src/runtime/mgcwork.go

// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package runtime

import (
	"runtime/internal/atomic"
	"runtime/internal/sys"
	"unsafe"
)

const (
	_WorkbufSize = 2048 // in bytes; larger values result in less contention
)

// Garbage collector work pool abstraction.
//
// This implements a producer/consumer model for pointers to grey
// objects. A grey object is one that is marked and on a work
// queue. A black object is marked and not on a work queue.
//
// Write barriers, root discovery, stack scanning, and object scanning
// produce pointers to grey objects. Scanning consumes pointers to
// grey objects, thus blackening them, and then scans them,
// potentially producing new pointers to grey objects.

// A wbufptr holds a workbuf*, but protects it from write barriers.
// workbufs never live on the heap, so write barriers are unnecessary.
// Write barriers on workbuf pointers may also be dangerous in the GC.
//
// TODO: Since workbuf is now go:notinheap, this isn't necessary.
type wbufptr uintptr

func wbufptrOf(w *workbuf) wbufptr {
	return wbufptr(unsafe.Pointer(w))
}

func (wp wbufptr) ptr() *workbuf {
	return (*workbuf)(unsafe.Pointer(wp))
}

// A gcWork provides the interface to produce and consume work for the
// garbage collector.
//
// A gcWork can be used on the stack as follows:
//
//     (preemption must be disabled)
//     gcw := &getg().m.p.ptr().gcw
//     .. call gcw.put() to produce and gcw.get() to consume ..
//     if gcBlackenPromptly {
//         gcw.dispose()
//     }
//
// It's important that any use of gcWork during the mark phase prevent
// the garbage collector from transitioning to mark termination since
// gcWork may locally hold GC work buffers. This can be done by
// disabling preemption (systemstack or acquirem).
type gcWork struct {
	// wbuf1 and wbuf2 are the primary and secondary work buffers.
	//
	// This can be thought of as a stack of both work buffers'
	// pointers concatenated. When we pop the last pointer, we
	// shift the stack up by one work buffer by bringing in a new
	// full buffer and discarding an empty one. When we fill both
	// buffers, we shift the stack down by one work buffer by
	// bringing in a new empty buffer and discarding a full one.
	// This way we have one buffer's worth of hysteresis, which
	// amortizes the cost of getting or putting a work buffer over
	// at least one buffer of work and reduces contention on the
	// global work lists.
	//
	// wbuf1 is always the buffer we're currently pushing to and
	// popping from and wbuf2 is the buffer that will be discarded
	// next.
	//
	// Invariant: Both wbuf1 and wbuf2 are nil or neither are.
	wbuf1, wbuf2 wbufptr

	// Bytes marked (blackened) on this gcWork. This is aggregated
	// into work.bytesMarked by dispose.
	bytesMarked uint64

	// Scan work performed on this gcWork. This is aggregated into
	// gcController by dispose and may also be flushed by callers.
	scanWork int64
}

func (w *gcWork) init() {
	w.wbuf1 = wbufptrOf(getempty())
	wbuf2 := trygetfull()
	if wbuf2 == nil {
		wbuf2 = getempty()
	}
	w.wbuf2 = wbufptrOf(wbuf2)
}

// put enqueues a pointer for the garbage collector to trace.
// obj must point to the beginning of a heap object or an oblet.
//go:nowritebarrier
func (w *gcWork) put(obj uintptr) {
	flushed := false
	wbuf := w.wbuf1.ptr()
	if wbuf == nil {
		w.init()
		wbuf = w.wbuf1.ptr()
		// wbuf is empty at this point.
	} else if wbuf.nobj == len(wbuf.obj) {
		w.wbuf1, w.wbuf2 = w.wbuf2, w.wbuf1
		wbuf = w.wbuf1.ptr()
		if wbuf.nobj == len(wbuf.obj) {
			putfull(wbuf)
			wbuf = getempty()
			w.wbuf1 = wbufptrOf(wbuf)
			flushed = true
		}
	}

	wbuf.obj[wbuf.nobj] = obj
	wbuf.nobj++

	// If we put a buffer on full, let the GC controller know so
	// it can encourage more workers to run. We delay this until
	// the end of put so that w is in a consistent state, since
	// enlistWorker may itself manipulate w.
	if flushed && gcphase == _GCmark {
		gcController.enlistWorker()
	}
}

// putFast does a put and returns true if it can be done quickly
// otherwise it returns false and the caller needs to call put.
//go:nowritebarrier
func (w *gcWork) putFast(obj uintptr) bool {
	wbuf := w.wbuf1.ptr()
	if wbuf == nil {
		return false
	} else if wbuf.nobj == len(wbuf.obj) {
		return false
	}

	wbuf.obj[wbuf.nobj] = obj
	wbuf.nobj++
	return true
}

// tryGet dequeues a pointer for the garbage collector to trace.
//
// If there are no pointers remaining in this gcWork or in the global
// queue, tryGet returns 0.  Note that there may still be pointers in
// other gcWork instances or other caches.
//go:nowritebarrier
func (w *gcWork) tryGet() uintptr {
	wbuf := w.wbuf1.ptr()
	if wbuf == nil {
		w.init()
		wbuf = w.wbuf1.ptr()
		// wbuf is empty at this point.
	}
	if wbuf.nobj == 0 {
		w.wbuf1, w.wbuf2 = w.wbuf2, w.wbuf1
		wbuf = w.wbuf1.ptr()
		if wbuf.nobj == 0 {
			owbuf := wbuf
			wbuf = trygetfull()
			if wbuf == nil {
				return 0
			}
			putempty(owbuf)
			w.wbuf1 = wbufptrOf(wbuf)
		}
	}

	wbuf.nobj--
	return wbuf.obj[wbuf.nobj]
}

// tryGetFast dequeues a pointer for the garbage collector to trace
// if one is readily available. Otherwise it returns 0 and
// the caller is expected to call tryGet().
//go:nowritebarrier
func (w *gcWork) tryGetFast() uintptr {
	wbuf := w.wbuf1.ptr()
	if wbuf == nil {
		return 0
	}
	if wbuf.nobj == 0 {
		return 0
	}

	wbuf.nobj--
	return wbuf.obj[wbuf.nobj]
}

// get dequeues a pointer for the garbage collector to trace, blocking
// if necessary to ensure all pointers from all queues and caches have
// been retrieved.  get returns 0 if there are no pointers remaining.
//go:nowritebarrier
func (w *gcWork) get() uintptr {
	wbuf := w.wbuf1.ptr()
	if wbuf == nil {
		w.init()
		wbuf = w.wbuf1.ptr()
		// wbuf is empty at this point.
	}
	if wbuf.nobj == 0 {
		w.wbuf1, w.wbuf2 = w.wbuf2, w.wbuf1
		wbuf = w.wbuf1.ptr()
		if wbuf.nobj == 0 {
			owbuf := wbuf
			wbuf = getfull()
			if wbuf == nil {
				return 0
			}
			putempty(owbuf)
			w.wbuf1 = wbufptrOf(wbuf)
		}
	}

	// TODO: This might be a good place to add prefetch code

	wbuf.nobj--
	return wbuf.obj[wbuf.nobj]
}

// dispose returns any cached pointers to the global queue.
// The buffers are being put on the full queue so that the
// write barriers will not simply reacquire them before the
// GC can inspect them. This helps reduce the mutator's
// ability to hide pointers during the concurrent mark phase.
//
//go:nowritebarrier
func (w *gcWork) dispose() {
	if wbuf := w.wbuf1.ptr(); wbuf != nil {
		if wbuf.nobj == 0 {
			putempty(wbuf)
		} else {
			putfull(wbuf)
		}
		w.wbuf1 = 0

		wbuf = w.wbuf2.ptr()
		if wbuf.nobj == 0 {
			putempty(wbuf)
		} else {
			putfull(wbuf)
		}
		w.wbuf2 = 0
	}
	if w.bytesMarked != 0 {
		// dispose happens relatively infrequently. If this
		// atomic becomes a problem, we should first try to
		// dispose less and if necessary aggregate in a per-P
		// counter.
		atomic.Xadd64(&work.bytesMarked, int64(w.bytesMarked))
		w.bytesMarked = 0
	}
	if w.scanWork != 0 {
		atomic.Xaddint64(&gcController.scanWork, w.scanWork)
		w.scanWork = 0
	}
}

// balance moves some work that's cached in this gcWork back on the
// global queue.
//go:nowritebarrier
func (w *gcWork) balance() {
	if w.wbuf1 == 0 {
		return
	}
	if wbuf := w.wbuf2.ptr(); wbuf.nobj != 0 {
		putfull(wbuf)
		w.wbuf2 = wbufptrOf(getempty())
	} else if wbuf := w.wbuf1.ptr(); wbuf.nobj > 4 {
		w.wbuf1 = wbufptrOf(handoff(wbuf))
	} else {
		return
	}
	// We flushed a buffer to the full list, so wake a worker.
	if gcphase == _GCmark {
		gcController.enlistWorker()
	}
}

// empty returns true if w has no mark work available.
//go:nowritebarrier
func (w *gcWork) empty() bool {
	return w.wbuf1 == 0 || (w.wbuf1.ptr().nobj == 0 && w.wbuf2.ptr().nobj == 0)
}

// Internally, the GC work pool is kept in arrays in work buffers.
// The gcWork interface caches a work buffer until full (or empty) to
// avoid contending on the global work buffer lists.

type workbufhdr struct {
	node lfnode // must be first
	nobj int
}

//go:notinheap
type workbuf struct {
	workbufhdr
	// account for the above fields
	obj [(_WorkbufSize - unsafe.Sizeof(workbufhdr{})) / sys.PtrSize]uintptr
}

// workbuf factory routines. These funcs are used to manage the
// workbufs.
// If the GC asks for some work these are the only routines that
// make wbufs available to the GC.

func (b *workbuf) checknonempty() {
	if b.nobj == 0 {
		throw("workbuf is empty")
	}
}

func (b *workbuf) checkempty() {
	if b.nobj != 0 {
		throw("workbuf is not empty")
	}
}

// getempty pops an empty work buffer off the work.empty list,
// allocating new buffers if none are available.
//go:nowritebarrier
func getempty() *workbuf {
	var b *workbuf
	if work.empty != 0 {
		b = (*workbuf)(lfstackpop(&work.empty))
		if b != nil {
			b.checkempty()
		}
	}
	if b == nil {
		b = (*workbuf)(persistentalloc(unsafe.Sizeof(*b), sys.CacheLineSize, &memstats.gc_sys))
	}
	return b
}

// putempty puts a workbuf onto the work.empty list.
// Upon entry this go routine owns b. The lfstackpush relinquishes ownership.
//go:nowritebarrier
func putempty(b *workbuf) {
	b.checkempty()
	lfstackpush(&work.empty, &b.node)
}

// putfull puts the workbuf on the work.full list for the GC.
// putfull accepts partially full buffers so the GC can avoid competing
// with the mutators for ownership of partially full buffers.
//go:nowritebarrier
func putfull(b *workbuf) {
	b.checknonempty()
	lfstackpush(&work.full, &b.node)
}

// trygetfull tries to get a full or partially empty workbuffer.
// If one is not immediately available return nil
//go:nowritebarrier
func trygetfull() *workbuf {
	b := (*workbuf)(lfstackpop(&work.full))
	if b != nil {
		b.checknonempty()
		return b
	}
	return b
}

// Get a full work buffer off the work.full list.
// If nothing is available wait until all the other gc helpers have
// finished and then return nil.
// getfull acts as a barrier for work.nproc helpers. As long as one
// gchelper is actively marking objects it
// may create a workbuffer that the other helpers can work on.
// The for loop either exits when a work buffer is found
// or when _all_ of the work.nproc GC helpers are in the loop
// looking for work and thus not capable of creating new work.
// This is in fact the termination condition for the STW mark
// phase.
//go:nowritebarrier
func getfull() *workbuf {
	b := (*workbuf)(lfstackpop(&work.full))
	if b != nil {
		b.checknonempty()
		return b
	}

	incnwait := atomic.Xadd(&work.nwait, +1)
	if incnwait > work.nproc {
		println("runtime: work.nwait=", incnwait, "work.nproc=", work.nproc)
		throw("work.nwait > work.nproc")
	}
	for i := 0; ; i++ {
		if work.full != 0 {
			decnwait := atomic.Xadd(&work.nwait, -1)
			if decnwait == work.nproc {
				println("runtime: work.nwait=", decnwait, "work.nproc=", work.nproc)
				throw("work.nwait > work.nproc")
			}
			b = (*workbuf)(lfstackpop(&work.full))
			if b != nil {
				b.checknonempty()
				return b
			}
			incnwait := atomic.Xadd(&work.nwait, +1)
			if incnwait > work.nproc {
				println("runtime: work.nwait=", incnwait, "work.nproc=", work.nproc)
				throw("work.nwait > work.nproc")
			}
		}
		if work.nwait == work.nproc && work.markrootNext >= work.markrootJobs {
			return nil
		}
		_g_ := getg()
		if i < 10 {
			_g_.m.gcstats.nprocyield++
			procyield(20)
		} else if i < 20 {
			_g_.m.gcstats.nosyield++
			osyield()
		} else {
			_g_.m.gcstats.nsleep++
			usleep(100)
		}
	}
}

//go:nowritebarrier
func handoff(b *workbuf) *workbuf {
	// Make new buffer with half of b's pointers.
	b1 := getempty()
	n := b.nobj / 2
	b.nobj -= n
	b1.nobj = n
	memmove(unsafe.Pointer(&b1.obj[0]), unsafe.Pointer(&b.obj[b.nobj]), uintptr(n)*unsafe.Sizeof(b1.obj[0]))
	_g_ := getg()
	_g_.m.gcstats.nhandoff++
	_g_.m.gcstats.nhandoffcnt += uint64(n)

	// Put b on full list - let first half of b get stolen.
	putfull(b)
	return b1
}