8.6 扫描标记与标记辅助

8.6 扫描标记与标记辅助

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func gcBgMarkWorker(_p_ *p) {
	gp := getg()

	type parkInfo struct {
		m      muintptr // Release this m on park.
		attach puintptr // If non-nil, attach to this p on park.
	}
	// We pass park to a gopark unlock function, so it can't be on
	// the stack (see gopark). Prevent deadlock from recursively
	// starting GC by disabling preemption.
	gp.m.preemptoff = "GC worker init"
	park := new(parkInfo)
	gp.m.preemptoff = ""

	park.m.set(acquirem())
	park.attach.set(_p_)
	// Inform gcBgMarkStartWorkers that this worker is ready.
	// After this point, the background mark worker is scheduled
	// cooperatively by gcController.findRunnable. Hence, it must
	// never be preempted, as this would put it into _Grunnable
	// and put it on a run queue. Instead, when the preempt flag
	// is set, this puts itself into _Gwaiting to be woken up by
	// gcController.findRunnable at the appropriate time.
	notewakeup(&work.bgMarkReady)

	for {
		// Go to sleep until woken by gcController.findRunnable.
		// We can't releasem yet since even the call to gopark
		// may be preempted.
		gopark(func(g *g, parkp unsafe.Pointer) bool {
			park := (*parkInfo)(parkp)

			// The worker G is no longer running, so it's
			// now safe to allow preemption.
			releasem(park.m.ptr())

			// If the worker isn't attached to its P,
			// attach now. During initialization and after
			// a phase change, the worker may have been
			// running on a different P. As soon as we
			// attach, the owner P may schedule the
			// worker, so this must be done after the G is
			// stopped.
			if park.attach != 0 {
				p := park.attach.ptr()
				park.attach.set(nil)
				// cas the worker because we may be
				// racing with a new worker starting
				// on this P.
				if !p.gcBgMarkWorker.cas(0, guintptr(unsafe.Pointer(g))) {
					// The P got a new worker.
					// Exit this worker.
					return false
				}
			}
			return true
		}, unsafe.Pointer(park), waitReasonGCWorkerIdle, traceEvGoBlock, 0)

		// Loop until the P dies and disassociates this
		// worker (the P may later be reused, in which case
		// it will get a new worker) or we failed to associate.
		if _p_.gcBgMarkWorker.ptr() != gp {
			break
		}

		// Disable preemption so we can use the gcw. If the
		// scheduler wants to preempt us, we'll stop draining,
		// dispose the gcw, and then preempt.
		park.m.set(acquirem())
		(...)

		startTime := nanotime()
		_p_.gcMarkWorkerStartTime = startTime

		decnwait := atomic.Xadd(&work.nwait, -1)
		(...)

		systemstack(func() {
			// Mark our goroutine preemptible so its stack
			// can be scanned. This lets two mark workers
			// scan each other (otherwise, they would
			// deadlock). We must not modify anything on
			// the G stack. However, stack shrinking is
			// disabled for mark workers, so it is safe to
			// read from the G stack.
			casgstatus(gp, _Grunning, _Gwaiting)
			switch _p_.gcMarkWorkerMode {
			(...)
			case gcMarkWorkerDedicatedMode:
				gcDrain(&_p_.gcw, gcDrainUntilPreempt|gcDrainFlushBgCredit)
				if gp.preempt {
					// We were preempted. This is
					// a useful signal to kick
					// everything out of the run
					// queue so it can run
					// somewhere else.
					lock(&sched.lock)
					for {
						gp, _ := runqget(_p_)
						if gp == nil {
							break
						}
						globrunqput(gp)
					}
					unlock(&sched.lock)
				}
				// Go back to draining, this time
				// without preemption.
				gcDrain(&_p_.gcw, gcDrainFlushBgCredit)
			case gcMarkWorkerFractionalMode:
				gcDrain(&_p_.gcw, gcDrainFractional|gcDrainUntilPreempt|gcDrainFlushBgCredit)
			case gcMarkWorkerIdleMode:
				gcDrain(&_p_.gcw, gcDrainIdle|gcDrainUntilPreempt|gcDrainFlushBgCredit)
			}
			casgstatus(gp, _Gwaiting, _Grunning)
		})

		// Account for time.
		duration := nanotime() - startTime
		switch _p_.gcMarkWorkerMode {
		case gcMarkWorkerDedicatedMode:
			atomic.Xaddint64(&gcController.dedicatedMarkTime, duration)
			atomic.Xaddint64(&gcController.dedicatedMarkWorkersNeeded, 1)
		case gcMarkWorkerFractionalMode:
			atomic.Xaddint64(&gcController.fractionalMarkTime, duration)
			atomic.Xaddint64(&_p_.gcFractionalMarkTime, duration)
		case gcMarkWorkerIdleMode:
			atomic.Xaddint64(&gcController.idleMarkTime, duration)
		}

		// Was this the last worker and did we run out
		// of work?
		incnwait := atomic.Xadd(&work.nwait, +1)
		if incnwait > work.nproc {
			(...)
		}

		// If this worker reached a background mark completion
		// point, signal the main GC goroutine.
		if incnwait == work.nproc && !gcMarkWorkAvailable(nil) {
			// Make this G preemptible and disassociate it
			// as the worker for this P so
			// findRunnableGCWorker doesn't try to
			// schedule it.
			_p_.gcBgMarkWorker.set(nil)
			releasem(park.m.ptr())

			gcMarkDone()

			// Disable preemption and prepare to reattach
			// to the P.
			//
			// We may be running on a different P at this
			// point, so we can't reattach until this G is
			// parked.
			park.m.set(acquirem())
			park.attach.set(_p_)
		}
	}
}

进一步阅读的参考文献