libct/intelrdt: elide parsing mountinfo

The intelrdt package needs to parse mountinfo for two things: to find
the mountpoint of the resctrl filesystem, and to check if the user
enabled the MBA Software Controller. Users are generally going to mount
the resctrl filesystem to the pre-created /sys/fs/resctrl directory and
neither libcontainer nor the rest of runc need to know whether the MBA
Software Controller is enabled, 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 it using a
single statfs syscall.

Signed-off-by: Cory Snider <[email protected]>

Author: corhere

libcontainer/intelrdt/intelrdt.go

@@ -181,8 +181,6 @@ var (
 	catEnabled bool
 	// The flag to indicate if Intel RDT/MBA is enabled
 	mbaEnabled bool
-	// The flag to indicate if Intel RDT/MBA Software Controller is enabled
-	mbaScEnabled bool
 
 	// For Intel RDT initialization
 	initOnce sync.Once
@@ -207,12 +205,7 @@ func featuresInit() {
 		if _, err := os.Stat(filepath.Join(root, "info", "L3")); err == nil {
 			catEnabled = true
 		}
-		if mbaScEnabled {
-			// We confirm MBA Software Controller is enabled in step 1,
-			// MBA should be enabled because MBA Software Controller
-			// depends on MBA
-			mbaEnabled = true
-		} else if _, err := os.Stat(filepath.Join(root, "info", "MB")); err == nil {
+		if _, err := os.Stat(filepath.Join(root, "info", "MB")); err == nil {
 			mbaEnabled = true
 		}
 		if _, err := os.Stat(filepath.Join(root, "info", "L3_MON")); err != nil {
@@ -231,28 +224,36 @@ func featuresInit() {
 	})
 }
 
-// Return the mount point path of 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
-		if m.FSType == "resctrl" {
-			return false, true // don't skip, stop
+// for findResctrlMountinfo() use only.
+var (
+	resctrlMountinfo  *mountinfo.Info
+	findMountinfoErr  error
+	findMountinfoOnce sync.Once
+)
+
+// findResctrlMountinfo returns the mount info for the mounted Intel RDT "resource control" filesystem.
+func findResctrlMountinfo() (*mountinfo.Info, error) {
+	findMountinfoOnce.Do(func() {
+		mi, err := mountinfo.GetMounts(func(m *mountinfo.Info) (bool, bool) {
+			// similar to mountinfo.FSTypeFilter but stops after the first match
+			if m.FSType == "resctrl" {
+				return false, true // don't skip, stop
+			}
+			return true, false // skip, keep going
+		})
+		if err != nil {
+			findMountinfoErr = err
+			return
+		}
+		if len(mi) < 1 {
+			findMountinfoErr = errNotFound
+			return
 		}
-		return true, false // skip, keep going
-	})
-	if err != nil {
-		return "", err
-	}
-	if len(mi) < 1 {
-		return "", errNotFound
-	}
 
-	// Check if MBA Software Controller is enabled through mount option "-o mba_MBps"
-	if strings.Contains(","+mi[0].VFSOptions+",", ",mba_MBps,") {
-		mbaScEnabled = true
-	}
+		resctrlMountinfo = mi[0]
+	})
 
-	return mi[0].Mountpoint, nil
+	return resctrlMountinfo, findMountinfoErr
 }
 
 // For Root() use only.
@@ -262,24 +263,37 @@ 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
+		}
+
+		// Has the resctrl fs been mounted to the default mount point?
+		if statfs.Type == unix.RDTGROUP_SUPER_MAGIC {
+			intelRdtRoot = defaultResctrlMountpoint
 			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()
+		// The resctrl fs could have been mounted somewhere nonstandard.
+		mi, err := findResctrlMountinfo()
 		if err != nil {
 			intelRdtRootErr = err
 			return
 		}
-
-		intelRdtRoot = root
+		intelRdtRoot = mi.Mountpoint
 	})
 
 	return intelRdtRoot, intelRdtRootErr
@@ -430,8 +444,14 @@ func IsMBAEnabled() bool {
 
 // Check if Intel RDT/MBA Software Controller is enabled
 func IsMBAScEnabled() bool {
-	featuresInit()
-	return mbaScEnabled
+	// Neither libcontainer nor runc need to check, and parsing mountinfo is
+	// the only way to check it. Parse it lazily.
+	mi, err := findResctrlMountinfo()
+	if err != nil {
+		return false
+	}
+	// Check if MBA Software Controller is enabled through mount option "-o mba_MBps"
+	return strings.Contains(","+mi.VFSOptions+",", ",mba_MBps,")
 }
 
 // Get the path of the clos group in "resource control" filesystem that the container belongs to