runtime: break out system-specific constants into package sys

[gostls13.git] / 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 (
        _Debugwbufs  = false // if true check wbufs consistency
        _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.
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:
//
//     var gcw gcWork
//     disable preemption
//     .. call gcw.put() to produce and gcw.get() to consume ..
//     gcw.dispose()
//     enable preemption
//
// Or from the per-P gcWork cache:
//
//     (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(101))
        wbuf2 := trygetfull(102)
        if wbuf2 == nil {
                wbuf2 = getempty(103)
        }
        w.wbuf2 = wbufptrOf(wbuf2)
}

// put enqueues a pointer for the garbage collector to trace.
// obj must point to the beginning of a heap object.
//go:nowritebarrier
func (ww *gcWork) put(obj uintptr) {
        w := (*gcWork)(noescape(unsafe.Pointer(ww))) // TODO: remove when escape analysis is fixed

        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, 132)
                        wbuf = getempty(133)
                        w.wbuf1 = wbufptrOf(wbuf)
                }
        }

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

// 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 (ww *gcWork) tryGet() uintptr {
        w := (*gcWork)(noescape(unsafe.Pointer(ww))) // TODO: remove when escape analysis is fixed

        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(167)
                        if wbuf == nil {
                                return 0
                        }
                        putempty(owbuf, 166)
                        w.wbuf1 = wbufptrOf(wbuf)
                }
        }

        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 (ww *gcWork) get() uintptr {
        w := (*gcWork)(noescape(unsafe.Pointer(ww))) // TODO: remove when escape analysis is fixed

        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(185)
                        if wbuf == nil {
                                return 0
                        }
                        putempty(owbuf, 184)
                        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, 212)
                } else {
                        putfull(wbuf, 214)
                }
                w.wbuf1 = 0

                wbuf = w.wbuf2.ptr()
                if wbuf.nobj == 0 {
                        putempty(wbuf, 218)
                } else {
                        putfull(wbuf, 220)
                }
                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, 246)
                w.wbuf2 = wbufptrOf(getempty(247))
        } else if wbuf := w.wbuf1.ptr(); wbuf.nobj > 4 {
                w.wbuf1 = wbufptrOf(handoff(wbuf))
        }
}

// 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
        inuse bool   // This workbuf is in use by some gorotuine and is not on the work.empty/full queues.
        log   [4]int // line numbers forming a history of ownership changes to workbuf
}

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.
// Each of the gets and puts also take an distinct integer that is used
// to record a brief history of changes to ownership of the workbuf.
// The convention is to use a unique line number but any encoding
// is permissible. For example if you want to pass in 2 bits of information
// you could simple add lineno1*100000+lineno2.

// logget records the past few values of entry to aid in debugging.
// logget checks the buffer b is not currently in use.
func (b *workbuf) logget(entry int) {
        if !_Debugwbufs {
                return
        }
        if b.inuse {
                println("runtime: logget fails log entry=", entry,
                        "b.log[0]=", b.log[0], "b.log[1]=", b.log[1],
                        "b.log[2]=", b.log[2], "b.log[3]=", b.log[3])
                throw("logget: get not legal")
        }
        b.inuse = true
        copy(b.log[1:], b.log[:])
        b.log[0] = entry
}

// logput records the past few values of entry to aid in debugging.
// logput checks the buffer b is currently in use.
func (b *workbuf) logput(entry int) {
        if !_Debugwbufs {
                return
        }
        if !b.inuse {
                println("runtime: logput fails log entry=", entry,
                        "b.log[0]=", b.log[0], "b.log[1]=", b.log[1],
                        "b.log[2]=", b.log[2], "b.log[3]=", b.log[3])
                throw("logput: put not legal")
        }
        b.inuse = false
        copy(b.log[1:], b.log[:])
        b.log[0] = entry
}

func (b *workbuf) checknonempty() {
        if b.nobj == 0 {
                println("runtime: nonempty check fails",
                        "b.log[0]=", b.log[0], "b.log[1]=", b.log[1],
                        "b.log[2]=", b.log[2], "b.log[3]=", b.log[3])
                throw("workbuf is empty")
        }
}

func (b *workbuf) checkempty() {
        if b.nobj != 0 {
                println("runtime: empty check fails",
                        "b.log[0]=", b.log[0], "b.log[1]=", b.log[1],
                        "b.log[2]=", b.log[2], "b.log[3]=", b.log[3])
                throw("workbuf is not empty")
        }
}

// getempty pops an empty work buffer off the work.empty list,
// allocating new buffers if none are available.
// entry is used to record a brief history of ownership.
//go:nowritebarrier
func getempty(entry int) *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))
        }
        b.logget(entry)
        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, entry int) {
        b.checkempty()
        b.logput(entry)
        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, entry int) {
        b.checknonempty()
        b.logput(entry)
        lfstackpush(&work.full, &b.node)

        // We just made more work available. Let the GC controller
        // know so it can encourage more workers to run.
        if gcphase == _GCmark {
                gcController.enlistWorker()
        }
}

// trygetfull tries to get a full or partially empty workbuffer.
// If one is not immediately available return nil
//go:nowritebarrier
func trygetfull(entry int) *workbuf {
        b := (*workbuf)(lfstackpop(&work.full))
        if b != nil {
                b.logget(entry)
                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(entry int) *workbuf {
        b := (*workbuf)(lfstackpop(&work.full))
        if b != nil {
                b.logget(entry)
                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.logget(entry)
                                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(915)
        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, 942)
        return b1
}