runtime: add page cache and tests

This change adds a page cache structure which owns a chunk of free pages
at a given base address. It also adds code to allocate to this cache
from the page allocator. Finally, it adds tests for both.

Notably this change does not yet integrate the code into the runtime,
just into runtime tests.

Updates #35112.

Change-Id: Ibe121498d5c3be40390fab58a3816295601670df
Reviewed-on: https://go-review.googlesource.com/c/go/+/196643
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>

src/runtime/export_test.go

@@ -684,6 +684,25 @@ func (d *PallocData) Scavenged() *PallocBits {
 // Expose fillAligned for testing.
 func FillAligned(x uint64, m uint) uint64 { return fillAligned(x, m) }
 
+// Expose pageCache for testing.
+type PageCache pageCache
+
+const PageCachePages = pageCachePages
+
+func NewPageCache(base uintptr, cache, scav uint64) PageCache {
+	return PageCache(pageCache{base: base, cache: cache, scav: scav})
+}
+func (c *PageCache) Empty() bool   { return (*pageCache)(c).empty() }
+func (c *PageCache) Base() uintptr { return (*pageCache)(c).base }
+func (c *PageCache) Cache() uint64 { return (*pageCache)(c).cache }
+func (c *PageCache) Scav() uint64  { return (*pageCache)(c).scav }
+func (c *PageCache) Alloc(npages uintptr) (uintptr, uintptr) {
+	return (*pageCache)(c).alloc(npages)
+}
+func (c *PageCache) Flush(s *PageAlloc) {
+	(*pageCache)(c).flush((*pageAlloc)(s))
+}
+
 // Expose chunk index type.
 type ChunkIdx chunkIdx
 
@@ -694,6 +713,9 @@ type PageAlloc pageAlloc
 func (p *PageAlloc) Alloc(npages uintptr) (uintptr, uintptr) {
 	return (*pageAlloc)(p).alloc(npages)
 }
+func (p *PageAlloc) AllocToCache() PageCache {
+	return PageCache((*pageAlloc)(p).allocToCache())
+}
 func (p *PageAlloc) Free(base, npages uintptr) {
 	(*pageAlloc)(p).free(base, npages)
 }

src/runtime/mpagecache.go

+// Copyright 2019 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 (
+	"math/bits"
+	"unsafe"
+)
+
+const pageCachePages = 8 * unsafe.Sizeof(pageCache{}.cache)
+
+// pageCache represents a per-p cache of pages the allocator can
+// allocate from without a lock. More specifically, it represents
+// a pageCachePages*pageSize chunk of memory with 0 or more free
+// pages in it.
+type pageCache struct {
+	base  uintptr // base address of the chunk
+	cache uint64  // 64-bit bitmap representing free pages (1 means free)
+	scav  uint64  // 64-bit bitmap representing scavenged pages (1 means scavenged)
+}
+
+// empty returns true if the pageCache has any free pages, and false
+// otherwise.
+func (c *pageCache) empty() bool {
+	return c.cache == 0
+}
+
+// alloc allocates npages from the page cache and is the main entry
+// point for allocation.
+//
+// Returns a base address and the amount of scavenged memory in the
+// allocated region in bytes.
+//
+// Returns a base address of zero on failure, in which case the
+// amount of scavenged memory should be ignored.
+func (c *pageCache) alloc(npages uintptr) (uintptr, uintptr) {
+	if c.cache == 0 {
+		return 0, 0
+	}
+	if npages == 1 {
+		i := uintptr(bits.TrailingZeros64(c.cache))
+		scav := (c.scav >> i) & 1
+		c.cache &^= 1 << i // set bit to mark in-use
+		c.scav &^= 1 << i  // clear bit to mark unscavenged
+		return c.base + i*pageSize, uintptr(scav) * pageSize
+	}
+	return c.allocN(npages)
+}
+
+// allocN is a helper which attempts to allocate npages worth of pages
+// from the cache. It represents the general case for allocating from
+// the page cache.
+//
+// Returns a base address and the amount of scavenged memory in the
+// allocated region in bytes.
+func (c *pageCache) allocN(npages uintptr) (uintptr, uintptr) {
+	i := findBitRange64(c.cache, uint(npages))
+	if i >= 64 {
+		return 0, 0
+	}
+	mask := ((uint64(1) << npages) - 1) << i
+	scav := bits.OnesCount64(c.scav & mask)
+	c.cache &^= mask // mark in-use bits
+	c.scav &^= mask  // clear scavenged bits
+	return c.base + uintptr(i*pageSize), uintptr(scav) * pageSize
+}
+
+// flush empties out unallocated free pages in the given cache
+// into s. Then, it clears the cache, such that empty returns
+// true.
+//
+// s.mheapLock must be held or the world must be stopped.
+func (c *pageCache) flush(s *pageAlloc) {
+	if c.empty() {
+		return
+	}
+	ci := chunkIndex(c.base)
+	pi := chunkPageIndex(c.base)
+
+	// This method is called very infrequently, so just do the
+	// slower, safer thing by iterating over each bit individually.
+	for i := uint(0); i < 64; i++ {
+		if c.cache&(1<<i) != 0 {
+			s.chunks[ci].free1(pi + i)
+		}
+		if c.scav&(1<<i) != 0 {
+			s.chunks[ci].scavenged.setRange(pi+i, 1)
+		}
+	}
+	// Since this is a lot like a free, we need to make sure
+	// we update the searchAddr just like free does.
+	if s.compareSearchAddrTo(c.base) < 0 {
+		s.searchAddr = c.base
+	}
+	s.update(c.base, pageCachePages, false, false)
+	*c = pageCache{}
+}
+
+// allocToCache acquires a pageCachePages-aligned chunk of free pages which
+// may not be contiguous, and returns a pageCache structure which owns the
+// chunk.
+//
+// s.mheapLock must be held.
+func (s *pageAlloc) allocToCache() pageCache {
+	// If the searchAddr refers to a region which has a higher address than
+	// any known chunk, then we know we're out of memory.
+	if chunkIndex(s.searchAddr) >= s.end {
+		return pageCache{}
+	}
+	c := pageCache{}
+	ci := chunkIndex(s.searchAddr) // chunk index
+	if s.summary[len(s.summary)-1][ci] != 0 {
+		// Fast path: there's free pages at or near the searchAddr address.
+		j, _ := s.chunks[ci].find(1, chunkPageIndex(s.searchAddr))
+		if j < 0 {
+			throw("bad summary data")
+		}
+		c = pageCache{
+			base:  chunkBase(ci) + alignDown(uintptr(j), 64)*pageSize,
+			cache: ^s.chunks[ci].pages64(j),
+			scav:  s.chunks[ci].scavenged.block64(j),
+		}
+	} else {
+		// Slow path: the searchAddr address had nothing there, so go find
+		// the first free page the slow way.
+		addr, _ := s.find(1)
+		if addr == 0 {
+			// We failed to find adequate free space, so mark the searchAddr as OoM
+			// and return an empty pageCache.
+			s.searchAddr = maxSearchAddr
+			return pageCache{}
+		}
+		ci := chunkIndex(addr)
+		c = pageCache{
+			base:  alignDown(addr, 64*pageSize),
+			cache: ^s.chunks[ci].pages64(chunkPageIndex(addr)),
+			scav:  s.chunks[ci].scavenged.block64(chunkPageIndex(addr)),
+		}
+	}
+
+	// Set the bits as allocated and clear the scavenged bits.
+	s.allocRange(c.base, pageCachePages)
+
+	// Update as an allocation, but note that it's not contiguous.
+	s.update(c.base, pageCachePages, false, true)
+
+	// We're always searching for the first free page, and we always know the
+	// up to pageCache size bits will be allocated, so we can always move the
+	// searchAddr past the cache.
+	s.searchAddr = c.base + pageSize*pageCachePages
+	return c
+}

src/runtime/mpagecache_test.go

+// Copyright 2019 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_test
+
+import (
+	"math/rand"
+	. "runtime"
+	"testing"
+)
+
+func checkPageCache(t *testing.T, got, want PageCache) {
+	if got.Base() != want.Base() {
+		t.Errorf("bad pageCache base: got 0x%x, want 0x%x", got.Base(), want.Base())
+	}
+	if got.Cache() != want.Cache() {
+		t.Errorf("bad pageCache bits: got %016x, want %016x", got.Base(), want.Base())
+	}
+	if got.Scav() != want.Scav() {
+		t.Errorf("bad pageCache scav: got %016x, want %016x", got.Scav(), want.Scav())
+	}
+}
+
+func TestPageCacheAlloc(t *testing.T) {
+	base := PageBase(BaseChunkIdx, 0)
+	type hit struct {
+		npages uintptr
+		base   uintptr
+		scav   uintptr
+	}
+	tests := map[string]struct {
+		cache PageCache
+		hits  []hit
+	}{
+		"Empty": {
+			cache: NewPageCache(base, 0, 0),
+			hits: []hit{
+				{1, 0, 0},
+				{2, 0, 0},
+				{3, 0, 0},
+				{4, 0, 0},
+				{5, 0, 0},
+				{11, 0, 0},
+				{12, 0, 0},
+				{16, 0, 0},
+				{27, 0, 0},
+				{32, 0, 0},
+				{43, 0, 0},
+				{57, 0, 0},
+				{64, 0, 0},
+				{121, 0, 0},
+			},
+		},
+		"Lo1": {
+			cache: NewPageCache(base, 0x1, 0x1),
+			hits: []hit{
+				{1, base, PageSize},
+				{1, 0, 0},
+				{10, 0, 0},
+			},
+		},
+		"Hi1": {
+			cache: NewPageCache(base, 0x1<<63, 0x1),
+			hits: []hit{
+				{1, base + 63*PageSize, 0},
+				{1, 0, 0},
+				{10, 0, 0},
+			},
+		},
+		"Swiss1": {
+			cache: NewPageCache(base, 0x20005555, 0x5505),
+			hits: []hit{
+				{2, 0, 0},
+				{1, base, PageSize},
+				{1, base + 2*PageSize, PageSize},
+				{1, base + 4*PageSize, 0},
+				{1, base + 6*PageSize, 0},
+				{1, base + 8*PageSize, PageSize},
+				{1, base + 10*PageSize, PageSize},
+				{1, base + 12*PageSize, PageSize},
+				{1, base + 14*PageSize, PageSize},
+				{1, base + 29*PageSize, 0},
+				{1, 0, 0},
+				{10, 0, 0},
+			},
+		},
+		"Lo2": {
+			cache: NewPageCache(base, 0x3, 0x2<<62),
+			hits: []hit{
+				{2, base, 0},
+				{2, 0, 0},
+				{1, 0, 0},
+			},
+		},
+		"Hi2": {
+			cache: NewPageCache(base, 0x3<<62, 0x3<<62),
+			hits: []hit{
+				{2, base + 62*PageSize, 2 * PageSize},
+				{2, 0, 0},
+				{1, 0, 0},
+			},
+		},
+		"Swiss2": {
+			cache: NewPageCache(base, 0x3333<<31, 0x3030<<31),
+			hits: []hit{
+				{2, base + 31*PageSize, 0},
+				{2, base + 35*PageSize, 2 * PageSize},
+				{2, base + 39*PageSize, 0},
+				{2, base + 43*PageSize, 2 * PageSize},
+				{2, 0, 0},
+			},
+		},
+		"Hi53": {
+			cache: NewPageCache(base, ((uint64(1)<<53)-1)<<10, ((uint64(1)<<16)-1)<<10),
+			hits: []hit{
+				{53, base + 10*PageSize, 16 * PageSize},
+				{53, 0, 0},
+				{1, 0, 0},
+			},
+		},
+		"Full53": {
+			cache: NewPageCache(base, ^uint64(0), ((uint64(1)<<16)-1)<<10),
+			hits: []hit{
+				{53, base, 16 * PageSize},
+				{53, 0, 0},
+				{1, base + 53*PageSize, 0},
+			},
+		},
+		"Full64": {
+			cache: NewPageCache(base, ^uint64(0), ^uint64(0)),
+			hits: []hit{
+				{64, base, 64 * PageSize},
+				{64, 0, 0},
+				{1, 0, 0},
+			},
+		},
+		"FullMixed": {
+			cache: NewPageCache(base, ^uint64(0), ^uint64(0)),
+			hits: []hit{
+				{5, base, 5 * PageSize},
+				{7, base + 5*PageSize, 7 * PageSize},
+				{1, base + 12*PageSize, 1 * PageSize},
+				{23, base + 13*PageSize, 23 * PageSize},
+				{63, 0, 0},
+				{3, base + 36*PageSize, 3 * PageSize},
+				{3, base + 39*PageSize, 3 * PageSize},
+				{3, base + 42*PageSize, 3 * PageSize},
+				{12, base + 45*PageSize, 12 * PageSize},
+				{11, 0, 0},
+				{4, base + 57*PageSize, 4 * PageSize},
+				{4, 0, 0},
+				{6, 0, 0},
+				{36, 0, 0},
+				{2, base + 61*PageSize, 2 * PageSize},
+				{3, 0, 0},
+				{1, base + 63*PageSize, 1 * PageSize},
+				{4, 0, 0},
+				{2, 0, 0},
+				{62, 0, 0},
+				{1, 0, 0},
+			},
+		},
+	}
+	for name, test := range tests {
+		test := test
+		t.Run(name, func(t *testing.T) {
+			c := test.cache
+			for i, h := range test.hits {
+				b, s := c.Alloc(h.npages)
+				if b != h.base {
+					t.Fatalf("bad alloc base #%d: got 0x%x, want 0x%x", i, b, h.base)
+				}
+				if s != h.scav {
+					t.Fatalf("bad alloc scav #%d: got %d, want %d", i, s, h.scav)
+				}
+			}
+		})
+	}
+}
+
+func TestPageCacheFlush(t *testing.T) {
+	bits64ToBitRanges := func(bits uint64, base uint) []BitRange {
+		var ranges []BitRange
+		start, size := uint(0), uint(0)
+		for i := 0; i < 64; i++ {
+			if bits&(1<<i) != 0 {
+				if size == 0 {
+					start = uint(i) + base
+				}
+				size++
+			} else {
+				if size != 0 {
+					ranges = append(ranges, BitRange{start, size})
+					size = 0
+				}
+			}
+		}
+		if size != 0 {
+			ranges = append(ranges, BitRange{start, size})
+		}
+		return ranges
+	}
+	runTest := func(t *testing.T, base uint, cache, scav uint64) {
+		// Set up the before state.
+		beforeAlloc := map[ChunkIdx][]BitRange{
+			BaseChunkIdx: {{base, 64}},
+		}
+		beforeScav := map[ChunkIdx][]BitRange{
+			BaseChunkIdx: {},
+		}
+		b := NewPageAlloc(beforeAlloc, beforeScav)
+		defer FreePageAlloc(b)
+
+		// Create and flush the cache.
+		c := NewPageCache(PageBase(BaseChunkIdx, base), cache, scav)
+		c.Flush(b)
+		if !c.Empty() {
+			t.Errorf("pageCache flush did not clear cache")
+		}
+
+		// Set up the expected after state.
+		afterAlloc := map[ChunkIdx][]BitRange{
+			BaseChunkIdx: bits64ToBitRanges(^cache, base),
+		}
+		afterScav := map[ChunkIdx][]BitRange{
+			BaseChunkIdx: bits64ToBitRanges(scav, base),
+		}
+		want := NewPageAlloc(afterAlloc, afterScav)
+		defer FreePageAlloc(want)
+
+		// Check to see if it worked.
+		checkPageAlloc(t, want, b)
+	}
+
+	// Empty.
+	runTest(t, 0, 0, 0)
+
+	// Full.
+	runTest(t, 0, ^uint64(0), ^uint64(0))
+
+	// Random.
+	for i := 0; i < 100; i++ {
+		// Generate random valid base within a chunk.
+		base := uint(rand.Intn(PallocChunkPages/64)) * 64
+
+		// Generate random cache.
+		cache := rand.Uint64()
+		scav := rand.Uint64() & cache
+
+		// Run the test.
+		runTest(t, base, cache, scav)
+	}
+}
+
+func TestPageAllocAllocToCache(t *testing.T) {
+	tests := map[string]struct {
+		before map[ChunkIdx][]BitRange
+		scav   map[ChunkIdx][]BitRange
+		hits   []PageCache // expected base addresses and patterns
+		after  map[ChunkIdx][]BitRange
+	}{
+		"AllFree": {
+			before: map[ChunkIdx][]BitRange{
+				BaseChunkIdx: {},
+			},
+			scav: map[ChunkIdx][]BitRange{
+				BaseChunkIdx: {{1, 1}, {64, 64}},
+			},
+			hits: []PageCache{
+				NewPageCache(PageBase(BaseChunkIdx, 0), ^uint64(0), 0x2),
+				NewPageCache(PageBase(BaseChunkIdx, 64), ^uint64(0), ^uint64(0)),
+				NewPageCache(PageBase(BaseChunkIdx, 128), ^uint64(0), 0),
+				NewPageCache(PageBase(BaseChunkIdx, 192), ^uint64(0), 0),
+			},
+			after: map[ChunkIdx][]BitRange{
+				BaseChunkIdx: {{0, 256}},
+			},
+		},
+		"ManyArena": {
+			before: map[ChunkIdx][]BitRange{
+				BaseChunkIdx:     {{0, PallocChunkPages}},
+				BaseChunkIdx + 1: {{0, PallocChunkPages}},
+				BaseChunkIdx + 2: {{0, PallocChunkPages - 64}},
+			},
+			scav: map[ChunkIdx][]BitRange{
+				BaseChunkIdx:     {{0, PallocChunkPages}},
+				BaseChunkIdx + 1: {{0, PallocChunkPages}},
+				BaseChunkIdx + 2: {},
+			},
+			hits: []PageCache{
+				NewPageCache(PageBase(BaseChunkIdx+2, PallocChunkPages-64), ^uint64(0), 0),
+			},
+			after: map[ChunkIdx][]BitRange{
+				BaseChunkIdx:     {{0, PallocChunkPages}},
+				BaseChunkIdx + 1: {{0, PallocChunkPages}},
+				BaseChunkIdx + 2: {{0, PallocChunkPages}},
+			},
+		},
+		"NotContiguous": {
+			before: map[ChunkIdx][]BitRange{
+				BaseChunkIdx:        {{0, PallocChunkPages}},
+				BaseChunkIdx + 0xff: {{0, 0}},
+			},
+			scav: map[ChunkIdx][]BitRange{
+				BaseChunkIdx:        {{0, PallocChunkPages}},
+				BaseChunkIdx + 0xff: {{31, 67}},
+			},
+			hits: []PageCache{
+				NewPageCache(PageBase(BaseChunkIdx+0xff, 0), ^uint64(0), ((uint64(1)<<33)-1)<<31),
+			},
+			after: map[ChunkIdx][]BitRange{
+				BaseChunkIdx:        {{0, PallocChunkPages}},
+				BaseChunkIdx + 0xff: {{0, 64}},
+			},
+		},
+		"First": {
+			before: map[ChunkIdx][]BitRange{
+				BaseChunkIdx: {{0, 32}, {33, 31}, {96, 32}},
+			},
+			scav: map[ChunkIdx][]BitRange{
+				BaseChunkIdx: {{1, 4}, {31, 5}, {66, 2}},
+			},
+			hits: []PageCache{
+				NewPageCache(PageBase(BaseChunkIdx, 0), 1<<32, 1<<32),
+				NewPageCache(PageBase(BaseChunkIdx, 64), (uint64(1)<<32)-1, 0x3<<2),
+			},
+			after: map[ChunkIdx][]BitRange{
+				BaseChunkIdx: {{0, 128}},
+			},
+		},
+		"Fail": {
+			before: map[ChunkIdx][]BitRange{
+				BaseChunkIdx: {{0, PallocChunkPages}},
+			},
+			hits: []PageCache{
+				NewPageCache(0, 0, 0),
+				NewPageCache(0, 0, 0),
+				NewPageCache(0, 0, 0),
+			},
+			after: map[ChunkIdx][]BitRange{
+				BaseChunkIdx: {{0, PallocChunkPages}},
+			},
+		},
+	}
+	for name, v := range tests {
+		v := v
+		t.Run(name, func(t *testing.T) {
+			b := NewPageAlloc(v.before, v.scav)
+			defer FreePageAlloc(b)
+
+			for _, expect := range v.hits {
+				checkPageCache(t, b.AllocToCache(), expect)
+				if t.Failed() {
+					return
+				}
+			}
+			want := NewPageAlloc(v.after, v.scav)
+			defer FreePageAlloc(want)
+
+			checkPageAlloc(t, want, b)
+		})
+	}
+}

src/runtime/mpallocbits.go

@@ -16,6 +16,11 @@ func (b *pageBits) get(i uint) uint {
 	return uint((b[i/64] >> (i % 64)) & 1)
 }
 
+// block64 returns the 64-bit aligned block of bits containing the i'th bit.
+func (b *pageBits) block64(i uint) uint64 {
+	return b[i/64]
+}
+
 // set sets bit i of pageBits.
 func (b *pageBits) set(i uint) {
 	b[i/64] |= 1 << (i % 64)
@@ -339,6 +344,13 @@ func (b *pallocBits) freeAll() {
 	(*pageBits)(b).clearAll()
 }
 
+// pages64 returns a 64-bit bitmap representing a block of 64 pages aligned
+// to 64 pages. The returned block of pages is the one containing the i'th
+// page in this pallocBits. Each bit represents whether the page is in-use.
+func (b *pallocBits) pages64(i uint) uint64 {
+	return (*pageBits)(b).block64(i)
+}
+
 // findBitRange64 returns the bit index of the first set of
 // n consecutive 1 bits. If no consecutive set of 1 bits of
 // size n may be found in c, then it returns an integer >= 64.