runtime: scavenge huge spans first

This change adds two new treap iteration types: one for large
unscavenged spans (contain at least one huge page) and one for small
unscavenged spans. This allows us to scavenge the huge spans first by
first iterating over the large ones, then the small ones.

Also, since we now depend on physHugePageSize being a power of two,
ensure that that's the case when it's retrieved from the OS.

For #30333.

Change-Id: I51662740205ad5e4905404a0856f5f2b2d2a5680
Reviewed-on: https://go-review.googlesource.com/c/go/+/174399
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

@@ -550,6 +550,7 @@ type TreapIterType treapIterType
 
 const (
 	TreapIterScav TreapIterType = TreapIterType(treapIterScav)
+	TreapIterHuge               = TreapIterType(treapIterHuge)
 	TreapIterBits               = treapIterBits
 )
 

src/runtime/mgclarge.go

@@ -273,6 +273,7 @@ type treapIterType uint8
 
 const (
 	treapIterScav treapIterType = 1 << iota // scavenged spans
+	treapIterHuge                           // spans containing at least one huge page
 	treapIterBits = iota
 )
 
@@ -318,6 +319,9 @@ func (s *mspan) treapFilter() treapIterFilter {
 	if s.scavenged {
 		have |= treapIterScav
 	}
+	if s.hugePages() > 0 {
+		have |= treapIterHuge
+	}
 	return treapIterFilter(uint32(1) << (0x1f & have))
 }
 

src/runtime/mheap.go

@@ -521,6 +521,25 @@ func (h *mheap) coalesce(s *mspan) {
 	}
 }
 
+// hugePages returns the number of aligned physical huge pages in the memory
+// regioned owned by this mspan.
+func (s *mspan) hugePages() uintptr {
+	if physHugePageSize == 0 || s.npages < physHugePageSize/pageSize {
+		return 0
+	}
+	start := s.base()
+	end := start + s.npages*pageSize
+	if physHugePageSize > pageSize {
+		// Round start and end in.
+		start = (start + physHugePageSize - 1) &^ (physHugePageSize - 1)
+		end &^= physHugePageSize - 1
+	}
+	if start < end {
+		return (end - start) / physHugePageSize
+	}
+	return 0
+}
+
 func (s *mspan) scavenge() uintptr {
 	// start and end must be rounded in, otherwise madvise
 	// will round them *out* and release more memory
@@ -1324,11 +1343,13 @@ func (h *mheap) scavengeLocked(nbytes uintptr) {
 		h.scavengeCredit -= nbytes
 		return
 	}
-	// Iterate over the unscavenged spans in the treap backwards (from highest
-	// address to lowest address) scavenging spans until we've reached our
-	// quota of nbytes.
 	released := uintptr(0)
-	for t := h.free.end(treapIterScav, 0); released < nbytes && t.valid(); {
+	// Iterate over spans with huge pages first, then spans without.
+	const mask = treapIterScav | treapIterHuge
+	for _, match := range []treapIterType{treapIterHuge, 0} {
+		// Iterate over the treap backwards (from highest address to lowest address)
+		// scavenging spans until we've reached our quota of nbytes.
+		for t := h.free.end(mask, match); released < nbytes && t.valid(); {
 			s := t.span()
 			start, end := s.physPageBounds()
 			if start >= end {
@@ -1346,6 +1367,7 @@ func (h *mheap) scavengeLocked(nbytes uintptr) {
 			t = n
 			h.free.insert(s)
 		}
+	}
 	// If we over-scavenged, turn that extra amount into credit.
 	if released > nbytes {
 		h.scavengeCredit += released - nbytes

src/runtime/os_linux.go

@@ -270,8 +270,8 @@ func getHugePageSize() uintptr {
 		return 0
 	}
 	n := read(fd, noescape(unsafe.Pointer(&numbuf[0])), int32(len(numbuf)))
-	if n <= 0 {
 	closefd(fd)
+	if n <= 0 {
 		return 0
 	}
 	l := n - 1 // remove trailing newline
@@ -279,7 +279,10 @@ func getHugePageSize() uintptr {
 	if !ok || v < 0 {
 		v = 0
 	}
-	closefd(fd)
+	if v&(v-1) != 0 {
+		// v is not a power of 2
+		return 0
+	}
 	return uintptr(v)
 }
 

src/runtime/treap_test.go

@@ -41,9 +41,11 @@ func TestTreapFilter(t *testing.T) {
 		mask, match runtime.TreapIterType
 		filter      runtime.TreapIterFilter // expected filter
 	}{
-		{0, 0, 0x3},
-		{runtime.TreapIterScav, 0, 0x1},
-		{runtime.TreapIterScav, runtime.TreapIterScav, 0x2},
+		{0, 0, 0xf},
+		{runtime.TreapIterScav, 0, 0x5},
+		{runtime.TreapIterScav, runtime.TreapIterScav, 0xa},
+		{runtime.TreapIterScav | runtime.TreapIterHuge, runtime.TreapIterHuge, 0x4},
+		{runtime.TreapIterScav | runtime.TreapIterHuge, 0, 0x1},
 		{0, runtime.TreapIterScav, 0x0},
 	}
 	for _, it := range iterTypes {