libct/cgroups/fs.GetStats: drop PathExists check

Half of controllers' GetStats just return nil, and most of the others
ignore ENOENT on files, so it will be cheaper to not check that the
path exists in the main GetStats method, offloading that to the
controllers.

Drop PathExists check from GetStats, add it to those controllers'
GetStats where it was missing.

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

libcontainer/cgroups/fs/cpuacct.go

@@ -54,6 +54,9 @@ func (s *CpuacctGroup) Set(path string, cgroup *configs.Cgroup) error {
 }
 
 func (s *CpuacctGroup) GetStats(path string, stats *cgroups.Stats) error {
+	if !cgroups.PathExists(path) {
+		return nil
+	}
 	userModeUsage, kernelModeUsage, err := getCpuUsageBreakdown(path)
 	if err != nil {
 		return err

libcontainer/cgroups/fs/fs.go

@@ -250,7 +250,7 @@ func (m *manager) GetStats() (*cgroups.Stats, error) {
 	stats := cgroups.NewStats()
 	for name, path := range m.paths {
 		sys, err := m.getSubsystems().Get(name)
-		if err == errSubsystemDoesNotExist || !cgroups.PathExists(path) {
+		if err == errSubsystemDoesNotExist {
 			continue
 		}
 		if err := sys.GetStats(path, stats); err != nil {

libcontainer/cgroups/fs/hugetlb.go

@@ -39,6 +39,9 @@ func (s *HugetlbGroup) Set(path string, cgroup *configs.Cgroup) error {
 
 func (s *HugetlbGroup) GetStats(path string, stats *cgroups.Stats) error {
 	hugetlbStats := cgroups.HugetlbStats{}
+	if !cgroups.PathExists(path) {
+		return nil
+	}
 	for _, pageSize := range HugePageSizes {
 		usage := strings.Join([]string{"hugetlb", pageSize, "usage_in_bytes"}, ".")
 		value, err := fscommon.GetCgroupParamUint(path, usage)

libcontainer/cgroups/fs/pids.go

@@ -45,6 +45,9 @@ func (s *PidsGroup) Set(path string, cgroup *configs.Cgroup) error {
 }
 
 func (s *PidsGroup) GetStats(path string, stats *cgroups.Stats) error {
+	if !cgroups.PathExists(path) {
+		return nil
+	}
 	current, err := fscommon.GetCgroupParamUint(path, "pids.current")
 	if err != nil {
 		return fmt.Errorf("failed to parse pids.current - %s", err)