Cgroup2: Reduce allocations in readHugeTlbStats

In the journey of continuing to reduce allocations in manager.Stat,
this sets its eyes on optimizing readHugeTlbStats. The number of allocs
shaved off here is tied to the number of files in the cgroup directory
as f.Readdir(-1) was being called which returns a slice of interfaces.
The change here is to just swap to f.Readdirnames as we were only using
the os.FileInfo's to call Name(), and that's what Readdirnames returns us.

Signed-off-by: Danny Canter <danny@dcantah.dev>

cgroup2/utils.go

@@ -389,41 +389,55 @@ func systemdUnitFromPath(path string) string {
 func readHugeTlbStats(path string) []*stats.HugeTlbStat {
 	usage := []*stats.HugeTlbStat{}
 	keyUsage := make(map[string]*stats.HugeTlbStat)
+
 	f, err := os.Open(path)
 	if err != nil {
 		return usage
 	}
-	files, err := f.Readdir(-1)
+
+	files, err := f.Readdirnames(-1)
 	f.Close()
 	if err != nil {
 		return usage
 	}
 
+	buf := make([]byte, 32)
 	for _, file := range files {
-		if strings.Contains(file.Name(), "hugetlb") &&
-			(strings.HasSuffix(file.Name(), "max") || strings.HasSuffix(file.Name(), "current")) {
-			var hugeTlb *stats.HugeTlbStat
-			var ok bool
-			fileName := strings.Split(file.Name(), ".")
+		if strings.Contains(file, "hugetlb") &&
+			(strings.HasSuffix(file, "max") || strings.HasSuffix(file, "current")) {
+			var (
+				hugeTlb *stats.HugeTlbStat
+				ok      bool
+			)
+			fileName := strings.Split(file, ".")
 			pageSize := fileName[1]
 			if hugeTlb, ok = keyUsage[pageSize]; !ok {
 				hugeTlb = &stats.HugeTlbStat{}
 			}
 			hugeTlb.Pagesize = pageSize
-			out, err := os.ReadFile(filepath.Join(path, file.Name()))
+
+			f, err := os.Open(filepath.Join(path, file))
 			if err != nil {
 				continue
 			}
+			defer f.Close()
+
+			n, err := f.Read(buf)
+			if err != nil {
+				continue
+			}
+
 			var value uint64
-			stringVal := strings.TrimSpace(string(out))
+			stringVal := strings.TrimSpace(string(buf[:n]))
 			if stringVal == "max" {
 				value = math.MaxUint64
 			} else {
 				value, err = strconv.ParseUint(stringVal, 10, 64)
+				if err != nil {
+					continue
+				}
 			}
-			if err != nil {
-				continue
-			}
+
 			switch fileName[2] {
 			case "max":
 				hugeTlb.Max = value