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JulianneRit
3年前发布

Go -调度器

   <p>写得稍微有点乱,主要是按自己看代码的顺序来记录的,也不是出书,就这样吧。</p>    <p>PS: 新人不推荐刚学 Golang 就去看调度器代码,这部分代码个人觉得写得很乱。</p>    <h2>调度</h2>    <h2>基本数据结构</h2>    <p>goroutine 在 runtime 中的数据结构:</p>    <pre>  <code class="language-go">// stack 描述的是 Go 的执行栈,下界和上界分别为 [lo, hi]  // 如果从传统内存布局的角度来讲,Go 的栈实际上是分配在 C 语言中的堆区的  // 所以才能比 ulimit -s 的 stack size 还要大(1GB)  type stack struct {      lo uintptr      hi uintptr  }    // g 的运行现场  type gobuf struct {      sp   uintptr    // sp 寄存器      pc   uintptr    // pc 寄存器      g    guintptr   // g 指针      ctxt unsafe.Pointer // 这个似乎是用来辅助 gc 的      ret  sys.Uintreg      lr   uintptr    // 这是在 arm 上用的寄存器,不用关心      bp   uintptr    // 开启 GOEXPERIMENT=framepointer,才会有这个  }      type g struct {      // 简单数据结构,lo 和 hi 成员描述了栈的下界和上界内存地址      stack       stack      // 在函数的栈增长 prologue 中用 sp 寄存器和 stackguard0 来做比较      // 如果 sp 比 stackguard0 小(因为栈向低地址方向增长),那么就触发栈拷贝和调度      // 正常情况下 stackguard0 = stack.lo + StackGuard      // 不过 stackguard0 在需要进行调度时,会被修改为 StackPreempt      // 以触发抢占s      stackguard0 uintptr      // stackguard1 是在 C 栈增长 prologue 作对比的对象      // 在 g0 和 gsignal 栈上,其值为 stack.lo+StackGuard      // 在其它的栈上这个值是 ~0(按 0 取反)以触发 morestack 调用(并 crash)      stackguard1 uintptr        _panic         *_panic      _defer         *_defer      m              *m             // 当前与 g 绑定的 m      sched          gobuf          // goroutine 的现场      syscallsp      uintptr        // if status==Gsyscall, syscallsp = sched.sp to use during gc      syscallpc      uintptr        // if status==Gsyscall, syscallpc = sched.pc to use during gc      stktopsp       uintptr        // expected sp at top of stack, to check in traceback      param          unsafe.Pointer // wakeup 时的传入参数      atomicstatus   uint32      stackLock      uint32 // sigprof/scang lock; TODO: fold in to atomicstatus      goid           int64  // goroutine id      waitsince      int64  // g 被阻塞之后的近似时间      waitreason     string // if status==Gwaiting      schedlink      guintptr      preempt        bool     // 抢占标记,这个为 true 时,stackguard0 是等于 stackpreempt 的      throwsplit     bool     // must not split stack      raceignore     int8     // ignore race detection events      sysblocktraced bool     // StartTrace has emitted EvGoInSyscall about this goroutine      sysexitticks   int64    // syscall 返回之后的 cputicks,用来做 tracing      traceseq       uint64   // trace event sequencer      tracelastp     puintptr // last P emitted an event for this goroutine      lockedm        muintptr // 如果调用了 LockOsThread,那么这个 g 会绑定到某个 m 上      sig            uint32      writebuf       []byte      sigcode0       uintptr      sigcode1       uintptr      sigpc          uintptr      gopc           uintptr // 创建该 goroutine 的语句的指令地址      startpc        uintptr // goroutine 函数的指令地址      racectx        uintptr      waiting        *sudog         // sudog structures this g is waiting on (that have a valid elem ptr); in lock order      cgoCtxt        []uintptr      // cgo traceback context      labels         unsafe.Pointer // profiler labels      timer          *timer         // time.Sleep 缓存的定时器      selectDone     uint32         // 该 g 是否正在参与 select,是否已经有人从 select 中胜出  }</code></pre>    <p>当 g 遇到阻塞,或需要等待的场景时,会被打包成 sudog 这样一个结构。一个 g 可能被打包为多个 sudog 分别挂在不同的等待队列上:</p>    <pre>  <code class="language-go">// sudog 代表在等待列表里的 g,比如向 channel 发送/接收内容时  // 之所以需要 sudog 是因为 g 和同步对象之间的关系是多对多的  // 一个 g 可能会在多个等待队列中,所以一个 g 可能被打包为多个 sudog  // 多个 g 也可以等待在同一个同步对象上  // 因此对于一个同步对象就会有很多 sudog 了  // sudog 是从一个特殊的池中进行分配的。用 acquireSudog 和 releaseSudog 来分配和释放 sudog  type sudog struct {        // 之后的这些字段都是被该 g 所挂在的 channel 中的 hchan.lock 来保护的      // shrinkstack depends on      // this for sudogs involved in channel ops.      g *g        // isSelect 表示一个 g 是否正在参与 select 操作      // 所以 g.selectDone 必须用 CAS 来操作,以胜出唤醒的竞争      isSelect bool      next     *sudog      prev     *sudog      elem     unsafe.Pointer // data element (may point to stack)        // 下面这些字段则永远都不会被并发访问      // 对于 channel 来说,waitlink 只会被 g 访问      // 对于信号量来说,所有的字段,包括上面的那些字段都只在持有 semaRoot 锁时才可以访问      acquiretime int64      releasetime int64      ticket      uint32      parent      *sudog // semaRoot binary tree      waitlink    *sudog // g.waiting list or semaRoot      waittail    *sudog // semaRoot      c           *hchan // channel  }</code></pre>    <p>线程在 runtime 中的结构,对应一个 pthread,pthread 也会对应唯一的内核线程(task_struct):</p>    <pre>  <code class="language-go">type m struct {      g0      *g     // 用来执行调度指令的 goroutine      morebuf gobuf  // gobuf arg to morestack      divmod  uint32 // div/mod denominator for arm - known to liblink        // Fields not known to debuggers.      procid        uint64       // for debuggers, but offset not hard-coded      gsignal       *g           // signal-handling g      goSigStack    gsignalStack // Go-allocated signal handling stack      sigmask       sigset       // storage for saved signal mask      tls           [6]uintptr   // thread-local storage (for x86 extern register)      mstartfn      func()      curg          *g       // 当前运行的用户 goroutine      caughtsig     guintptr // goroutine running during fatal signal      p             puintptr // attached p for executing go code (nil if not executing go code)      nextp         puintptr      id            int64      mallocing     int32      throwing      int32      preemptoff    string // 该字段不等于空字符串的话,要保持 curg 始终在这个 m 上运行      locks         int32      softfloat     int32      dying         int32      profilehz     int32      helpgc        int32      spinning      bool // m 失业了,正在积极寻找工作~      blocked       bool // m 正阻塞在 note 上      inwb          bool // m 正在执行 write barrier      newSigstack   bool // minit on C thread called sigaltstack      printlock     int8      incgo         bool   // m 正在执行 cgo call      freeWait      uint32 // if == 0, safe to free g0 and delete m (atomic)      fastrand      [2]uint32      needextram    bool      traceback     uint8      ncgocall      uint64      // cgo 调用总计数      ncgo          int32       // 当前正在执行的 cgo 订单计数      cgoCallersUse uint32      // if non-zero, cgoCallers in use temporarily      cgoCallers    *cgoCallers // cgo traceback if crashing in cgo call      park          note      alllink       *m // on allm      schedlink     muintptr      mcache        *mcache      lockedg       guintptr      createstack   [32]uintptr    // stack that created this thread.      freglo        [16]uint32     // d[i] lsb and f[i]      freghi        [16]uint32     // d[i] msb and f[i+16]      fflag         uint32         // floating point compare flags      lockedExt     uint32         // tracking for external LockOSThread      lockedInt     uint32         // tracking for internal lockOSThread      nextwaitm     muintptr       // 正在等待锁的下一个 m      waitunlockf   unsafe.Pointer // todo go func(*g, unsafe.pointer) bool      waitlock      unsafe.Pointer      waittraceev   byte      waittraceskip int      startingtrace bool      syscalltick   uint32      thread        uintptr // thread handle      freelink      *m      // on sched.freem        // these are here because they are too large to be on the stack      // of low-level NOSPLIT functions.      libcall   libcall      libcallpc uintptr // for cpu profiler      libcallsp uintptr      libcallg  guintptr      syscall   libcall // 存储 windows 平台的 syscall 参数        mOS  }</code></pre>    <p>抽象数据结构,可以认为是 processor 的抽象,代表了任务执行时的上下文,m 必须获得 p 才能执行:</p>    <pre>  <code class="language-go">type p struct {      lock mutex        id          int32      status      uint32 // one of pidle/prunning/...      link        puintptr      schedtick   uint32     // 每次调用 schedule 时会加一      syscalltick uint32     // 每次系统调用时加一      sysmontick  sysmontick // 上次 sysmon 观察到的 tick 时间      m           muintptr   // 和相关联的 m 的反向指针,如果 p 是 idle 的话,那这个指针是 nil      mcache      *mcache      racectx     uintptr        deferpool    [5][]*_defer // pool of available defer structs of different sizes (see panic.go)      deferpoolbuf [5][32]*_defer        // Cache of goroutine ids, amortizes accesses to runtime·sched.goidgen.      goidcache    uint64      goidcacheend uint64        // runnable 状态的 goroutine。访问时是不加锁的      runqhead uint32      runqtail uint32      runq     [256]guintptr      // runnext 非空时,代表的是一个 runnable 状态的 G,      // 这个 G 是被 当前 G 修改为 ready 状态的,      // 并且相比在 runq 中的 G 有更高的优先级      // 如果当前 G 的还有剩余的可用时间,那么就应该运行这个 G      // 运行之后,该 G 会继承当前 G 的剩余时间      // If a set of goroutines is locked in a      // communicate-and-wait pattern, this schedules that set as a      // unit and eliminates the (potentially large) scheduling      // latency that otherwise arises from adding the ready'd      // goroutines to the end of the run queue.      runnext guintptr        // Available G's (status == Gdead)      gfree    *g      gfreecnt int32        sudogcache []*sudog      sudogbuf   [128]*sudog        tracebuf traceBufPtr        // traceSweep indicates the sweep events should be traced.      // This is used to defer the sweep start event until a span      // has actually been swept.      traceSweep bool      // traceSwept and traceReclaimed track the number of bytes      // swept and reclaimed by sweeping in the current sweep loop.      traceSwept, traceReclaimed uintptr        palloc persistentAlloc // per-P to avoid mutex        // Per-P GC state      gcAssistTime         int64 // Nanoseconds in assistAlloc      gcFractionalMarkTime int64 // Nanoseconds in fractional mark worker      gcBgMarkWorker       guintptr      gcMarkWorkerMode     gcMarkWorkerMode        // 当前标记 worker 的开始时间,单位纳秒      gcMarkWorkerStartTime int64        // gcw is this P's GC work buffer cache. The work buffer is      // filled by write barriers, drained by mutator assists, and      // disposed on certain GC state transitions.      gcw gcWork        // wbBuf is this P's GC write barrier buffer.      //      // TODO: Consider caching this in the running G.      wbBuf wbBuf        runSafePointFn uint32 // if 1, run sched.safePointFn at next safe point        pad [sys.CacheLineSize]byte  }</code></pre>    <p>全局调度器,全局只有一个 schedt 类型的实例:</p>    <pre>  <code class="language-go">type schedt struct {      // 下面两个变量需以原子访问访问。保持在 struct 顶部,以使其在 32 位系统上可以对齐      goidgen  uint64      lastpoll uint64        lock mutex        // 当修改 nmidle,nmidlelocked,nmsys,nmfreed 这些数值时      // 需要记得调用 checkdead        midle        muintptr // idle m's waiting for work      nmidle       int32    // 当前等待工作的空闲 m 计数      nmidlelocked int32    // 当前等待工作的被 lock 的 m 计数      mnext        int64    // 当前预缴创建的 m 数,并且该值会作为下一个创建的 m 的 ID      maxmcount    int32    // 允许创建的最大的 m 数量      nmsys        int32    // number of system m's not counted for deadlock      nmfreed      int64    // cumulative number of freed m's        ngsys uint32 // number of system goroutines; updated atomically        pidle      puintptr // 空闲 p's      npidle     uint32      nmspinning uint32 // See "Worker thread parking/unparking" comment in proc.go.        // 全局的可运行 g 队列      runqhead guintptr      runqtail guintptr      runqsize int32        // dead G 的全局缓存      gflock       mutex      gfreeStack   *g      gfreeNoStack *g      ngfree       int32        // sudog 结构的集中缓存      sudoglock  mutex      sudogcache *sudog        // 不同大小的可用的 defer struct 的集中缓存池      deferlock mutex      deferpool [5]*_defer        // 被设置了 m.exited 标记之后的 m,这些 m 正在 freem 这个链表上等待被 free      // 链表用 m.freelink 字段进行链接      freem *m        gcwaiting  uint32 // gc is waiting to run      stopwait   int32      stopnote   note      sysmonwait uint32      sysmonnote note        // safepointFn should be called on each P at the next GC      // safepoint if p.runSafePointFn is set.      safePointFn   func(*p)      safePointWait int32      safePointNote note        profilehz int32 // cpu profiling rate        procresizetime int64 // 上次修改 gomaxprocs 的纳秒时间      totaltime      int64 // ∫gomaxprocs dt up to procresizetime  }</code></pre>    <h2>g/p/m 的关系</h2>    <p>Go 实现了所谓的 M:N 模型,执行用户代码的 goroutine 可以认为都是对等的 goroutine。不考虑 g0 和 gsignal 的话,我们可以简单地认为调度就是将 m 绑定到 p,然后在 m 中不断循环执行调度函数(runtime.schedule),寻找可用的 g 来执行,下图为 m 绑定到 p 时,可能得到的 g 的来源:</p>    <pre>  <code class="language-go">+--------------+                                                  |    binded    +-------------------------------------+                                                  +-------+------+                                     |  +------------------------------------+                  |                                            v                         +------------------------------------+  |                                    |                  |                         +------------------------------------+       |                                    |  |             +------------------+   |                  |                         |                                    |       |            +------------------+    |  |             | Local Run Queue  |   |                  |                         |             +------------------+   |       |            | Global Run Queue |    |  |   other P   +-+-+-+-+-+-+-+-+--+   |                  |                         |             | Local Run Queue  |   |       |  schedt    +--+-+-+-+-+-+-+---+    |  |               |G|G|G|G|G|G|G|      |                  |                         |    P        +-+-+-+-+-+-+-+-+--+   |       |               |G|G|G|G|G|G|        |  |               +-+-+-+-+-+-+-+      |                  |                         |               |G|G|G|G|G|G|G|      |       |               +-+-+-+-+-+-+        |  |                ^                   |                  |                         |               +-+-+-+-+-+-+-+      |       |                ^                   |  +----------------+-------------------+                  |                         |                ^                   |       +----------------+-------------------+                   |                                      |                         +----------------+-------------------+                        |                   |                                      |                                          |                                            |                   |                                      |                                          |                                            |                   |                                      |                                          |                                            |                   |                                      |                                          |                                            |                   |                                      |                                          |                                            |                   |                                      |                                          |                                            |                   |                                      |                                          |                                            |                   |                                      |                                          |                                            |                   |                                      |                                          |                                            |                   |                                      |                                          |                                            |                   |                                      |                                          |                                            |                   |                                      v                                          |                                            |            +------+-------+                             .-.      +----------------+                 |                                            |            |    steal     +----------------------------( M )-----+    runqget     +-----------------+                                            |            +--------------+                             `-'      +----------------+                                                              |                                                          |                                                                                       |                                                          |                                                                           +-----------+-----+                                                          +---------------------------------------------------------------------------+   globrunqget   |                                                          |                                                                           +-----------------+                                                          |                                                          |                                                          |                                                          |                                                          |                                                          |                                               +----------+--------+                                               |   get netpoll g   |                                               +----------+--------+                                                          |                                                          |                                                          |                                                          |                                                          |                                           +--------------+--------------------+                                           |              |                    |                                           |              |                    |                                           |   netpoll    v                    |                                           |             +-+-+-+-+             |                                           |             |G|G|G|G|             |                                           |             +-+-+-+-+             |                                           |                                   |                                           +-----------------------------------+</code></pre>    <p>这张图展示了 g、p、m 三者之间的大致关系。m 是执行实体,对应的是操作系统线程。可以看到 m 会从绑定的 p 的本地队列、sched 中的全局队列、netpoll 中获取可运行的 g,实在找不着还会去其它的 p 那里去偷。</p>    <h2>p 如何初始化</h2>    <p>程序启动时,会依次调用:</p>    <pre>  <code class="language-go">graph TD  runtime.schedinit -->  runtime.procresize</code></pre>    <p>在 procresize 中会将全局 p 数组初始化,并将这些 p 串成链表放进 sched 全局调度器的 pidle 队列中:</p>    <pre>  <code class="language-go">for i := nprocs - 1; i >= 0; i-- {      p := allp[i]        // ...      // 设置 p 的状态      p.status = _Pidle      // 初始化时,所有 p 的 runq 都是空的,所以一定会走这个 if      if runqempty(p) {          // 将 p 放到全局调度器的 pidle 队列中          pidleput(p)      } else {          // ...      }  }</code></pre>    <p>pidleput 也比较简单,没啥可说的:</p>    <pre>  <code class="language-go">func pidleput(_p_ *p) {      if !runqempty(_p_) {          throw("pidleput: P has non-empty run queue")      }      // 简单的链表操作      _p_.link = sched.pidle      sched.pidle.set(_p_)        // pidle count + 1      atomic.Xadd(&sched.npidle, 1)  }</code></pre>    <p>所有 p 在程序启动的时候就已经被初始化完毕了,除非手动调用 runtime.GOMAXPROCS。</p>    <pre>  <code class="language-go">func GOMAXPROCS(n int) int {      lock(&sched.lock)      ret := int(gomaxprocs)      unlock(&sched.lock)      if n <= 0 || n == ret {          return ret      }        stopTheWorld("GOMAXPROCS")        // newprocs will be processed by startTheWorld      newprocs = int32(n)        startTheWorld()      return ret  }</code></pre>    <p>在 startTheWorld 中会调用 procresize。</p>    <h2>g 如何创建</h2>    <p>在用户代码里一般这么写:</p>    <pre>  <code class="language-go">go func() {      // do the stuff  }()</code></pre>    <p>实际上会被翻译成 runtime.newproc ,特权语法只是个语法糖。如果你要在其它语言里实现类似的东西,只要实现编译器翻译之后的内容就好了。具体流程:</p>    <pre>  <code class="language-go">graph TD  runtime.newproc --> runtime.newproc1</code></pre>    <p>newproc 干的事情也比较简单</p>    <pre>  <code class="language-go">func newproc(siz int32, fn *funcval) {      // add 是一个指针运算,跳过函数指针      // 把栈上的参数起始地址找到      argp := add(unsafe.Pointer(&fn), sys.PtrSize)      pc := getcallerpc()      systemstack(func() {          newproc1(fn, (*uint8)(argp), siz, pc)      })  }    // funcval 是一个变长结构,第一个成员是函数指针  // 所以上面的 add 是跳过这个 fn  type funcval struct {      fn uintptr      // variable-size, fn-specific data here  }</code></pre>    <p>runtime 里比较常见的 getcallerpc 和 getcallersp,代码里的注释写的比较明白了:</p>    <pre>  <code class="language-go">// For example:  //  // func f(arg1, arg2, arg3 int) {  //    pc := getcallerpc()  //    sp := getcallersp(unsafe.Pointer(&arg1))  //}  //  // These two lines find the PC and SP immediately following  // the call to f (where f will return).  //</code></pre>    <p>getcallerpc 返回的是调用函数之后的那条程序指令的地址,即 callee 函数返回时要执行的下一条指令的地址。</p>    <p>systemstack 在 runtime 中用的也比较多,其功能为让 m 切换到 g0 上执行各种调度函数。至于啥是 g0,在讲 m 的时候再说。</p>    <p>newproc1 的工作流程也比较简单:</p>    <pre>  <code class="language-go">graph TD  newproc1 --> newg  newg[gfget] --> nil{is nil?}  nil -->|yes|E[init stack]  nil -->|no|C[malg]  C --> D[set g status=> idle->dead]  D --> allgadd  E --> G[set g status=> dead-> runnable]  allgadd --> G  G --> runqput</code></pre>    <p>删掉了不关心的细节后的代码:</p>    <pre>  <code class="language-go">func newproc1(fn *funcval, argp *uint8, narg int32, callerpc uintptr) {      _g_ := getg()        if fn == nil {          _g_.m.throwing = -1 // do not dump full stacks          throw("go of nil func value")      }      _g_.m.locks++ // disable preemption because it can be holding p in a local var      siz := narg      siz = (siz + 7) &^ 7          _p_ := _g_.m.p.ptr()      newg := gfget(_p_)      if newg == nil {          newg = malg(_StackMin)          casgstatus(newg, _Gidle, _Gdead)          allgadd(newg) // publishes with a g->status of Gdead so GC scanner doesn't look at uninitialized stack.      }        totalSize := 4*sys.RegSize + uintptr(siz) + sys.MinFrameSize // extra space in case of reads slightly beyond frame      totalSize += -totalSize & (sys.SpAlign - 1)                  // align to spAlign      sp := newg.stack.hi - totalSize      spArg := sp        // 初始化 g,g 的 gobuf 现场,g 的 m 的 curg      // 以及各种寄存器      memclrNoHeapPointers(unsafe.Pointer(&newg.sched), unsafe.Sizeof(newg.sched))      newg.sched.sp = sp      newg.stktopsp = sp      newg.sched.pc = funcPC(goexit) + sys.PCQuantum // +PCQuantum so that previous instruction is in same function      newg.sched.g = guintptr(unsafe.Pointer(newg))      gostartcallfn(&newg.sched, fn)      newg.gopc = callerpc      newg.startpc = fn.fn      if _g_.m.curg != nil {          newg.labels = _g_.m.curg.labels      }        casgstatus(newg, _Gdead, _Grunnable)        newg.goid = int64(_p_.goidcache)      _p_.goidcache++      runqput(_p_, newg, true)        if atomic.Load(&sched.npidle) != 0 && atomic.Load(&sched.nmspinning) == 0 && mainStarted {          wakep()      }      _g_.m.locks--      if _g_.m.locks == 0 && _g_.preempt { // restore the preemption request in case we've cleared it in newstack          _g_.stackguard0 = stackPreempt      }  }</code></pre>    <p>所以 go func 执行的结果是调用 runqput 将 g 放进了执行队列。但在放队列之前还做了点小动作:</p>    <pre>  <code class="language-go">newg.sched.pc = funcPC(goexit) + sys.PCQuantum // +PCQuantum so that previous instruction is in same function</code></pre>    <h3>gostartcallfn</h3>    <pre>  <code class="language-go">// adjust Gobuf as if it executed a call to fn  // and then did an immediate gosave.  func gostartcallfn(gobuf *gobuf, fv *funcval) {      var fn unsafe.Pointer      if fv != nil {          fn = unsafe.Pointer(fv.fn)      } else {          fn = unsafe.Pointer(funcPC(nilfunc))      }      gostartcall(gobuf, fn, unsafe.Pointer(fv))  }    // adjust Gobuf as if it executed a call to fn with context ctxt  // and then did an immediate gosave.  func gostartcall(buf *gobuf, fn, ctxt unsafe.Pointer) {      sp := buf.sp      if sys.RegSize > sys.PtrSize {          sp -= sys.PtrSize          *(*uintptr)(unsafe.Pointer(sp)) = 0      }      sp -= sys.PtrSize      *(*uintptr)(unsafe.Pointer(sp)) = buf.pc // 注意这里,这个,这里的 buf.pc 实际上是 goexit 的 pc      buf.sp = sp      buf.pc = uintptr(fn)      buf.ctxt = ctxt  }</code></pre>    <p>在 gostartcall 中把 newproc1 时设置到 buf.pc 中的 goexit 的函数地址放到了 goroutine 的栈顶,然后重新设置 buf.pc 为 goroutine 函数的位置。这样做的目的是为了在执行完任何 goroutine 的函数时,通过 RET 指令,都能从栈顶把 sp 保存的 goexit 的指令 pop 到 pc 寄存器,效果相当于任何 goroutine 执行函数执行完之后,都会去执行 runtime.goexit,完成一些清理工作后再进入 schedule。</p>    <p>在之后的 m 的 schedule 讲解中会看到更详细的调度循环过程。</p>    <h3>runqput</h3>    <p>因为是放 runq 而不是直接执行,因而什么时候开始执行并不是用户代码能决定得了的。再看看 runqput 这个函数:</p>    <pre>  <code class="language-go">// runqput 尝试把 g 放到本地执行队列中  // next 参数如果是 false 的话,runqput 会将 g 放到运行队列的尾部  // If next if false, runqput adds g to the tail of the runnable queue.  // If next is true, runqput puts g in the _p_.runnext slot.  // If the run queue is full, runnext puts g on the global queue.  // Executed only by the owner P.  func runqput(_p_ *p, gp *g, next bool) {      if randomizeScheduler && next && fastrand()%2 == 0 {          next = false      }        if next {      retryNext:          oldnext := _p_.runnext          if !_p_.runnext.cas(oldnext, guintptr(unsafe.Pointer(gp))) {              goto retryNext          }          if oldnext == 0 {              return          }          // 把之前的 runnext 踢到正常的 runq 中          gp = oldnext.ptr()      }    retry:      h := atomic.Load(&_p_.runqhead) // load-acquire, synchronize with consumers      t := _p_.runqtail      if t-h < uint32(len(_p_.runq)) {          _p_.runq[t%uint32(len(_p_.runq))].set(gp)          atomic.Store(&_p_.runqtail, t+1) // store-release, makes the item available for consumption          return      }      if runqputslow(_p_, gp, h, t) {          return      }      // 队列没有满的话,上面的 put 操作会成功      goto retry  }</code></pre>    <h3>runqputslow</h3>    <pre>  <code class="language-go">// 因为 slow,所以会一次性把本地队列里的多个 g (包含当前的这个) 放到全局队列  // 只会被 g 的 owner P 执行  func runqputslow(_p_ *p, gp *g, h, t uint32) bool {      var batch [len(_p_.runq)/2 + 1]*g        // 先从本地队列抓一批 g      n := t - h      n = n / 2      if n != uint32(len(_p_.runq)/2) {          throw("runqputslow: queue is not full")      }      for i := uint32(0); i < n; i++ {          batch[i] = _p_.runq[(h+i)%uint32(len(_p_.runq))].ptr()      }      if !atomic.Cas(&_p_.runqhead, h, h+n) { // cas-release, commits consume          return false      }      batch[n] = gp        if randomizeScheduler {          for i := uint32(1); i <= n; i++ {              j := fastrandn(i + 1)              batch[i], batch[j] = batch[j], batch[i]          }      }        // 把这些 goroutine 构造成链表      for i := uint32(0); i < n; i++ {          batch[i].schedlink.set(batch[i+1])      }        // 将链表放到全局队列中      lock(&sched.lock)      globrunqputbatch(batch[0], batch[n], int32(n+1))      unlock(&sched.lock)      return true  }</code></pre>    <p>操作全局 sched 时,需要获取全局 sched.lock 锁,全局锁争抢的开销较大,所以才称之为 slow。p 和 g 在 m 中交互时,因为现场永远是单线程,所以很多时候不用加锁。</p>    <h2>m 工作机制</h2>    <p>在 runtime 中有三种线程,一种是主线程,一种是用来跑 sysmon 的线程,一种是普通的用户线程。主线程在 runtime 由对应的全局变量: runtime.m0 来表示。用户线程就是普通的线程了,和 p 绑定,执行 g 中的任务。虽然说是有三种,实际上前两种线程整个 runtime 就只有一个实例。用户线程才会有很多实例。</p>    <h3>主线程 m0</h3>    <p>主线程中用来跑 runtime.main ,流程线性执行,没有跳转:</p>    <pre>  <code class="language-go">graph TD  runtime.main --> A[init max stack size]  A --> B[systemstack execute -> newm -> sysmon]  B --> runtime.lockOsThread  runtime.lockOsThread --> runtime.init  runtime.init --> runtime.gcenable  runtime.gcenable --> main.init  main.init --> main.main</code></pre>    <h3>sysmon 线程</h3>    <p>sysmon 是在 runtime.main 中启动的,不过需要注意的是 sysmon 并不是在 m0 上执行的。因为:</p>    <pre>  <code class="language-go">systemstack(func() {      newm(sysmon, nil)  })</code></pre>    <p>创建了新的 m,但这个 m 又与普通的线程不一样,因为不需要绑定 p 就可以执行。是与整个调度系统脱离的。</p>    <p>sysmon 内部是个死循环,主要负责以下几件事情:</p>    <ol>     <li> <p>checkdead,检查是否所有 goroutine 都已经锁死,如果是的话,直接调用 runtime.throw,强制退出。这个操作只在启动的时候做一次</p> </li>     <li> <p>将 netpoll 返回的结果注入到全局 sched 的任务队列</p> </li>     <li> <p>收回因为 syscall 而长时间阻塞的 p,同时抢占那些执行时间过长的 g</p> </li>     <li> <p>如果 span 内存闲置超过 5min,那么释放掉</p> </li>    </ol>    <p>流程图:</p>    <pre>  <code class="language-go">graph TD  sysmon --> usleep  usleep --> checkdead  checkdead --> |every 10ms|C[netpollinited && lastpoll != 0]  C --> |yes|netpoll  netpoll --> injectglist  injectglist --> retake  C --> |no|retake  retake --> A[check forcegc needed]  A --> B[scavenge heap once in a while]  B --> usleep</code></pre>    <pre>  <code class="language-go">// sysmon 不需要绑定 P 就可以运行,所以不允许 write barriers  //  //go:nowritebarrierrec  func sysmon() {      lock(&sched.lock)      sched.nmsys++      checkdead()      unlock(&sched.lock)        // 如果一个 heap span 在一次GC 之后 5min 都没有被使用过      // 那么把它交还给操作系统      scavengelimit := int64(5 * 60 * 1e9)        if debug.scavenge > 0 {          // Scavenge-a-lot for testing.          forcegcperiod = 10 * 1e6          scavengelimit = 20 * 1e6      }        lastscavenge := nanotime()      nscavenge := 0        lasttrace := int64(0)      idle := 0 // how many cycles in succession we had not wokeup somebody      delay := uint32(0)      for {          if idle == 0 { // 初始化时 20us sleep              delay = 20          } else if idle > 50 { // start doubling the sleep after 1ms...              delay *= 2          }          if delay > 10*1000 { // 最多到 10ms              delay = 10 * 1000          }          usleep(delay)          if debug.schedtrace <= 0 && (sched.gcwaiting != 0 || atomic.Load(&sched.npidle) == uint32(gomaxprocs)) {              lock(&sched.lock)              if atomic.Load(&sched.gcwaiting) != 0 || atomic.Load(&sched.npidle) == uint32(gomaxprocs) {                  atomic.Store(&sched.sysmonwait, 1)                  unlock(&sched.lock)                  // Make wake-up period small enough                  // for the sampling to be correct.                  maxsleep := forcegcperiod / 2                  if scavengelimit < forcegcperiod {                      maxsleep = scavengelimit / 2                  }                  shouldRelax := true                  if osRelaxMinNS > 0 {                      next := timeSleepUntil()                      now := nanotime()                      if next-now < osRelaxMinNS {                          shouldRelax = false                      }                  }                  if shouldRelax {                      osRelax(true)                  }                  notetsleep(&sched.sysmonnote, maxsleep)                  if shouldRelax {                      osRelax(false)                  }                  lock(&sched.lock)                  atomic.Store(&sched.sysmonwait, 0)                  noteclear(&sched.sysmonnote)                  idle = 0                  delay = 20              }              unlock(&sched.lock)          }          // trigger libc interceptors if needed          if *cgo_yield != nil {              asmcgocall(*cgo_yield, nil)          }          // 如果 10ms 没有 poll 过 network,那么就 netpoll 一次          lastpoll := int64(atomic.Load64(&sched.lastpoll))          now := nanotime()          if netpollinited() && lastpoll != 0 && lastpoll+10*1000*1000 < now {              atomic.Cas64(&sched.lastpoll, uint64(lastpoll), uint64(now))              gp := netpoll(false) // 非阻塞 -- 返回一个 goroutine 的列表              if gp != nil {                  // Need to decrement number of idle locked M's                  // (pretending that one more is running) before injectglist.                  // Otherwise it can lead to the following situation:                  // injectglist grabs all P's but before it starts M's to run the P's,                  // another M returns from syscall, finishes running its G,                  // observes that there is no work to do and no other running M's                  // and reports deadlock.                  incidlelocked(-1)                  injectglist(gp)                  incidlelocked(1)              }          }          // 接收在 syscall 状态阻塞的 P          // 抢占长时间运行的 G          if retake(now) != 0 {              idle = 0          } else {              idle++          }          // 检查是否需要 force GC(两分钟一次的)          if t := (gcTrigger{kind: gcTriggerTime, now: now}); t.test() && atomic.Load(&forcegc.idle) != 0 {              lock(&forcegc.lock)              forcegc.idle = 0              forcegc.g.schedlink = 0              injectglist(forcegc.g)              unlock(&forcegc.lock)          }          // 每过一段时间扫描一次堆          if lastscavenge+scavengelimit/2 < now {              mheap_.scavenge(int32(nscavenge), uint64(now), uint64(scavengelimit))              lastscavenge = now              nscavenge++          }          if debug.schedtrace > 0 && lasttrace+int64(debug.schedtrace)*1000000 <= now {              lasttrace = now              schedtrace(debug.scheddetail > 0)          }      }  }</code></pre>    <p>checkdead</p>    <pre>  <code class="language-go">// 检查死锁的场景  // 该检查基于当前正在运行的 M 的数量,如果 0,那么就是 deadlock 了  // 检查的时候必须持有 sched.lock 锁  func checkdead() {      // 对于 -buildmode=c-shared 或者 -buildmode=c-archive 来说      // 没有 goroutine 正在运行也是 OK 的。因为调用这个库的程序应该是在运行的      if islibrary || isarchive {          return      }        // If we are dying because of a signal caught on an already idle thread,      // freezetheworld will cause all running threads to block.      // And runtime will essentially enter into deadlock state,      // except that there is a thread that will call exit soon.      if panicking > 0 {          return      }        run := mcount() - sched.nmidle - sched.nmidlelocked - sched.nmsys      if run > 0 {          return      }      if run < 0 {          print("runtime: checkdead: nmidle=", sched.nmidle, " nmidlelocked=", sched.nmidlelocked, " mcount=", mcount(), " nmsys=", sched.nmsys, "\n")          throw("checkdead: inconsistent counts")      }        grunning := 0      lock(&allglock)      for i := 0; i < len(allgs); i++ {          gp := allgs[i]          if isSystemGoroutine(gp) {              continue          }          s := readgstatus(gp)          switch s &^ _Gscan {          case _Gwaiting:              grunning++          case _Grunnable,              _Grunning,              _Gsyscall:              unlock(&allglock)              print("runtime: checkdead: find g ", gp.goid, " in status ", s, "\n")              throw("checkdead: runnable g")          }      }      unlock(&allglock)      if grunning == 0 { // possible if main goroutine calls runtime·Goexit()          throw("no goroutines (main called runtime.Goexit) - deadlock!")      }        // Maybe jump time forward for playground.      gp := timejump()      if gp != nil {          casgstatus(gp, _Gwaiting, _Grunnable)          globrunqput(gp)          _p_ := pidleget()          if _p_ == nil {              throw("checkdead: no p for timer")          }          mp := mget()          if mp == nil {              // There should always be a free M since              // nothing is running.              throw("checkdead: no m for timer")          }          mp.nextp.set(_p_)          notewakeup(&mp.park)          return      }        getg().m.throwing = -1 // do not dump full stacks      throw("all goroutines are asleep - deadlock!")  }</code></pre>    <p>retake</p>    <pre>  <code class="language-go">// forcePreemptNS is the time slice given to a G before it is  // preempted.  const forcePreemptNS = 10 * 1000 * 1000 // 10ms    func retake(now int64) uint32 {      n := 0      // Prevent allp slice changes. This lock will be completely      // uncontended unless we're already stopping the world.      lock(&allpLock)      // We can't use a range loop over allp because we may      // temporarily drop the allpLock. Hence, we need to re-fetch      // allp each time around the loop.      for i := 0; i < len(allp); i++ {          _p_ := allp[i]          if _p_ == nil {              // 在 procresize 修改了 allp 但还没有创建新的 p 的时候              // 会有这种情况              continue          }          pd := &_p_.sysmontick          s := _p_.status          if s == _Psyscall {              // 从 syscall 接管 P,如果它进行 syscall 已经经过了一个 sysmon 的 tick(至少 20us)              t := int64(_p_.syscalltick)              if int64(pd.syscalltick) != t {                  pd.syscalltick = uint32(t)                  pd.syscallwhen = now                  continue              }              // 一方面如果没有其它工作可做的话,我们不想接管 p              // 但另一方面为了避免 sysmon 线程陷入沉睡,我们最终还是会接管这些 p              if runqempty(_p_) && atomic.Load(&sched.nmspinning)+atomic.Load(&sched.npidle) > 0 && pd.syscallwhen+10*1000*1000 > now {                  continue              }              // 解开 allplock 的锁,然后就可以持有 sched.lock 锁了              unlock(&allpLock)              // Need to decrement number of idle locked M's              // (pretending that one more is running) before the CAS.              // Otherwise the M from which we retake can exit the syscall,              // increment nmidle and report deadlock.              incidlelocked(-1)              if atomic.Cas(&_p_.status, s, _Pidle) {                  if trace.enabled {                      traceGoSysBlock(_p_)                      traceProcStop(_p_)                  }                  n++                  _p_.syscalltick++                  handoffp(_p_)              }              incidlelocked(1)              lock(&allpLock)          } else if s == _Prunning {              // 如果 G 运行时间太长,那么抢占它              t := int64(_p_.schedtick)              if int64(pd.schedtick) != t {                  pd.schedtick = uint32(t)                  pd.schedwhen = now                  continue              }              if pd.schedwhen+forcePreemptNS > now {                  continue              }              preemptone(_p_)          }      }      unlock(&allpLock)      return uint32(n)  }</code></pre>    <h3>普通线程</h3>    <p>普通线程就是我们 G/P/M 模型里的 M 了,M 对应的就是操作系统的线程。</p>    <p>线程创建</p>    <p>上面在创建 sysmon 线程的时候也看到了,创建线程的函数是 newm。</p>    <pre>  <code class="language-go">graph TD  newm --> newm1  newm1 --> newosproc  newosproc --> clone</code></pre>    <p>最终会走到 linux 创建线程的系统调用 clone ,代码里大段和 cgo 相关的内容我们就不关心了,摘掉 cgo 相关的逻辑后的代码如下:</p>    <pre>  <code class="language-go">// 创建一个新的 m。该 m 会在启动时调用函数 fn,或者 schedule 函数  // fn 需要是 static 类型,且不能是在堆上分配的闭包。  // 运行 m 时,m.p 是有可能为 nil 的,所以不允许 write barriers  //go:nowritebarrierrec  func newm(fn func(), _p_ *p) {      mp := allocm(_p_, fn)      mp.nextp.set(_p_)      mp.sigmask = initSigmask      newm1(mp)  }</code></pre>    <p>传入的 p 会被赋值给 m 的 nextp 成员,在 m 执行 schedule 时,会将 nextp 拿出来,进行之后真正的绑定操作(其实就是把 nextp 赋值为 nil,并把这个 nextp 赋值给 m.p,把 m 赋值给 p.m)。</p>    <pre>  <code class="language-go">func newm1(mp *m) {      execLock.rlock() // Prevent process clone.      newosproc(mp, unsafe.Pointer(mp.g0.stack.hi))      execLock.runlock()  }</code></pre>    <pre>  <code class="language-go">func newosproc(mp *m, stk unsafe.Pointer) {      // Disable signals during clone, so that the new thread starts      // with signals disabled. It will enable them in minit.      var oset sigset      sigprocmask(_SIG_SETMASK, &sigset_all, &oset)      ret := clone(cloneFlags, stk, unsafe.Pointer(mp), unsafe.Pointer(mp.g0), unsafe.Pointer(funcPC(mstart)))      sigprocmask(_SIG_SETMASK, &oset, nil)        if ret < 0 {          print("runtime: failed to create new OS thread (have ", mcount(), " already; errno=", -ret, ")\n")          if ret == -_EAGAIN {              println("runtime: may need to increase max user processes (ulimit -u)")          }          throw("newosproc")      }  }</code></pre>    <p>工作流程</p>    <p>首先空闲的 m 会被丢进全局调度器的 midle 队列中,在需要 m 的时候,会先从这里取:</p>    <pre>  <code class="language-go">//go:nowritebarrierrec  // 尝试从 midle 列表中获取一个 m  // 必须锁全局的 sched  // 可能在 STW 期间执行,所以不允许 write barriers  func mget() *m {      mp := sched.midle.ptr()      if mp != nil {          sched.midle = mp.schedlink          sched.nmidle--      }      return mp  }</code></pre>    <p>取不到的话就会调用之前提到的 newm 来创建新线程,创建的线程是不会被销毁的,哪怕之后不需要这么多 m 了,也就只是会把 m 放在 midle 中。</p>    <p>什么时候会创建线程呢,可以追踪一下 newm 的调用方:</p>    <pre>  <code class="language-go">graph TD  main --> |sysmon|newm  startTheWorld --> startTheWorldWithSema  gcMarkTermination --> startTheWorldWithSema  gcStart--> startTheWorldWithSema  startTheWorldWithSema --> |helpgc|newm  startTheWorldWithSema --> |run p|newm  startm --> mget  mget --> |if no free m|newm  startTemplateThread --> |templateThread|newm  LockOsThread --> startTemplateThread  main --> |iscgo|startTemplateThread  handoffp --> startm  wakep --> startm  injectglist --> startm</code></pre>    <p>基本上来讲,m 都是按需创建的。如果 sched.midle 中没有空闲的 m 了,现在又需要,那么就会去创建一个。</p>    <p>创建好的线程需要绑定到 p 之后才会开始执行,执行过程中也可能被剥夺掉 p。比如前面 retake 的流程,就会将 g 的 stackguard0 修改为 stackPreempt,待下一次进入 newstack 时,会判断是否有该抢占标记,有的话,就会放弃运行。这也就是所谓的 协作式抢占 。</p>    <p>工作线程执行的内容核心其实就只有俩: schedule() 和 findrunnable() 。</p>    <p>schedule</p>    <pre>  <code class="language-go">graph TD  schedule --> A[schedtick%61 == 0]  A --> |yes|globrunqget  A --> |no|runqget  globrunqget --> C[gp == nil]  C --> |no|execute  C --> |yes|runqget  runqget --> B[gp == nil]  B --> |no|execute  B --> |yes|findrunnable  findrunnable --> execute</code></pre>    <pre>  <code class="language-go">// 调度器调度一轮要执行的函数: 寻找一个 runnable 状态的 goroutine,并 execute 它  // 调度函数是循环,永远都不会返回  func schedule() {      _g_ := getg()        if _g_.m.locks != 0 {          throw("schedule: holding locks")      }        if _g_.m.lockedg != 0 {          stoplockedm()          execute(_g_.m.lockedg.ptr(), false) // Never returns.      }        // 执行 cgo 调用的 g 不能被 schedule 走      // 因为 cgo 调用使用 m 的 g0 栈      if _g_.m.incgo {          throw("schedule: in cgo")      }    top:      if sched.gcwaiting != 0 {          gcstopm()          goto top      }      if _g_.m.p.ptr().runSafePointFn != 0 {          runSafePointFn()      }        var gp *g      var inheritTime bool      if trace.enabled || trace.shutdown {          gp = traceReader()          if gp != nil {              casgstatus(gp, _Gwaiting, _Grunnable)              traceGoUnpark(gp, 0)          }      }      if gp == nil && gcBlackenEnabled != 0 {          gp = gcController.findRunnableGCWorker(_g_.m.p.ptr())      }      if gp == nil {          // 每调度几次就检查一下全局的 runq 来确保公平          // 否则两个 goroutine 就可以通过互相调用          // 完全占用本地的 runq 了          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 {          gp, inheritTime = runqget(_g_.m.p.ptr())          if gp != nil && _g_.m.spinning {              throw("schedule: spinning with local work")          }      }      if gp == nil {          gp, inheritTime = findrunnable() // 在找到 goroutine 之前会一直阻塞下去      }        // 当前线程将要执行 goroutine,并且不会再进入 spinning 状态      // 所以如果它被标记为 spinning,我们需要 reset 这个状态      // 可能会重启一个新的 spinning 状态的 M      if _g_.m.spinning {          resetspinning()      }        if gp.lockedm != 0 {          // Hands off own p to the locked m,          // then blocks waiting for a new p.          startlockedm(gp)          goto top      }        execute(gp, inheritTime)  }</code></pre>    <p>m 中所谓的调度循环实际上就是一直在执行下图中的 loop:</p>    <pre>  <code class="language-go">graph TD  schedule --> execute  execute --> gogo  gogo --> goexit  goexit --> goexit1  goexit1 --> goexit0  goexit0 --> schedule</code></pre>    <p>execute</p>    <pre>  <code class="language-go">// 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() // 这个可能是 m 的 g0        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 // 把当前 g 的位置让给 m      gp.m = _g_.m // 把 gp 指向 m,建立双向关系        gogo(&gp.sched)  }</code></pre>    <p>比较简单,绑定 g 和 m,然后 gogo 执行绑定的 g 中的函数。</p>    <p>gogo</p>    <p>runtime.gogo 是汇编完成的,功能就是执行 go func() 的这个 func() ,可以看到功能主要是把 g 对象的 gobuf 里的内容搬到寄存器里。然后从 gobuf.pc 寄存器存储的指令位置开始继续向后执行。</p>    <pre>  <code class="language-go">// 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</code></pre>    <p>当然,这里还是有一些和手写汇编不太一样的,看着比较奇怪的地方, gobuf_sp(BX) 这种写法按说标准 plan9 汇编中 gobuf_sp 只是个 symbol ,没有任何偏移量的意思,但这里却用名字来代替了其偏移量,这是怎么回事呢?</p>    <p>实际上这是 runtime 的特权,是需要链接器配合完成的,再来看看 gobuf 在 runtime 中的 struct 定义开头部分的注释:</p>    <pre>  <code class="language-go">// The offsets of sp, pc, and g are known to (hard-coded in) libmach.</code></pre>    <p>这下知道怎么回事了吧,链接器会帮助我们把这个换成偏移量。。</p>    <p>Goexit</p>    <p>Goexit :</p>    <pre>  <code class="language-go">// Goexit terminates the goroutine that calls it. No other goroutine is affected.  // Goexit runs all deferred calls before terminating the goroutine. Because Goexit  // is not a panic, any recover calls in those deferred functions will return nil.  //  // Calling Goexit from the main goroutine terminates that goroutine  // without func main returning. Since func main has not returned,  // the program continues execution of other goroutines.  // If all other goroutines exit, the program crashes.  func Goexit() {      // Run all deferred functions for the current goroutine.      // This code is similar to gopanic, see that implementation      // for detailed comments.      gp := getg()      for {          d := gp._defer          if d == nil {              break          }          if d.started {              if d._panic != nil {                  d._panic.aborted = true                  d._panic = nil              }              d.fn = nil              gp._defer = d.link              freedefer(d)              continue          }          d.started = true          reflectcall(nil, unsafe.Pointer(d.fn), deferArgs(d), uint32(d.siz), uint32(d.siz))          if gp._defer != d {              throw("bad defer entry in Goexit")          }          d._panic = nil          d.fn = nil          gp._defer = d.link          freedefer(d)          // Note: we ignore recovers here because Goexit isn't a panic      }      goexit1()  }    // Finishes execution of the current goroutine.  func goexit1() {      if raceenabled {          racegoend()      }      if trace.enabled {          traceGoEnd()      }      mcall(goexit0)  }</code></pre>    <pre>  <code class="language-go">// The top-most function running on a goroutine  // returns to goexit+PCQuantum.  TEXT runtime·goexit(SB),NOSPLIT,$0-0      BYTE    $0x90    // NOP      CALL    runtime·goexit1(SB)    // does not return      // traceback from goexit1 must hit code range of goexit      BYTE    $0x90    // NOP</code></pre>    <p>mcall :</p>    <pre>  <code class="language-go">// func mcall(fn func(*g))  // Switch to m->g0's stack, call fn(g).  // Fn must never return. It should gogo(&g->sched)  // to keep running g.  TEXT runtime·mcall(SB), NOSPLIT, $0-8      MOVQ    fn+0(FP), DI        get_tls(CX)      MOVQ    g(CX), AX    // save state in g->sched      MOVQ    0(SP), BX    // caller's PC      MOVQ    BX, (g_sched+gobuf_pc)(AX)      LEAQ    fn+0(FP), BX    // caller's SP      MOVQ    BX, (g_sched+gobuf_sp)(AX)      MOVQ    AX, (g_sched+gobuf_g)(AX)      MOVQ    BP, (g_sched+gobuf_bp)(AX)        // switch to m->g0 & its stack, call fn      MOVQ    g(CX), BX      MOVQ    g_m(BX), BX      MOVQ    m_g0(BX), SI      CMPQ    SI, AX    // if g == m->g0 call badmcall      JNE    3(PC)      MOVQ    $runtime·badmcall(SB), AX      JMP    AX      MOVQ    SI, g(CX)    // g = m->g0      MOVQ    (g_sched+gobuf_sp)(SI), SP    // sp = m->g0->sched.sp      PUSHQ    AX      MOVQ    DI, DX      MOVQ    0(DI), DI      CALL    DI      POPQ    AX      MOVQ    $runtime·badmcall2(SB), AX      JMP    AX      RET</code></pre>    <p>wakep</p>    <pre>  <code class="language-go">// Tries to add one more P to execute G's.  // Called when a G is made runnable (newproc, ready).  func wakep() {      // be conservative about spinning threads      if !atomic.Cas(&sched.nmspinning, 0, 1) {          return      }      startm(nil, true)  }    // Schedules some M to run the p (creates an M if necessary).  // If p==nil, tries to get an idle P, if no idle P's does nothing.  // May run with m.p==nil, so write barriers are not allowed.  // If spinning is set, the caller has incremented nmspinning and startm will  // either decrement nmspinning or set m.spinning in the newly started M.  //go:nowritebarrierrec  func startm(_p_ *p, spinning bool) {      lock(&sched.lock)      if _p_ == nil {          _p_ = pidleget()          if _p_ == nil {               unlock(&sched.lock)               if spinning {                   // The caller incremented nmspinning, but there are no idle Ps,                   // so it's okay to just undo the increment and give up.                   if int32(atomic.Xadd(&sched.nmspinning, -1)) < 0 {                       throw("startm: negative nmspinning")                   }               }               return          }      }      mp := mget()      unlock(&sched.lock)      if mp == nil {          var fn func()          if spinning {              // The caller incremented nmspinning, so set m.spinning in the new M.              fn = mspinning          }          newm(fn, _p_)          return      }      if mp.spinning {          throw("startm: m is spinning")      }      if mp.nextp != 0 {          throw("startm: m has p")      }      if spinning && !runqempty(_p_) {          throw("startm: p has runnable gs")      }      // The caller incremented nmspinning, so set m.spinning in the new M.      mp.spinning = spinning      mp.nextp.set(_p_)      notewakeup(&mp.park)  }</code></pre>    <p>goroutine 挂起</p>    <pre>  <code class="language-go">// Puts the current goroutine into a waiting state and calls unlockf.  // If unlockf returns false, the goroutine is resumed.  // unlockf must not access this G's stack, as it may be moved between  // the call to gopark and the call to unlockf.  func gopark(unlockf func(*g, unsafe.Pointer) bool, lock unsafe.Pointer, reason string, traceEv byte, traceskip int) {      mp := acquirem()      gp := mp.curg      status := readgstatus(gp)      if status != _Grunning && status != _Gscanrunning {          throw("gopark: bad g status")      }      mp.waitlock = lock      mp.waitunlockf = *(*unsafe.Pointer)(unsafe.Pointer(&unlockf))      gp.waitreason = reason      mp.waittraceev = traceEv      mp.waittraceskip = traceskip      releasem(mp)      // can't do anything that might move the G between Ms here.      mcall(park_m)  }    func goready(gp *g, traceskip int) {      systemstack(func() {          ready(gp, traceskip, true)      })  }    // Mark gp ready to run.  func ready(gp *g, traceskip int, next bool) {      if trace.enabled {          traceGoUnpark(gp, traceskip)      }        status := readgstatus(gp)        // Mark runnable.      _g_ := getg()      _g_.m.locks++ // disable preemption because it can be holding p in a local var      if status&^_Gscan != _Gwaiting {          dumpgstatus(gp)          throw("bad g->status in ready")      }        // status is Gwaiting or Gscanwaiting, make Grunnable and put on runq      casgstatus(gp, _Gwaiting, _Grunnable)      runqput(_g_.m.p.ptr(), gp, next)      if atomic.Load(&sched.npidle) != 0 && atomic.Load(&sched.nmspinning) == 0 {          wakep()      }      _g_.m.locks--      if _g_.m.locks == 0 && _g_.preempt { // restore the preemption request in Case we've cleared it in newstack          _g_.stackguard0 = stackPreempt      }  }</code></pre>    <pre>  <code class="language-go">func notesleep(n *note) {      gp := getg()      if gp != gp.m.g0 {          throw("notesleep not on g0")      }      ns := int64(-1)      if *cgo_yield != nil {          // Sleep for an arbitrary-but-moderate interval to poll libc interceptors.          ns = 10e6      }      for atomic.Load(key32(&n.key)) == 0 {          gp.m.blocked = true          futexsleep(key32(&n.key), 0, ns)          if *cgo_yield != nil {              asmcgocall(*cgo_yield, nil)          }          gp.m.blocked = false      }  }    // One-time notifications.  func noteclear(n *note) {      n.key = 0  }    func notewakeup(n *note) {      old := atomic.Xchg(key32(&n.key), 1)      if old != 0 {          print("notewakeup - double wakeup (", old, ")\n")          throw("notewakeup - double wakeup")      }      futexwakeup(key32(&n.key), 1)  }</code></pre>    <p>findrunnable</p>    <p>findrunnable 比较复杂,流程图先把 gc 相关的省略掉了:</p>    <pre>  <code class="language-go">graph TD  runqget --> A[gp == nil]  A --> |no|return  A --> |yes|globrunqget  globrunqget --> B[gp == nil]  B --> |no| return  B --> |yes| C[netpollinited && lastpoll != 0]  C --> |yes|netpoll  netpoll --> K[gp == nil]  K --> |no|return  K --> |yes|runqsteal  C --> |no|runqsteal  runqsteal --> D[gp == nil]  D --> |no|return  D --> |yes|E[globrunqget]  E --> F[gp == nil]  F --> |no| return  F --> |yes| G[check all p's runq]  G --> H[runq is empty]  H --> |no|runqget  H --> |yes|I[netpoll]  I --> J[gp == nil]  J --> |no| return  J --> |yes| stopm  stopm --> runqget</code></pre>    <pre>  <code class="language-go">// 找到一个可执行的 goroutine 来 execute  // 会尝试从其它的 P 那里偷 g,从全局队列中拿,或者 network 中 poll  func findrunnable() (gp *g, inheritTime bool) {      _g_ := getg()        // The conditions here and in handoffp must agree: if      // findrunnable would return a G to run, handoffp must start      // an M.    top:      _p_ := _g_.m.p.ptr()      if sched.gcwaiting != 0 {          gcstopm()          goto top      }      if _p_.runSafePointFn != 0 {          runSafePointFn()      }      if fingwait && fingwake {          if gp := wakefing(); gp != nil {              ready(gp, 0, true)          }      }      if *cgo_yield != nil {          asmcgocall(*cgo_yield, nil)      }        // 本地 runq      if gp, inheritTime := runqget(_p_); gp != nil {          return gp, inheritTime      }        // 全局 runq      if sched.runqsize != 0 {          lock(&sched.lock)          gp := globrunqget(_p_, 0)          unlock(&sched.lock)          if gp != nil {              return gp, false          }      }        // Poll network.      // netpoll 是我们执行 work-stealing 之前的一个优化      // 如果没有任何的 netpoll 等待者,或者线程被阻塞在 netpoll 中,我们可以安全地跳过这段逻辑      // 如果在阻塞的线程中存在任何逻辑上的竞争(e.g. 已经从 netpoll 中返回,但还没有设置 lastpoll)      // 该线程还是会将下面的 netpoll 阻塞住      if netpollinited() && atomic.Load(&netpollWaiters) > 0 && atomic.Load64(&sched.lastpoll) != 0 {          if gp := netpoll(false); gp != nil { // 非阻塞              // netpoll 返回 goroutine 链表,用 schedlink 连接              injectglist(gp.schedlink.ptr())              casgstatus(gp, _Gwaiting, _Grunnable)              if trace.enabled {                  traceGoUnpark(gp, 0)              }              return gp, false          }      }        // 从其它 p 那里偷 g      procs := uint32(gomaxprocs)      if atomic.Load(&sched.npidle) == procs-1 {          // GOMAXPROCS=1 或者除了我们其它的 p 都是 idle          // 新的工作可能从 syscall/cgocall,网络或者定时器中来。          // 上面这些任务都不会被放到本地的 runq,所有没有可以 stealing 的点          goto stop      }      // 如果正在自旋的 M 的数量 >= 忙着的 P,那么阻塞      // 这是为了      // 当 GOMAXPROCS 远大于 1,但程序的并行度又很低的时候      // 防止过量的 CPU 消耗      if !_g_.m.spinning && 2*atomic.Load(&sched.nmspinning) >= procs-atomic.Load(&sched.npidle) {          goto stop      }      if !_g_.m.spinning {          _g_.m.spinning = true          atomic.Xadd(&sched.nmspinning, 1)      }      for i := 0; i < 4; i++ {          for enum := stealOrder.start(fastrand()); !enum.done(); enum.next() {              if sched.gcwaiting != 0 {                  goto top              }              stealRunNextG := i > 2 // first look for ready queues with more than 1 g              if gp := runqsteal(_p_, allp[enum.position()], stealRunNextG); gp != nil {                  return gp, false              }          }      }    stop:        // 没有可以干的事情。如果我们正在 GC 的标记阶段,可以安全地扫描和加深对象的颜色,      // 这样可以进行空闲时间的标记,而不是直接放弃 P      if gcBlackenEnabled != 0 && _p_.gcBgMarkWorker != 0 && gcMarkWorkAvailable(_p_) {          _p_.gcMarkWorkerMode = gcMarkWorkerIdleMode          gp := _p_.gcBgMarkWorker.ptr()          casgstatus(gp, _Gwaiting, _Grunnable)          if trace.enabled {              traceGoUnpark(gp, 0)          }          return gp, false      }        // Before we drop our P, make a snapshot of the allp slice,      // which can change underfoot once we no longer block      // safe-points. We don't need to snapshot the contents because      // everything up to cap(allp) is immutable.      allpSnapshot := allp        // 返回 P 并阻塞      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      }      if releasep() != _p_ {          throw("findrunnable: wrong p")      }      pidleput(_p_)      unlock(&sched.lock)        // Delicate dance: thread transitions from spinning to non-spinning state,      // potentially concurrently with submission of new goroutines. We must      // drop nmspinning first and then check all per-P queues again (with      // #StoreLoad memory barrier in between). If we do it the other way around,      // another thread can submit a goroutine after we've checked all run queues      // but before we drop nmspinning; as the result nobody will unpark a thread      // to run the goroutine.      // If we discover new work below, we need to restore m.spinning as a signal      // for resetspinning to unpark a new worker thread (because there can be more      // than one starving goroutine). However, if after discovering new work      // we also observe no idle Ps, it is OK to just park the current thread:      // the system is fully loaded so no spinning threads are required.      // Also see "Worker thread parking/unparking" comment at the top of the file.      wasSpinning := _g_.m.spinning      if _g_.m.spinning {          _g_.m.spinning = false          if int32(atomic.Xadd(&sched.nmspinning, -1)) < 0 {              throw("findrunnable: negative nmspinning")          }      }        // 再检查一下所有的 runq      for _, _p_ := range allpSnapshot {          if !runqempty(_p_) {              lock(&sched.lock)              _p_ = pidleget()              unlock(&sched.lock)              if _p_ != nil {                  acquirep(_p_)                  if wasSpinning {                      _g_.m.spinning = true                      atomic.Xadd(&sched.nmspinning, 1)                  }                  goto top              }              break          }      }        // 再检查 gc 空闲 g      if gcBlackenEnabled != 0 && gcMarkWorkAvailable(nil) {          lock(&sched.lock)          _p_ = pidleget()          if _p_ != nil && _p_.gcBgMarkWorker == 0 {              pidleput(_p_)              _p_ = nil          }          unlock(&sched.lock)          if _p_ != nil {              acquirep(_p_)              if wasSpinning {                  _g_.m.spinning = true                  atomic.Xadd(&sched.nmspinning, 1)              }              // Go back to idle GC check.              goto stop          }      }        // poll network      if netpollinited() && atomic.Load(&netpollWaiters) > 0 && atomic.Xchg64(&sched.lastpoll, 0) != 0 {          if _g_.m.p != 0 {              throw("findrunnable: netpoll with p")          }          if _g_.m.spinning {              throw("findrunnable: netpoll with spinning")          }          gp := netpoll(true) // 阻塞到返回为止          atomic.Store64(&sched.lastpoll, uint64(nanotime()))          if gp != nil {              lock(&sched.lock)              _p_ = pidleget()              unlock(&sched.lock)              if _p_ != nil {                  acquirep(_p_)                  injectglist(gp.schedlink.ptr())                  casgstatus(gp, _Gwaiting, _Grunnable)                  if trace.enabled {                      traceGoUnpark(gp, 0)                  }                  return gp, false              }              injectglist(gp)          }      }      stopm()      goto top  }</code></pre>    <h2>m 和 p 解绑定</h2>    <h3>handoffp</h3>    <pre>  <code class="language-go">graph TD    mexit --> A[is m0?]  A --> |yes|B[handoffp]  A --> |no| C[iterate allm]  C --> |m found|handoffp  C --> |m not found| throw    forEachP --> |p status == syscall| handoffp    stoplockedm --> handoffp    entersyscallblock --> entersyscallblock_handoff  entersyscallblock_handoff --> handoffp    retake --> |p status == syscall| handoffp</code></pre>    <p>最终会把 p 放回全局的 pidle 队列中:</p>    <pre>  <code class="language-go">// Hands off P from syscall or locked M.  // Always runs without a P, so write barriers are not allowed.  //go:nowritebarrierrec  func handoffp(_p_ *p) {   // handoffp must start an M in any situation where   // findrunnable would return a G to run on _p_.     // if it has local work, start it straight away   if !runqempty(_p_) || sched.runqsize != 0 {    startm(_p_, false)    return   }   // if it has GC work, start it straight away   if gcBlackenEnabled != 0 && gcMarkWorkAvailable(_p_) {    startm(_p_, false)    return   }   // no local work, check that there are no spinning/idle M's,   // otherwise our help is not required   if atomic.Load(&sched.nmspinning)+atomic.Load(&sched.npidle) == 0 && atomic.Cas(&sched.nmspinning, 0, 1) { // TODO: fast atomic    startm(_p_, true)    return   }   lock(&sched.lock)   if sched.gcwaiting != 0 {    _p_.status = _Pgcstop    sched.stopwait--    if sched.stopwait == 0 {     notewakeup(&sched.stopnote)    }    unlock(&sched.lock)    return   }   if _p_.runSafePointFn != 0 && atomic.Cas(&_p_.runSafePointFn, 1, 0) {    sched.safePointFn(_p_)    sched.safePointWait--    if sched.safePointWait == 0 {     notewakeup(&sched.safePointNote)    }   }   if sched.runqsize != 0 {    unlock(&sched.lock)    startm(_p_, false)    return   }   // If this is the last running P and nobody is polling network,   // need to wakeup another M to poll network.   if sched.npidle == uint32(gomaxprocs-1) && atomic.Load64(&sched.lastpoll) != 0 {    unlock(&sched.lock)    startm(_p_, false)    return   }   pidleput(_p_)   unlock(&sched.lock)  }</code></pre>    <h2>g 的状态迁移</h2>    <pre>  <code class="language-go">graph LR  start{newg} --> Gidle  Gidle --> |oneNewExtraM|Gdead  Gidle --> |newproc1|Gdead    Gdead --> |newproc1|Grunnable  Gdead --> |needm|Gsyscall    Gscanrunning --> |scang|Grunning    Grunnable --> |execute|Grunning    Gany --> |casgcopystack|Gcopystack    Gcopystack --> |todotodo|Grunning    Gsyscall --> |dropm|Gdead  Gsyscall --> |exitsyscall0|Grunnable  Gsyscall --> |exitsyscall|Grunning    Grunning --> |goschedImpl|Grunnable  Grunning --> |goexit0|Gdead  Grunning --> |newstack|Gcopystack  Grunning --> |reentersyscall|Gsyscall  Grunning --> |entersyscallblock|Gsyscall  Grunning --> |markroot|Gwaiting  Grunning --> |gcAssistAlloc1|Gwaiting  Grunning --> |park_m|Gwaiting  Grunning --> |gcMarkTermination|Gwaiting  Grunning --> |gcBgMarkWorker|Gwaiting  Grunning --> |newstack|Gwaiting    Gwaiting --> |gcMarkTermination|Grunning  Gwaiting --> |gcBgMarkWorker|Grunning  Gwaiting --> |markroot|Grunning  Gwaiting --> |gcAssistAlloc1|Grunning  Gwaiting --> |newstack|Grunning  Gwaiting --> |findRunnableGCWorker|Grunnable  Gwaiting --> |ready|Grunnable  Gwaiting --> |findrunnable|Grunnable  Gwaiting --> |injectglist|Grunnable  Gwaiting --> |schedule|Grunnable  Gwaiting --> |park_m|Grunnable  Gwaiting --> |procresize|Grunnable  Gwaiting --> |checkdead|Grunnable</code></pre>    <p>图上的 Gany 代表任意状态,GC 时的状态切换比较多,如果只关注正常情况下的状态转换,可以把 markroot、gcMark 之类的先忽略掉。</p>    <h2>p 的状态迁移</h2>    <pre>  <code class="language-go">graph LR    Pidle --> |acquirep1|Prunning    Psyscall --> |retake|Pidle  Psyscall --> |entersyscall_gcwait|Pgcstop  Psyscall --> |exitsyscallfast|Prunning    Pany --> |gcstopm|Pgcstop  Pany --> |forEachP|Pidle  Pany --> |releasep|Pidle  Pany --> |handoffp|Pgcstop  Pany --> |procresize release current p use allp 0|Pidle  Pany --> |procresize when init|Pgcstop  Pany --> |procresize when free old p| Pdead  Pany --> |procresize after resize use current p|Prunning  Pany --> |reentersyscall|Psyscall  Pany --> |stopTheWorldWithSema|Pgcstop</code></pre>    <h2>抢占流程</h2>    <p>函数执行是在 goroutine 的栈上,这个栈在函数执行期间是有可能溢出的,我们前面也看到了,如果一个函数用到了栈,会将 stackguard0 和 sp 寄存器进行比较,如果 sp > stackguard0,说明栈已经增长到溢出,因为栈是从内存高地址向低地址方向增长的。</p>    <p>那么这个比较过程是在哪里完成的呢?这一步是由编译器完成的,我们看看一个函数编译后的结果,这段代码来自 go-internals:</p>    <pre>  <code class="language-go">0x0000 TEXT    "".main(SB), $24-0    ;; stack-split prologue    0x0000 MOVQ    (TLS), CX    0x0009 CMPQ    SP, 16(CX)    0x000d JLS    58      0x000f SUBQ    $24, SP    0x0013 MOVQ    BP, 16(SP)    0x0018 LEAQ    16(SP), BP    ;; ...omitted FUNCDATA stuff...    0x001d MOVQ    $137438953482, AX    0x0027 MOVQ    AX, (SP)    ;; ...omitted PCDATA stuff...    0x002b CALL    "".add(SB)    0x0030 MOVQ    16(SP), BP    0x0035 ADDQ    $24, SP    0x0039 RET      ;; stack-split epilogue    0x003a NOP    ;; ...omitted PCDATA stuff...    0x003a CALL    runtime.morestack_noctxt(SB)    0x003f JMP    0</code></pre>    <p>函数开头被插的这段指令,即是将 g struct 中的 stackguard 与 SP 寄存器进行对比,JLS 表示 SP < 16(CX) 的话即跳转。</p>    <pre>  <code class="language-go">;; stack-split prologue    0x0000 MOVQ    (TLS), CX    0x0009 CMPQ    SP, 16(CX)    0x000d JLS    58</code></pre>    <p>这里因为 CX 寄存器存储的是 g 的起始地址,而 16(CX) 指的是 g 结构体偏移 16 个字节的位置,可以回顾一下 g 结构体定义,16 个字节恰好是跳过了第一个成员 stack(16字节) 之后的 stackguard0 的位置。</p>    <p>58 转为 16 进制即是 0x3a。</p>    <pre>  <code class="language-go">;; stack-split epilogue    0x003a NOP    ;; ...omitted PCDATA stuff...    0x003a CALL    runtime.morestack_noctxt(SB)    0x003f JMP    0</code></pre>    <p>morestack_noctxt:</p>    <pre>  <code class="language-go">// morestack but not preserving ctxt.  TEXT runtime·morestack_noctxt(SB),NOSPLIT,$0      MOVL    $0, DX      JMP    runtime·morestack(SB)</code></pre>    <p>morestack:</p>    <pre>  <code class="language-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)      INT    $3        // Cannot grow signal stack (m->gsignal).      MOVQ    m_gsignal(BX), SI      CMPQ    g(CX), SI      JNE    3(PC)      CALL    runtime·badmorestackgsignal(SB)      INT    $3        // Called from f.      // Set m->morebuf to f's caller.      MOVQ    8(SP), AX    // f's caller's PC      MOVQ    AX, (m_morebuf+gobuf_pc)(BX)      LEAQ    16(SP), AX    // f's caller's SP      MOVQ    AX, (m_morebuf+gobuf_sp)(BX)      get_tls(CX)      MOVQ    g(CX), SI      MOVQ    SI, (m_morebuf+gobuf_g)(BX)        // Set g->sched to context in f.      MOVQ    0(SP), AX // f's PC      MOVQ    AX, (g_sched+gobuf_pc)(SI)      MOVQ    SI, (g_sched+gobuf_g)(SI)      LEAQ    8(SP), AX // f's SP      MOVQ    AX, (g_sched+gobuf_sp)(SI)      MOVQ    BP, (g_sched+gobuf_bp)(SI)      MOVQ    DX, (g_sched+gobuf_ctxt)(SI)        // Call newstack on m->g0's stack.      MOVQ    m_g0(BX), BX      MOVQ    BX, g(CX)      MOVQ    (g_sched+gobuf_sp)(BX), SP      CALL    runtime·newstack(SB)      MOVQ    $0, 0x1003    // crash if newstack returns      RET</code></pre>    <p>newstack:</p>    <pre>  <code class="language-go">// Called from runtime·morestack when more stack is needed.  // Allocate larger stack and relocate to new stack.  // Stack growth is multiplicative, for constant amortized cost.  //  // g->atomicstatus will be Grunning or Gscanrunning upon entry.  // If the GC is trying to stop this g then it will set preemptscan to true.  //  // This must be nowritebarrierrec because it can be called as part of  // stack growth from other nowritebarrierrec functions, but the  // compiler doesn't check this.  //  //go:nowritebarrierrec  func newstack() {      thisg := getg()      // TODO: double check all gp. shouldn't be getg().      if thisg.m.morebuf.g.ptr().stackguard0 == stackFork {          throw("stack growth after fork")      }      if thisg.m.morebuf.g.ptr() != thisg.m.curg {          print("runtime: newstack called from g=", hex(thisg.m.morebuf.g), "\n"+"\tm=", thisg.m, " m->curg=", thisg.m.curg, " m->g0=", thisg.m.g0, " m->gsignal=", thisg.m.gsignal, "\n")          morebuf := thisg.m.morebuf          traceback(morebuf.pc, morebuf.sp, morebuf.lr, morebuf.g.ptr())          throw("runtime: wrong goroutine in newstack")      }        gp := thisg.m.curg        if thisg.m.curg.throwsplit {          // Update syscallsp, syscallpc in case traceback uses them.          morebuf := thisg.m.morebuf          gp.syscallsp = morebuf.sp          gp.syscallpc = morebuf.pc          pcname, pcoff := "(unknown)", uintptr(0)          f := findfunc(gp.sched.pc)          if f.valid() {              pcname = funcname(f)              pcoff = gp.sched.pc - f.entry          }          print("runtime: newstack at ", pcname, "+", hex(pcoff),              " sp=", hex(gp.sched.sp), " stack=[", hex(gp.stack.lo), ", ", hex(gp.stack.hi), "]\n",              "\tmorebuf={pc:", hex(morebuf.pc), " sp:", hex(morebuf.sp), " lr:", hex(morebuf.lr), "}\n",              "\tsched={pc:", hex(gp.sched.pc), " sp:", hex(gp.sched.sp), " lr:", hex(gp.sched.lr), " ctxt:", gp.sched.ctxt, "}\n")            thisg.m.traceback = 2 // Include runtime frames          traceback(morebuf.pc, morebuf.sp, morebuf.lr, gp)          throw("runtime: stack split at bad time")      }        morebuf := thisg.m.morebuf      thisg.m.morebuf.pc = 0      thisg.m.morebuf.lr = 0      thisg.m.morebuf.sp = 0      thisg.m.morebuf.g = 0        // NOTE: stackguard0 may change underfoot, if another thread      // is about to try to preempt gp. Read it just once and use that same      // value now and below.      preempt := atomic.Loaduintptr(&gp.stackguard0) == stackPreempt        // Be conservative about where we preempt.      // We are interested in preempting user Go code, not runtime code.      // If we're holding locks, mallocing, or preemption is disabled, don't      // preempt.      // This check is very early in newstack so that even the status change      // from Grunning to Gwaiting and back doesn't happen in this case.      // That status change by itself can be viewed as a small preemption,      // because the GC might change Gwaiting to Gscanwaiting, and then      // this goroutine has to wait for the GC to finish before continuing.      // If the GC is in some way dependent on this goroutine (for example,      // it needs a lock held by the goroutine), that small preemption turns      // into a real deadlock.      if preempt {          if thisg.m.locks != 0 || thisg.m.mallocing != 0 || thisg.m.preemptoff != "" || thisg.m.p.ptr().status != _Prunning {              // Let the goroutine keep running for now.              // gp->preempt is set, so it will be preempted next time.              gp.stackguard0 = gp.stack.lo + _StackGuard              gogo(&gp.sched) // never return          }      }        if gp.stack.lo == 0 {          throw("missing stack in newstack")      }      sp := gp.sched.sp      if sys.ArchFamily == sys.AMD64 || sys.ArchFamily == sys.I386 {          // The call to morestack cost a word.          sp -= sys.PtrSize      }      if stackDebug >= 1 || sp < gp.stack.lo {          print("runtime: newstack sp=", hex(sp), " stack=[", hex(gp.stack.lo), ", ", hex(gp.stack.hi), "]\n",              "\tmorebuf={pc:", hex(morebuf.pc), " sp:", hex(morebuf.sp), " lr:", hex(morebuf.lr), "}\n",              "\tsched={pc:", hex(gp.sched.pc), " sp:", hex(gp.sched.sp), " lr:", hex(gp.sched.lr), " ctxt:", gp.sched.ctxt, "}\n")      }      if sp < gp.stack.lo {          print("runtime: gp=", gp, ", gp->status=", hex(readgstatus(gp)), "\n ")          print("runtime: split stack overflow: ", hex(sp), " < ", hex(gp.stack.lo), "\n")          throw("runtime: split stack overflow")      }        if preempt {          if gp == thisg.m.g0 {              throw("runtime: preempt g0")          }          if thisg.m.p == 0 && thisg.m.locks == 0 {              throw("runtime: g is running but p is not")          }          // Synchronize with scang.          casgstatus(gp, _Grunning, _Gwaiting)          if gp.preemptscan {              for !castogscanstatus(gp, _Gwaiting, _Gscanwaiting) {                  // Likely to be racing with the GC as                  // it sees a _Gwaiting and does the                  // stack scan. If so, gcworkdone will                  // be set and gcphasework will simply                  // return.              }              if !gp.gcscandone {                  // gcw is safe because we're on the                  // system stack.                  gcw := &gp.m.p.ptr().gcw                  scanstack(gp, gcw)                  if gcBlackenPromptly {                      gcw.dispose()                  }                  gp.gcscandone = true              }              gp.preemptscan = false              gp.preempt = false              casfrom_Gscanstatus(gp, _Gscanwaiting, _Gwaiting)              // This clears gcscanvalid.              casgstatus(gp, _Gwaiting, _Grunning)              gp.stackguard0 = gp.stack.lo + _StackGuard              gogo(&gp.sched) // never return          }            // Act like goroutine called runtime.Gosched.          casgstatus(gp, _Gwaiting, _Grunning)          gopreempt_m(gp) // never return      }        // Allocate a bigger segment and move the stack.      oldsize := gp.stack.hi - gp.stack.lo      newsize := oldsize * 2      if newsize > maxstacksize {          print("runtime: goroutine stack exceeds ", maxstacksize, "-byte limit\n")          throw("stack overflow")      }        // The goroutine must be executing in order to call newstack,      // so it must be Grunning (or Gscanrunning).      casgstatus(gp, _Grunning, _Gcopystack)        // The concurrent GC will not scan the stack while we are doing the copy since      // the gp is in a Gcopystack status.      copystack(gp, newsize, true)      if stackDebug >= 1 {          print("stack grow done\n")      }      casgstatus(gp, _Gcopystack, _Grunning)      gogo(&gp.sched)  }</code></pre>    <p>总结一下流程:</p>    <pre>  <code class="language-go">graph TD  start[entering func] --> cmp[sp < stackguard0]  cmp --> |yes| morestack_noctxt  cmp --> |no|final[execute func]  morestack_noctxt --> morestack  morestack --> newstack  newstack --> preempt</code></pre>    <p>抢占都是在 newstack 中完成,但抢占标记是在 Go 源代码中的其它位置来进行标记的:</p>    <p>我们来看看 stackPreempt 是在哪些位置赋值给 stackguard0 的:</p>    <pre>  <code class="language-go">graph LR    unlock --> |in case cleared in newstack|restorePreempt  ready --> |in case cleared in newstack|restorePreempt  startTheWorldWithSema --> |in case cleared in newstack|restorePreempt  allocm --> |in case cleared in newstack|restorePreempt  exitsyscall --> |in case cleared in newstack|restorePreempt  newproc1--> |in case cleared in newstack|restorePreempt  releasem -->  |in case cleared in newstack|restorePreempt    scang --> setPreempt  reentersyscall --> setPreempt  entersyscallblock --> setPreempt  preemptone--> setPreempt    enlistWorker --> preemptone  retake --> preemptone  preemptall --> preemptone  freezetheworld --> preemptall  stopTheWorldWithSema --> preemptall  forEachP --> preemptall  startpanic_m --> freezetheworld  gcMarkDone --> forEachP</code></pre>    <p>可见只有 gc 和 retake 才会去真正地抢占 g,并没有其它的入口,其它的地方就只是恢复一下可能在 newstack 中被清除掉的抢占标记。</p>    <p>当然,这里 entersyscall 和 entersyscallblock 比较特殊,虽然这俩函数的实现中有设置抢占标记,但实际上这两段逻辑是不会被走到的。因为 syscall 执行时是在 m 的 g0 栈上,如果在执行时被抢占,那么会直接 throw,而无法恢复。</p>    <p> </p>    <p>来自:http://xargin.com/go-scheduler/</p>    <p> </p>    
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