cgroupv1/systemd: rework Apply/joinCgroups

We call joinCgroups() from Apply, and in there we iterate through the
list of subsystems, calling getSubsystemPath() for each. This is
expensive, since every getSubsystemPath() involves parsing mountinfo.

At the end of Apply(), we iterate through the list of subsystems to fill
the m.paths, again calling getSubsystemPath() for every subsystem.

As a result, we parse mountinfo about 20 times here.

Let's find the paths first and reuse m.paths in joinCgroups().

Signed-off-by: Kir Kolyshkin <[email protected]>

libcontainer/cgroups/systemd/v1.go

@@ -204,14 +204,16 @@ func (m *legacyManager) Apply(pid int) error {
 		return err
 	}
 
-	if err := joinCgroups(c, pid); err != nil {
-		return err
-	}
-
 	paths := make(map[string]string)
 	for _, s := range legacySubsystems {
 		subsystemPath, err := getSubsystemPath(m.cgroups, s.Name())
 		if err != nil {
+			// Even if it's `not found` error, we'll return err
+			// because devices cgroup is hard requirement for
+			// container security.
+			if s.Name() == "devices" {
+				return err
+			}
 			// Don't fail if a cgroup hierarchy was not found, just skip this subsystem
 			if cgroupv1.IsNotFound(err) {
 				continue
@@ -221,6 +223,11 @@ func (m *legacyManager) Apply(pid int) error {
 		paths[s.Name()] = subsystemPath
 	}
 	m.paths = paths
+
+	if err := m.joinCgroups(pid); err != nil {
+		return err
+	}
+
 	return nil
 }
 
@@ -249,48 +256,25 @@ func (m *legacyManager) Path(subsys string) string {
 	return m.paths[subsys]
 }
 
-func join(c *configs.Cgroup, subsystem string, pid int) (string, error) {
-	path, err := getSubsystemPath(c, subsystem)
-	if err != nil {
-		return "", err
-	}
-
-	if err := os.MkdirAll(path, 0755); err != nil {
-		return "", err
-	}
-	if err := cgroups.WriteCgroupProc(path, pid); err != nil {
-		return "", err
-	}
-	return path, nil
-}
-
-func joinCgroups(c *configs.Cgroup, pid int) error {
+func (m *legacyManager) joinCgroups(pid int) error {
 	for _, sys := range legacySubsystems {
 		name := sys.Name()
 		switch name {
 		case "name=systemd":
 			// let systemd handle this
 		case "cpuset":
-			path, err := getSubsystemPath(c, name)
-			if err != nil && !cgroupv1.IsNotFound(err) {
-				return err
-			}
-			s := &fs.CpusetGroup{}
-			if err := s.ApplyDir(path, c, pid); err != nil {
-				return err
+			if path, ok := m.paths[name]; ok {
+				s := &fs.CpusetGroup{}
+				if err := s.ApplyDir(path, m.cgroups, pid); err != nil {
+					return err
+				}
 			}
 		default:
-			_, err := join(c, name, pid)
-			if err != nil {
-				// Even if it's `not found` error, we'll return err
-				// because devices cgroup is hard requirement for
-				// container security.
-				if name == "devices" {
+			if path, ok := m.paths[name]; ok {
+				if err := os.MkdirAll(path, 0755); err != nil {
 					return err
 				}
-				// For other subsystems, omit the `not found` error
-				// because they are optional.
-				if !cgroupv1.IsNotFound(err) {
+				if err := cgroups.WriteCgroupProc(path, pid); err != nil {
 					return err
 				}
 			}