From 2a322e91ecaf642f339adac794dc3a7e361342bd Mon Sep 17 00:00:00 2001
From: Kir Kolyshkin <kolyshkin@gmail.com>
Date: Wed, 3 Jun 2020 09:11:20 -0700
Subject: [PATCH] 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      | 19 ++++---------------
 libcontainer/cgroups/systemd/v1.go | 19 ++++---------------
 2 files changed, 8 insertions(+), 30 deletions(-)

diff --git a/libcontainer/cgroups/fs/fs.go b/libcontainer/cgroups/fs/fs.go
index 0f0a7a865..dd62dc1e3 100644
--- a/libcontainer/cgroups/fs/fs.go
+++ b/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 {
diff --git a/libcontainer/cgroups/systemd/v1.go b/libcontainer/cgroups/systemd/v1.go
index c3cc9c8b3..7217b0af7 100644
--- a/libcontainer/cgroups/systemd/v1.go
+++ b/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 {