cgroupv1: remove subsystemSet.Get()

Instead of iterating over m.paths, iterate over subsystems and look up
the path for each. This is faster since a map lookup is faster than
iterating over the names in Get. A quick benchmark shows that the new
way is 2.5x faster than the old one.

Note though that this is not done to make things faster, as savings are
negligible, but to make things simpler by removing some code.

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

libcontainer/cgroups/fs/fs.go

@@ -18,7 +18,7 @@ import (
 )
 
 var (
-	subsystems = subsystemSet{
+	subsystems = []subsystem{
 		&CpusetGroup{},
 		&DevicesGroup{},
 		&MemoryGroup{},
@@ -38,17 +38,6 @@ var (
 
 var errSubsystemDoesNotExist = errors.New("cgroup: subsystem does not exist")
 
-type subsystemSet []subsystem
-
-func (s subsystemSet) Get(name string) (subsystem, error) {
-	for _, ss := range s {
-		if ss.Name() == name {
-			return ss, nil
-		}
-	}
-	return nil, errSubsystemDoesNotExist
-}
-
 type subsystem interface {
 	// Name returns the name of the subsystem.
 	Name() string
@@ -244,9 +233,9 @@ func (m *manager) GetStats() (*cgroups.Stats, error) {
 	m.mu.Lock()
 	defer m.mu.Unlock()
 	stats := cgroups.NewStats()
-	for name, path := range m.paths {
-		sys, err := subsystems.Get(name)
-		if err == errSubsystemDoesNotExist {
+	for _, sys := range subsystems {
+		path := m.paths[sys.Name()]
+		if path == "" {
 			continue
 		}
 		if err := sys.GetStats(path, stats); err != nil {

libcontainer/cgroups/systemd/v1.go

@@ -41,18 +41,7 @@ type subsystem interface {
 
 var errSubsystemDoesNotExist = errors.New("cgroup: subsystem does not exist")
 
-type subsystemSet []subsystem
-
-func (s subsystemSet) Get(name string) (subsystem, error) {
-	for _, ss := range s {
-		if ss.Name() == name {
-			return ss, nil
-		}
-	}
-	return nil, errSubsystemDoesNotExist
-}
-
-var legacySubsystems = subsystemSet{
+var legacySubsystems = []subsystem{
 	&fs.CpusetGroup{},
 	&fs.DevicesGroup{},
 	&fs.MemoryGroup{},
@@ -355,9 +344,9 @@ func (m *legacyManager) GetStats() (*cgroups.Stats, error) {
 	m.mu.Lock()
 	defer m.mu.Unlock()
 	stats := cgroups.NewStats()
-	for name, path := range m.paths {
-		sys, err := legacySubsystems.Get(name)
-		if err == errSubsystemDoesNotExist || !cgroups.PathExists(path) {
+	for _, sys := range legacySubsystems {
+		path := m.paths[sys.Name()]
+		if path == "" {
 			continue
 		}
 		if err := sys.GetStats(path, stats); err != nil {