跳转到gp.sched.pc所指的指令地址（runtime.main）处执行。
func main() {
g := getg() // _g_ = main_goroutine
// Racectx of m0->g0 is used only as the parent of the main goroutine.
// It must not be used for anything else.
g.m.g0.racectx = 0
// golang栈的最大值
// Max stack size is 1 GB on 64-bit, 250 MB on 32-bit.
// Using decimal instead of binary GB and MB because
// they look nicer in the stack overflow failure message.
if goarch.PtrSize == 8 {
maxstacksize = 1000000000
} else {
maxstacksize = 250000000
}
// An upper limit for max stack size. Used to avoid random crashes
// after calling SetMaxStack and trying to allocate a stack that is too big,
// since stackalloc works with 32-bit sizes.
maxstackceiling = 2 * maxstacksize
// Allow newproc to start new Ms.
mainStarted = true
// 需要切换到g0栈去执行newm
// 创建监控线程，该线程独立于调度器，无需与P关联
if GOARCH != "wasm" { // no threads on wasm yet, so no sysmon
systemstack(func() {
newm(sysmon, nil, -1)
})
}
// Lock the main goroutine onto this, the main OS thread,
// during initialization. Most programs won't care, but a few
// do require certain calls to be made by the main thread.
// Those can arrange for main.main to run in the main thread
// by calling runtime.LockOSThread during initialization
// to preserve the lock.
lockOSThread()
if g.m != &m0 {
throw("runtime.main not on m0")
}
// Record when the world started.
// Must be before doInit for tracing init.
runtimeInitTime = nanotime()
if runtimeInitTime == 0 {
throw("nanotime returning zero")
}
if debug.inittrace != 0 {
inittrace.id = getg().goid
inittrace.active = true
}
// runtime包的init
doInit(&runtime_inittask) // Must be before defer.
// Defer unlock so that runtime.Goexit during init does the unlock too.
needUnlock := true
defer func() {
if needUnlock {
unlockOSThread()
}
}()
gcenable()
main_init_done = make(chan bool)
if iscgo {
if _cgo_thread_start == nil {
throw("_cgo_thread_start missing")
}
if GOOS != "windows" {
if _cgo_setenv == nil {
throw("_cgo_setenv missing")
}
if _cgo_unsetenv == nil {
throw("_cgo_unsetenv missing")
}
}
if _cgo_notify_runtime_init_done == nil {
throw("_cgo_notify_runtime_init_done missing")
}
// Start the template thread in case we enter Go from
// a C-created thread and need to create a new thread.
startTemplateThread()
cgocall(_cgo_notify_runtime_init_done, nil)
}
doInit(&main_inittask) // main包的init，会递归调用import的包的初始化函数
// Disable init tracing after main init done to avoid overhead
// of collecting statistics in malloc and newproc
inittrace.active = false
close(main_init_done)
needUnlock = false
unlockOSThread()
if isarchive || islibrary {
// A program compiled with -buildmode=c-archive or c-shared
// has a main, but it is not executed.
return
}
fn := main_main // make an indirect call, as the linker doesn't know the address of the main package when laying down the runtime
fn() // 执行main函数
if raceenabled {
racefini()
}
// Make racy client program work: if panicking on
// another goroutine at the same time as main returns,
// let the other goroutine finish printing the panic trace.
// Once it does, it will exit. See issues 3934 and 20018.
if atomic.Load(&runningPanicDefers) != 0 {
// Running deferred functions should not take long.
for c := 0; c < 1000; c++ {
if atomic.Load(&runningPanicDefers) == 0 {
break
}
Gosched()
}
}
if atomic.Load(&panicking) != 0 {
gopark(nil, nil, waitReasonPanicWait, traceEvGoStop, 1)
}
exit(0)
for {
var x *int32
*x = 0
}
}runtime.main函数的主要工作是：
启动一个sysmon系统监控线程，该线程负责程序的gc、抢占调度等；
执行runtime包和所有包的初始化；
执行main.main函数；
最后调用exit系统调用退出进程，之前提到的注入goexit程序对main goroutine不起作用，是为了其他线程的回收而做的。
以上就是“Golang并发编程之main goroutine的创建与调度的方法是什么”这篇文章的所有内容，感谢各位的阅读！相信大家阅读完这篇文章都有很大的收获，小编每天都会为大家更新不同的知识，如果还想学习更多的知识，请关注亿速云行业资讯频道。
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