runtime: add per-p page allocation cache

This change adds a per-p free page cache which the page allocator may
allocate out of without a lock. The change also introduces a completely
lockless page allocator fast path.

Although the cache contains at most 64 pages (and usually less), the
vast majority (85%+) of page allocations are exactly 1 page in size.

Updates #35112.

Change-Id: I170bf0a9375873e7e3230845eb1df7e5cf741b78
Reviewed-on: https://go-review.googlesource.com/c/go/+/195701
Run-TryBot: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Austin Clements <austin@google.com>

src/runtime/export_test.go

@@ -7,6 +7,7 @@
 package runtime
 
 import (
+	"math/bits"
 	"runtime/internal/atomic"
 	"runtime/internal/sys"
 	"unsafe"
@@ -358,6 +359,10 @@ func ReadMemStatsSlow() (base, slow MemStats) {
 			pg := mheap_.pages.chunks[i].scavenged.popcntRange(0, pallocChunkPages)
 			slow.HeapReleased += uint64(pg) * pageSize
 		}
+		for _, p := range allp {
+			pg := bits.OnesCount64(p.pcache.scav)
+			slow.HeapReleased += uint64(pg) * pageSize
+		}
 
 		// Unused space in the current arena also counts as released space.
 		slow.HeapReleased += uint64(mheap_.curArena.end - mheap_.curArena.base)
@@ -879,3 +884,20 @@ func CheckScavengedBitsCleared(mismatches []BitsMismatch) (n int, ok bool) {
 	})
 	return
 }
+
+func PageCachePagesLeaked() (leaked uintptr) {
+	stopTheWorld("PageCachePagesLeaked")
+
+	// Walk over destroyed Ps and look for unflushed caches.
+	deadp := allp[len(allp):cap(allp)]
+	for _, p := range deadp {
+		// Since we're going past len(allp) we may see nil Ps.
+		// Just ignore them.
+		if p != nil {
+			leaked += uintptr(bits.OnesCount64(p.pcache.cache))
+		}
+	}
+
+	startTheWorld()
+	return
+}

src/runtime/malloc_test.go

@@ -168,6 +168,14 @@ func TestTinyAlloc(t *testing.T) {
 	}
 }
 
+func TestPageCacheLeak(t *testing.T) {
+	defer GOMAXPROCS(GOMAXPROCS(1))
+	leaked := PageCachePagesLeaked()
+	if leaked != 0 {
+		t.Fatalf("found %d leaked pages in page caches", leaked)
+	}
+}
+
 func TestPhysicalMemoryUtilization(t *testing.T) {
 	got := runTestProg(t, "testprog", "GCPhys")
 	want := "OK\n"

src/runtime/mheap.go

@@ -1073,28 +1073,60 @@ func (h *mheap) allocSpan(npages uintptr, manual bool, spanclass spanClass, sysS
 	gp := getg()
 	base, scav := uintptr(0), uintptr(0)
 
-	// Try to allocate a cached span.
-	s = h.tryAllocMSpan()
+	// If the allocation is small enough, try the page cache!
+	pp := gp.m.p.ptr()
+	if pp != nil && npages < pageCachePages/4 {
+		c := &pp.pcache
 
-	// We failed to do what we need to do without the lock.
+		// If the cache is empty, refill it.
+		if c.empty() {
+			lock(&h.lock)
+			*c = h.pages.allocToCache()
+			unlock(&h.lock)
+		}
+
+		// Try to allocate from the cache.
+		base, scav = c.alloc(npages)
+		if base != 0 {
+			s = h.tryAllocMSpan()
+
+			if s != nil && gcBlackenEnabled == 0 && (manual || spanclass.sizeclass() != 0) {
+				goto HaveSpan
+			}
+			// We're either running duing GC, failed to acquire a mspan,
+			// or the allocation is for a large object. This means we
+			// have to lock the heap and do a bunch of extra work,
+			// so go down the HaveBaseLocked path.
+			//
+			// We must do this during GC to avoid skew with heap_scan
+			// since we flush mcache stats whenever we lock.
+			//
+			// TODO(mknyszek): It would be nice to not have to
+			// lock the heap if it's a large allocation, but
+			// it's fine for now. The critical section here is
+			// short and large object allocations are relatively
+			// infrequent.
+		}
+	}
+
+	// For one reason or another, we couldn't get the
+	// whole job done without the heap lock.
 	lock(&h.lock)
 
-	// Try to acquire a base address.
-	base, scav = h.pages.alloc(npages)
-	if base != 0 {
-		goto HaveBase
+	if base == 0 {
+		// Try to acquire a base address.
+		base, scav = h.pages.alloc(npages)
+		if base == 0 {
+			if !h.grow(npages) {
+				unlock(&h.lock)
+				return nil
+			}
+			base, scav = h.pages.alloc(npages)
+			if base == 0 {
+				throw("grew heap, but no adequate free space found")
+			}
+		}
 	}
-	if !h.grow(npages) {
-		unlock(&h.lock)
-		return nil
-	}
-	base, scav = h.pages.alloc(npages)
-	if base != 0 {
-		goto HaveBase
-	}
-	throw("grew heap, but no adequate free space found")
-
-HaveBase:
 	if s == nil {
 		// We failed to get an mspan earlier, so grab
 		// one now that we have the heap lock.
@@ -1124,7 +1156,9 @@ HaveBase:
 	}
 	unlock(&h.lock)
 
-	// Initialize the span.
+HaveSpan:
+	// At this point, both s != nil and base != 0, and the heap
+	// lock is no longer held. Initialize the span.
 	s.init(base, npages)
 	if h.allocNeedsZero(base, npages) {
 		s.needzero = 1

src/runtime/proc.go

@@ -4088,6 +4088,7 @@ func (pp *p) destroy() {
 			mheap_.spanalloc.free(unsafe.Pointer(pp.mspancache.buf[i]))
 		}
 		pp.mspancache.len = 0
+		pp.pcache.flush(&mheap_.pages)
 	})
 	freemcache(pp.mcache)
 	pp.mcache = nil

src/runtime/runtime2.go

@@ -555,6 +555,7 @@ type p struct {
 	sysmontick  sysmontick // last tick observed by sysmon
 	m           muintptr   // back-link to associated m (nil if idle)
 	mcache      *mcache
+	pcache      pageCache
 	raceprocctx uintptr
 
 	deferpool    [5][]*_defer // pool of available defer structs of different sizes (see panic.go)
@@ -611,7 +612,7 @@ type p struct {
 
 	palloc persistentAlloc // per-P to avoid mutex
 
-	// _ uint32 // Alignment for atomic fields below
+	_ uint32 // Alignment for atomic fields below
 
 	// Per-P GC state
 	gcAssistTime         int64    // Nanoseconds in assistAlloc