container: skip checking for files in non-existent directories.

Inspired by profile data from @brancz at https://pprof.me/5dfadaf/ and
confirmed via `strace`. On cgroup v1 systems about half of Stat calls
turn out to be for files within paths that don't exist.

I have also added very basic benchmarks for getSpecInternal.

container/common/helpers.go

@@ -75,7 +75,11 @@ func getSpecInternal(cgroupPaths map[string]string, machineInfoFactory info.Mach
 		dir, err := os.Stat(cgroupPathDir)
 		if err == nil && dir.ModTime().Before(lowestTime) {
 			lowestTime = dir.ModTime()
+		} else if os.IsNotExist(err) {
+			// Directory does not exist, skip checking for files within.
+			continue
 		}
+
 		// The modified time of the cgroup directory sometimes changes whenever a subcontainer is created.
 		// eg. /docker will have creation time matching the creation of latest docker container.
 		// Use clone_children/events as a workaround as it isn't usually modified. It is only likely changed

container/common/helpers_test.go

@@ -247,3 +247,40 @@ func TestRemoveNetMetrics(t *testing.T) {
 		}
 	}
 }
+
+func BenchmarkGetSpecCgroupV2(b *testing.B) {
+	root, err := os.Getwd()
+	if err != nil {
+		b.Fatalf("getwd: %s", err)
+	}
+
+	cgroupPaths := map[string]string{
+		"": filepath.Join(root, "test_resources/cgroup_v2/test1"),
+	}
+
+	for i := 0; i < b.N; i++ {
+		_, err := getSpecInternal(cgroupPaths, &mockInfoProvider{}, false, false, true)
+		assert.Nil(b, err)
+	}
+
+}
+
+func BenchmarkGetSpecCgroupV1(b *testing.B) {
+	root, err := os.Getwd()
+	if err != nil {
+		b.Fatalf("getwd: %s", err)
+	}
+
+	cgroupPaths := map[string]string{
+		"memory": filepath.Join(root, "test_resources/cgroup_v1/test1/memory"),
+		"cpu":    filepath.Join(root, "test_resources/cgroup_v1/test1/cpu"),
+		"cpuset": filepath.Join(root, "test_resources/cgroup_v1/test1/cpuset"),
+		"pids":   filepath.Join(root, "test_resources/cgroup_v1/test1/pids"),
+	}
+
+	for i := 0; i < b.N; i++ {
+		_, err := getSpecInternal(cgroupPaths, &mockInfoProvider{}, false, false, false)
+		assert.Nil(b, err)
+	}
+
+}