libct/cg: lazy init for HugePageSizes

I have noticed that libct/cg/fs allocates 8K during init on every runc
execution:

> init github.com/opencontainers/runc/libcontainer/cgroups/fs @1.5 ms, 0.028 ms clock, 8512 bytes, 13 allocs

Apparently this is caused by global HugePageSizes variable init, which
is only used from GetStats (i.e. it is never used by runc itself).

Remove it, and use HugePageSizes() directly instead. Make it init-once,
so that GetStats (which, I guess, is periodically called by kubernetes)
does not re-read huge page sizes over and over.

This also removes 12 allocs and 8K from libct/cg/fs init section:

> $ time GODEBUG=inittrace=1 ./runc --help 2>&1 | grep cgroups/fs
> init github.com/opencontainers/runc/libcontainer/cgroups/fs @1.5 ms, 0.003 ms clock, 16 bytes, 1 allocs

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

libcontainer/cgroups/fs/fs.go

@@ -13,25 +13,22 @@ import (
 	"github.com/opencontainers/runc/libcontainer/configs"
 )
 
-var (
-	subsystems = []subsystem{
-		&CpusetGroup{},
-		&DevicesGroup{},
-		&MemoryGroup{},
-		&CpuGroup{},
-		&CpuacctGroup{},
-		&PidsGroup{},
-		&BlkioGroup{},
-		&HugetlbGroup{},
-		&NetClsGroup{},
-		&NetPrioGroup{},
-		&PerfEventGroup{},
-		&FreezerGroup{},
-		&RdmaGroup{},
-		&NameGroup{GroupName: "name=systemd", Join: true},
-	}
-	HugePageSizes = cgroups.HugePageSizes()
-)
+var subsystems = []subsystem{
+	&CpusetGroup{},
+	&DevicesGroup{},
+	&MemoryGroup{},
+	&CpuGroup{},
+	&CpuacctGroup{},
+	&PidsGroup{},
+	&BlkioGroup{},
+	&HugetlbGroup{},
+	&NetClsGroup{},
+	&NetPrioGroup{},
+	&PerfEventGroup{},
+	&FreezerGroup{},
+	&RdmaGroup{},
+	&NameGroup{GroupName: "name=systemd", Join: true},
+}
 
 var errSubsystemDoesNotExist = errors.New("cgroup: subsystem does not exist")
 

libcontainer/cgroups/fs/hugetlb.go

@@ -29,11 +29,11 @@ func (s *HugetlbGroup) Set(path string, r *configs.Resources) error {
 }
 
 func (s *HugetlbGroup) GetStats(path string, stats *cgroups.Stats) error {
-	hugetlbStats := cgroups.HugetlbStats{}
 	if !cgroups.PathExists(path) {
 		return nil
 	}
-	for _, pageSize := range HugePageSizes {
+	hugetlbStats := cgroups.HugetlbStats{}
+	for _, pageSize := range cgroups.HugePageSizes() {
 		usage := "hugetlb." + pageSize + ".usage_in_bytes"
 		value, err := fscommon.GetCgroupParamUint(path, usage)
 		if err != nil {

libcontainer/cgroups/fs/hugetlb_test.go

@@ -31,14 +31,14 @@ func TestHugetlbSetHugetlb(t *testing.T) {
 		hugetlbAfter  = 512
 	)
 
-	for _, pageSize := range HugePageSizes {
+	for _, pageSize := range cgroups.HugePageSizes() {
 		writeFileContents(t, path, map[string]string{
 			fmt.Sprintf(limit, pageSize): strconv.Itoa(hugetlbBefore),
 		})
 	}
 
 	r := &configs.Resources{}
-	for _, pageSize := range HugePageSizes {
+	for _, pageSize := range cgroups.HugePageSizes() {
 		r.HugetlbLimit = []*configs.HugepageLimit{
 			{
 				Pagesize: pageSize,
@@ -51,7 +51,7 @@ func TestHugetlbSetHugetlb(t *testing.T) {
 		}
 	}
 
-	for _, pageSize := range HugePageSizes {
+	for _, pageSize := range cgroups.HugePageSizes() {
 		limit := fmt.Sprintf(limit, pageSize)
 		value, err := fscommon.GetCgroupParamUint(path, limit)
 		if err != nil {
@@ -65,7 +65,7 @@ func TestHugetlbSetHugetlb(t *testing.T) {
 
 func TestHugetlbStats(t *testing.T) {
 	path := tempDir(t, "hugetlb")
-	for _, pageSize := range HugePageSizes {
+	for _, pageSize := range cgroups.HugePageSizes() {
 		writeFileContents(t, path, map[string]string{
 			fmt.Sprintf(usage, pageSize):    hugetlbUsageContents,
 			fmt.Sprintf(maxUsage, pageSize): hugetlbMaxUsageContents,
@@ -80,7 +80,7 @@ func TestHugetlbStats(t *testing.T) {
 		t.Fatal(err)
 	}
 	expectedStats := cgroups.HugetlbStats{Usage: 128, MaxUsage: 256, Failcnt: 100}
-	for _, pageSize := range HugePageSizes {
+	for _, pageSize := range cgroups.HugePageSizes() {
 		expectHugetlbStatEquals(t, expectedStats, actualStats.HugetlbStats[pageSize])
 	}
 }
@@ -101,7 +101,7 @@ func TestHugetlbStatsNoUsageFile(t *testing.T) {
 
 func TestHugetlbStatsNoMaxUsageFile(t *testing.T) {
 	path := tempDir(t, "hugetlb")
-	for _, pageSize := range HugePageSizes {
+	for _, pageSize := range cgroups.HugePageSizes() {
 		writeFileContents(t, path, map[string]string{
 			fmt.Sprintf(usage, pageSize): hugetlbUsageContents,
 		})
@@ -117,7 +117,7 @@ func TestHugetlbStatsNoMaxUsageFile(t *testing.T) {
 
 func TestHugetlbStatsBadUsageFile(t *testing.T) {
 	path := tempDir(t, "hugetlb")
-	for _, pageSize := range HugePageSizes {
+	for _, pageSize := range cgroups.HugePageSizes() {
 		writeFileContents(t, path, map[string]string{
 			fmt.Sprintf(usage, pageSize): "bad",
 			maxUsage:                     hugetlbMaxUsageContents,

libcontainer/cgroups/fs2/hugetlb.go

@@ -26,10 +26,8 @@ func setHugeTlb(dirPath string, r *configs.Resources) error {
 }
 
 func statHugeTlb(dirPath string, stats *cgroups.Stats) error {
-	hugePageSizes := cgroups.HugePageSizes()
 	hugetlbStats := cgroups.HugetlbStats{}
-
-	for _, pagesize := range hugePageSizes {
+	for _, pagesize := range cgroups.HugePageSizes() {
 		value, err := fscommon.GetCgroupParamUint(dirPath, "hugetlb."+pagesize+".current")
 		if err != nil {
 			return err

libcontainer/cgroups/utils.go

@@ -302,18 +302,26 @@ func RemovePaths(paths map[string]string) (err error) {
 	return fmt.Errorf("Failed to remove paths: %v", paths)
 }
 
-func HugePageSizes() []string {
-	dir, err := os.OpenFile("/sys/kernel/mm/hugepages", unix.O_DIRECTORY|unix.O_RDONLY, 0)
-	if err != nil {
-		return nil
-	}
-	files, err := dir.Readdirnames(0)
-	dir.Close()
-	if err != nil {
-		return nil
-	}
+var (
+	hugePageSizes []string
+	initHPSOnce   sync.Once
+)
+
+func HugePageSizes() []string {
+	initHPSOnce.Do(func() {
+		dir, err := os.OpenFile("/sys/kernel/mm/hugepages", unix.O_DIRECTORY|unix.O_RDONLY, 0)
+		if err != nil {
+			return
+		}
+		files, err := dir.Readdirnames(0)
+		dir.Close()
+		if err != nil {
+			return
+		}
+
+		hugePageSizes, _ = getHugePageSizeFromFilenames(files)
+	})
 
-	hugePageSizes, _ := getHugePageSizeFromFilenames(files)
 	return hugePageSizes
 }
 

libcontainer/cgroups/utils_test.go

@@ -448,16 +448,6 @@ func TestFindCgroupMountpointAndRoot(t *testing.T) {
 	}
 }
 
-func BenchmarkHugePageSizes(b *testing.B) {
-	var output []string
-	for i := 0; i < b.N; i++ {
-		output = HugePageSizes()
-	}
-	if len(output) == 0 {
-		b.Fatal("unexpected results")
-	}
-}
-
 func BenchmarkGetHugePageSizeImpl(b *testing.B) {
 	var (
 		input  = []string{"hugepages-1048576kB", "hugepages-2048kB", "hugepages-32768kB", "hugepages-64kB"}