libct/intelrdt: elide parsing mountinfo

The intelrdt package only needs to parse mountinfo to find the mount
point of the resctrl filesystem. Users are generally going to mount the
resctrl filesystem to the pre-created /sys/fs/resctrl directory, so
there is a common case where mountinfo parsing is not required. Optimize
for the common case with a fast path which checks both for the existence
of the /sys/fs/resctrl directory and whether the resctrl filesystem was
mounted to that path using a single statfs syscall.

Signed-off-by: Cory Snider <csnider@mirantis.com>
(cherry picked from commit c156bde)
Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

libcontainer/intelrdt/intelrdt.go

@@ -224,7 +224,7 @@ func featuresInit() {
 	})
 }
 
-// Return the mount point path of Intel RDT "resource control" filesystem.
+// findIntelRdtMountpointDir returns the mount point of the Intel RDT "resource control" filesystem.
 func findIntelRdtMountpointDir() (string, error) {
 	mi, err := mountinfo.GetMounts(func(m *mountinfo.Info) (bool, bool) {
 		// similar to mountinfo.FSTypeFilter but stops after the first match
@@ -250,24 +250,32 @@ var (
 	rootOnce        sync.Once
 )
 
+// The kernel creates this (empty) directory if resctrl is supported by the
+// hardware and kernel. The user is responsible for mounting the resctrl
+// filesystem, and they could mount it somewhere else if they wanted to.
+const defaultResctrlMountpoint = "/sys/fs/resctrl"
+
 // Root returns the Intel RDT "resource control" filesystem mount point.
 func Root() (string, error) {
 	rootOnce.Do(func() {
-		// If resctrl is available, kernel creates this directory.
-		if unix.Access("/sys/fs/resctrl", unix.F_OK) != nil {
-			intelRdtRootErr = errNotFound
+		// Does this system support resctrl?
+		var statfs unix.Statfs_t
+		if err := unix.Statfs(defaultResctrlMountpoint, &statfs); err != nil {
+			if errors.Is(err, unix.ENOENT) {
+				err = errNotFound
+			}
+			intelRdtRootErr = err
 			return
 		}
 
-		// NB: ideally, we could just do statfs and RDTGROUP_SUPER_MAGIC check, but
-		// we have to parse mountinfo since we're also interested in mount options.
-		root, err := findIntelRdtMountpointDir()
-		if err != nil {
-			intelRdtRootErr = err
+		// Has the resctrl fs been mounted to the default mount point?
+		if statfs.Type == unix.RDTGROUP_SUPER_MAGIC {
+			intelRdtRoot = defaultResctrlMountpoint
 			return
 		}
 
-		intelRdtRoot = root
+		// The resctrl fs could have been mounted somewhere nonstandard.
+		intelRdtRoot, intelRdtRootErr = findIntelRdtMountpointDir()
 	})
 
 	return intelRdtRoot, intelRdtRootErr