runtime: flush mcaches lazily

Currently, all mcaches are flushed during STW mark termination as a
root marking job. This is currently necessary because all spans must
be out of these caches before sweeping begins to avoid races with
allocation and to ensure the spans are in the state expected by
sweeping. We do it as a root marking job because mcache flushing is
somewhat expensive and O(GOMAXPROCS) and this parallelizes the work
across the Ps. However, it's also the last remaining root marking job
performed during mark termination.

This CL moves mcache flushing out of mark termination and performs it
lazily. We keep track of the last sweepgen at which each mcache was
flushed and as each P is woken from STW, it observes that its mcache
is out-of-date and flushes it.

The introduces a complication for spans cached in stale mcaches. These
may now be observed by background or proportional sweeping or when
attempting to add a finalizer, but aren't in a stable state. For
example, they are likely to be on the wrong mcentral list. To fix
this, this CL extends the sweepgen protocol to also capture whether a
span is cached and, if so, whether or not its cache is stale. This
protocol blocks asynchronous sweeping from touching cached spans and
makes it the responsibility of mcache flushing to sweep the flushed
spans.

This eliminates the last mark termination root marking job, which
means we can now eliminate that entire infrastructure.

Updates #26903. This implements lazy mcache flushing.

Change-Id: Iadda7aabe540b2026cffc5195da7be37d5b4125e
Reviewed-on: https://go-review.googlesource.com/c/134783
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>

src/runtime/mcache.go

@@ -4,7 +4,10 @@
 
 package runtime
 
-import "unsafe"
+import (
+	"runtime/internal/atomic"
+	"unsafe"
+)
 
 // Per-thread (in Go, per-P) cache for small objects.
 // No locking needed because it is per-thread (per-P).
@@ -42,6 +45,12 @@ type mcache struct {
 	local_largefree  uintptr                  // bytes freed for large objects (>maxsmallsize)
 	local_nlargefree uintptr                  // number of frees for large objects (>maxsmallsize)
 	local_nsmallfree [_NumSizeClasses]uintptr // number of frees for small objects (<=maxsmallsize)
+
+	// flushGen indicates the sweepgen during which this mcache
+	// was last flushed. If flushGen != mheap_.sweepgen, the spans
+	// in this mcache are stale and need to the flushed so they
+	// can be swept. This is done in acquirep.
+	flushGen uint32
 }
 
 // A gclink is a node in a linked list of blocks, like mlink,
@@ -76,6 +85,7 @@ var emptymspan mspan
 func allocmcache() *mcache {
 	lock(&mheap_.lock)
 	c := (*mcache)(mheap_.cachealloc.alloc())
+	c.flushGen = mheap_.sweepgen
 	unlock(&mheap_.lock)
 	for i := range c.alloc {
 		c.alloc[i] = &emptymspan
@@ -113,9 +123,12 @@ func (c *mcache) refill(spc spanClass) {
 	if uintptr(s.allocCount) != s.nelems {
 		throw("refill of span with free space remaining")
 	}
-
 	if s != &emptymspan {
-		s.incache = false
+		// Mark this span as no longer cached.
+		if s.sweepgen != mheap_.sweepgen+3 {
+			throw("bad sweepgen in refill")
+		}
+		atomic.Store(&s.sweepgen, mheap_.sweepgen)
 	}
 
 	// Get a new cached span from the central lists.
@@ -128,6 +141,10 @@ func (c *mcache) refill(spc spanClass) {
 		throw("span has no free space")
 	}
 
+	// Indicate that this span is cached and prevent asynchronous
+	// sweeping in the next sweep phase.
+	s.sweepgen = mheap_.sweepgen + 3
+
 	c.alloc[spc] = s
 }
 
@@ -143,3 +160,26 @@ func (c *mcache) releaseAll() {
 	c.tiny = 0
 	c.tinyoffset = 0
 }
+
+// prepareForSweep flushes c if the system has entered a new sweep phase
+// since c was populated. This must happen between the sweep phase
+// starting and the first allocation from c.
+func (c *mcache) prepareForSweep() {
+	// Alternatively, instead of making sure we do this on every P
+	// between starting the world and allocating on that P, we
+	// could leave allocate-black on, allow allocation to continue
+	// as usual, use a ragged barrier at the beginning of sweep to
+	// ensure all cached spans are swept, and then disable
+	// allocate-black. However, with this approach it's difficult
+	// to avoid spilling mark bits into the *next* GC cycle.
+	sg := mheap_.sweepgen
+	if c.flushGen == sg {
+		return
+	} else if c.flushGen != sg-2 {
+		println("bad flushGen", c.flushGen, "in prepareForSweep; sweepgen", sg)
+		throw("bad flushGen")
+	}
+	c.releaseAll()
+	stackcache_clear(c)
+	atomic.Store(&c.flushGen, mheap_.sweepgen) // Synchronizes with gcStart
+}

src/runtime/mcentral.go

@@ -135,7 +135,6 @@ havespan:
 		// heap_live changed.
 		gcController.revise()
 	}
-	s.incache = true
 	freeByteBase := s.freeindex &^ (64 - 1)
 	whichByte := freeByteBase / 8
 	// Init alloc bits cache.
@@ -150,29 +149,55 @@ havespan:
 
 // Return span from an MCache.
 func (c *mcentral) uncacheSpan(s *mspan) {
-	lock(&c.lock)
-
-	s.incache = false
-
 	if s.allocCount == 0 {
 		throw("uncaching span but s.allocCount == 0")
 	}
 
 	cap := int32((s.npages << _PageShift) / s.elemsize)
 	n := cap - int32(s.allocCount)
+
+	// cacheSpan updated alloc assuming all objects on s were
+	// going to be allocated. Adjust for any that weren't. We must
+	// do this before potentially sweeping the span.
 	if n > 0 {
-		c.empty.remove(s)
-		c.nonempty.insert(s)
-		// mCentral_CacheSpan conservatively counted
-		// unallocated slots in heap_live. Undo this.
-		atomic.Xadd64(&memstats.heap_live, -int64(n)*int64(s.elemsize))
-		// cacheSpan updated alloc assuming all objects on s
-		// were going to be allocated. Adjust for any that
-		// weren't.
 		atomic.Xadd64(&c.nmalloc, -int64(n))
 	}
+
+	sg := mheap_.sweepgen
+	stale := s.sweepgen == sg+1
+	if stale {
+		// Span was cached before sweep began. It's our
+		// responsibility to sweep it.
+		//
+		// Set sweepgen to indicate it's not cached but needs
+		// sweeping. sweep will set s.sweepgen to indicate s
+		// is swept.
+		s.sweepgen = sg - 1
+		s.sweep(true)
+		// sweep may have freed objects, so recompute n.
+		n = cap - int32(s.allocCount)
+	} else {
+		// Indicate that s is no longer cached.
+		s.sweepgen = sg
+	}
+
+	if n > 0 {
+		lock(&c.lock)
+		c.empty.remove(s)
+		c.nonempty.insert(s)
+		if !stale {
+			// mCentral_CacheSpan conservatively counted
+			// unallocated slots in heap_live. Undo this.
+			//
+			// If this span was cached before sweep, then
+			// heap_live was totally recomputed since
+			// caching this span, so we don't do this for
+			// stale spans.
+			atomic.Xadd64(&memstats.heap_live, -int64(n)*int64(s.elemsize))
+		}
 		unlock(&c.lock)
 	}
+}
 
 // freeSpan updates c and s after sweeping s.
 // It sets s's sweepgen to the latest generation,
@@ -183,13 +208,13 @@ func (c *mcentral) uncacheSpan(s *mspan) {
 // If preserve=true, it does not move s (the caller
 // must take care of it).
 func (c *mcentral) freeSpan(s *mspan, preserve bool, wasempty bool) bool {
-	if s.incache {
+	if sg := mheap_.sweepgen; s.sweepgen == sg+1 || s.sweepgen == sg+3 {
 		throw("freeSpan given cached span")
 	}
 	s.needzero = 1
 
 	if preserve {
-		// preserve is set only when called from MCentral_CacheSpan above,
+		// preserve is set only when called from (un)cacheSpan above,
 		// the span must be in the empty list.
 		if !s.inList() {
 			throw("can't preserve unlinked span")

src/runtime/mgc.go

@@ -1262,6 +1262,14 @@ func gcStart(trigger gcTrigger) {
 		traceGCStart()
 	}
 
+	// Check that all Ps have finished deferred mcache flushes.
+	for _, p := range allp {
+		if fg := atomic.Load(&p.mcache.flushGen); fg != mheap_.sweepgen {
+			println("runtime: p", p.id, "flushGen", fg, "!= sweepgen", mheap_.sweepgen)
+			throw("p mcache not flushed")
+		}
+	}
+
 	gcBgMarkStartWorkers()
 
 	gcResetMarkState()
@@ -1606,6 +1614,16 @@ func gcMarkTermination(nextTriggerRatio float64) {
 	// Free stack spans. This must be done between GC cycles.
 	systemstack(freeStackSpans)
 
+	// Ensure all mcaches are flushed. Each P will flush its own
+	// mcache before allocating, but idle Ps may not. Since this
+	// is necessary to sweep all spans, we need to ensure all
+	// mcaches are flushed before we start the next GC cycle.
+	systemstack(func() {
+		forEachP(func(_p_ *p) {
+			_p_.mcache.prepareForSweep()
+		})
+	})
+
 	// Print gctrace before dropping worldsema. As soon as we drop
 	// worldsema another cycle could start and smash the stats
 	// we're trying to print.

src/runtime/mgcmark.go

@@ -52,11 +52,7 @@ const (
 //
 //go:nowritebarrier
 func gcMarkRootPrepare() {
-	if gcphase == _GCmarktermination {
-		work.nFlushCacheRoots = int(gomaxprocs)
-	} else {
 	work.nFlushCacheRoots = 0
-	}
 
 	// Compute how many data and BSS root blocks there are.
 	nBlocks := func(bytes uintptr) int {
@@ -344,7 +340,8 @@ func markrootSpans(gcw *gcWork, shard int) {
 		if s.state != mSpanInUse {
 			continue
 		}
-		if !useCheckmark && s.sweepgen != sg {
+		// Check that this span was swept (it may be cached or uncached).
+		if !useCheckmark && !(s.sweepgen == sg || s.sweepgen == sg+3) {
 			// sweepgen was updated (+2) during non-checkmark GC pass
 			print("sweep ", s.sweepgen, " ", sg, "\n")
 			throw("gc: unswept span")

src/runtime/mgcsweep.go

@@ -161,7 +161,8 @@ func (s *mspan) ensureSwept() {
 	}
 
 	sg := mheap_.sweepgen
-	if atomic.Load(&s.sweepgen) == sg {
+	spangen := atomic.Load(&s.sweepgen)
+	if spangen == sg || spangen == sg+3 {
 		return
 	}
 	// The caller must be sure that the span is a mSpanInUse span.
@@ -170,7 +171,11 @@ func (s *mspan) ensureSwept() {
 		return
 	}
 	// unfortunate condition, and we don't have efficient means to wait
-	for atomic.Load(&s.sweepgen) != sg {
+	for {
+		spangen := atomic.Load(&s.sweepgen)
+		if spangen == sg || spangen == sg+3 {
+			break
+		}
 		osyield()
 	}
 }

src/runtime/mheap.go

@@ -317,6 +317,8 @@ type mspan struct {
 	// if sweepgen == h->sweepgen - 2, the span needs sweeping
 	// if sweepgen == h->sweepgen - 1, the span is currently being swept
 	// if sweepgen == h->sweepgen, the span is swept and ready to use
+	// if sweepgen == h->sweepgen + 1, the span was cached before sweep began and is still cached, and needs sweeping
+	// if sweepgen == h->sweepgen + 3, the span was swept and then cached and is still cached
 	// h->sweepgen is incremented by 2 after every GC
 
 	sweepgen    uint32
@@ -324,7 +326,6 @@ type mspan struct {
 	baseMask    uint16     // if non-0, elemsize is a power of 2, & this will get object allocation base
 	allocCount  uint16     // number of allocated objects
 	spanclass   spanClass  // size class and noscan (uint8)
-	incache     bool       // being used by an mcache
 	state       mSpanState // mspaninuse etc
 	needzero    uint8      // needs to be zeroed before allocation
 	divShift    uint8      // for divide by elemsize - divMagic.shift
@@ -1185,7 +1186,6 @@ func (span *mspan) init(base uintptr, npages uintptr) {
 	span.npages = npages
 	span.allocCount = 0
 	span.spanclass = 0
-	span.incache = false
 	span.elemsize = 0
 	span.state = mSpanDead
 	span.unusedsince = 0

src/runtime/proc.go

@@ -4119,6 +4119,7 @@ func procresize(nprocs int32) *p {
 	if _g_.m.p != 0 && _g_.m.p.ptr().id < nprocs {
 		// continue to use the current P
 		_g_.m.p.ptr().status = _Prunning
+		_g_.m.p.ptr().mcache.prepareForSweep()
 	} else {
 		// release the current P and acquire allp[0]
 		if _g_.m.p != 0 {
@@ -4169,6 +4170,10 @@ func acquirep(_p_ *p) {
 	_g_ := getg()
 	_g_.m.mcache = _p_.mcache
 
+	// Perform deferred mcache flush before this P can allocate
+	// from a potentially stale mcache.
+	_p_.mcache.prepareForSweep()
+
 	if trace.enabled {
 		traceProcStart()
 	}