GMP 调度器现场

type g struct {
	// Stack parameters.
	// stack describes the actual stack memory: [stack.lo, stack.hi).
	// stackguard0 is the stack pointer compared in the Go stack growth prologue.
	// It is stack.lo+StackGuard normally, but can be StackPreempt to trigger a preemption.
	// stackguard1 is the stack pointer compared in the //go:systemstack stack growth prologue.
	// It is stack.lo+StackGuard on g0 and gsignal stacks.
	// It is ~0 on other goroutine stacks, to trigger a call to morestackc (and crash).
	stack       stack   // offset known to runtime/cgo
	stackguard0 uintptr // offset known to cmd/internal/obj/*
	stackguard1 uintptr // offset known to cmd/internal/obj/*

	_panic    *_panic // innermost panic
	_defer    *_defer // innermost defer
	m         *m      // current m
	sched     gobuf
	syscallsp uintptr // if status==Gsyscall, syscallsp = sched.sp to use during gc
	syscallpc uintptr // if status==Gsyscall, syscallpc = sched.pc to use during gc
	syscallbp uintptr // if status==Gsyscall, syscallbp = sched.bp to use in fpTraceback
	stktopsp  uintptr // expected sp at top of stack, to check in traceback

type m struct {
	g0      *g     // goroutine with scheduling stack
	morebuf gobuf  // gobuf arg to morestack
	divmod  uint32 // div/mod denominator for arm - known to liblink (cmd/internal/obj/arm/obj5.go)

	// Fields whose offsets are 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        [tlsSlots]uintptr // thread-local storage (for x86 extern register)
	mstartfn   func()
	curg       *g       // current running goroutine
	caughtsig  guintptr // goroutine running during fatal signal

	// Indicates whether we've received a signal while
	// running in secret mode.
	signalSecret bool

	// p is the currently attached P for executing Go code, nil if not executing user Go code.
	//
	// A non-nil p implies exclusive ownership of the P, unless curg is in _Gsyscall.
	// In _Gsyscall the scheduler may mutate this instead. The point of synchronization
	// is the _Gscan bit on curg's status. The scheduler must arrange to prevent curg
	// from transitioning out of _Gsyscall if it intends to mutate p.
	p puintptr

	nextp           puintptr // The next P to install before executing. Implies exclusive ownership of this P.
	oldp            puintptr // The P that was attached before executing a syscall.
	id              int64

type p struct {
	id          int32
	status      uint32 // one of pidle/prunning/...
	link        puintptr
	schedtick   uint32     // incremented on every scheduler call
	syscalltick uint32     // incremented on every system call
	sysmontick  sysmontick // last tick observed by sysmon
	m           muintptr   // back-link to associated m (nil if idle)
	...

	// Queue of runnable goroutines. Accessed without lock.
	runqhead uint32
	runqtail uint32
	runq     [256]guintptr
	// runnext, if non-nil, is a runnable G that was ready'd by
	// the current G and should be run next instead of what's in
	// runq if there's time remaining in the running G's time
	// slice. It will inherit the time left in the current time
	// slice. 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.
	//
	// Note that while other P's may atomically CAS this to zero,
	// only the owner P can CAS it to a valid G.
	runnext guintptr

func stealWork(now int64) (gp *g, inheritTime bool, rnow, pollUntil int64, newWork bool) {
	pp := getg().m.p.ptr()

	ranTimer := false

	const stealTries = 4
	for i := 0; i < stealTries; i++ {
		stealTimersOrRunNextG := i == stealTries-1

		for enum := stealOrder.start(cheaprand()); !enum.done(); enum.next() {
			...

			// Don't bother to attempt to steal if p2 is idle.
			if !idlepMask.read(enum.position()) {
				if gp := runqsteal(pp, p2, stealTimersOrRunNextG); gp != nil {
					return gp, false, now, pollUntil, ranTimer
				}
			}
		}
	}

		// Try to prevent the P from continuing in the syscall, if it's in one at all.
		thread, ok := setBlockOnExitSyscall(pp)
		if !ok {
			// Not in a syscall, or something changed out from under us.
			goto done
		}

		// Retake the P if it's there for more than 1 sysmon tick (at least 20us).
		if syst := int64(pp.syscalltick); !sysretake && int64(pd.syscalltick) != syst {
			pd.syscalltick = uint32(syst)
			pd.syscallwhen = now
			thread.resume()
			goto done
		}

		// On the one hand we don't want to retake Ps if there is no other work to do,
		// but on the other hand we want to retake them eventually
		// because they can prevent the sysmon thread from deep sleep.
		if runqempty(pp) && sched.nmspinning.Load()+sched.npidle.Load() > 0 && pd.syscallwhen+10*1000*1000 > now {
			thread.resume()
			goto done
		}

		// Take the P. Note: because we have the scan bit, the goroutine
		// is at worst stuck spinning in exitsyscall.
		thread.takeP()
		thread.resume()
		n++

		// Handoff the P for some other thread to run it.
		handoffp(pp)