Use simpler parsing of /proc/self/mountinfo for FindCgroupMountpoint

It sped up this function x10 times(because of not using Ssprintf).
It was one of two major performance drawbacks in docker, because we're parsing
this file pretty often.

Signed-off-by: Alexander Morozov <lk4d4@docker.com>

cgroups/utils.go

@@ -19,20 +19,23 @@ import (
 
 // https://www.kernel.org/doc/Documentation/cgroups/cgroups.txt
 func FindCgroupMountpoint(subsystem string) (string, error) {
-	mounts, err := mount.GetMounts()
+	f, err := os.Open("/proc/self/mountinfo")
 	if err != nil {
 		return "", err
 	}
-
-	for _, mount := range mounts {
-		if mount.Fstype == "cgroup" {
-			for _, opt := range strings.Split(mount.VfsOpts, ",") {
-				if opt == subsystem {
-					return mount.Mountpoint, nil
-				}
+	scanner := bufio.NewScanner(f)
+	for scanner.Scan() {
+		txt := scanner.Text()
+		fields := strings.Split(txt, " ")
+		for _, opt := range strings.Split(fields[len(fields)-1], ",") {
+			if opt == subsystem {
+				return fields[4], nil
 			}
 		}
 	}
+	if err := scanner.Err(); err != nil {
+		return "", err
+	}
 
 	return "", NewNotFoundError(subsystem)
 }