Fix cpuset issue with cpuset.cpu_exclusive

This PR fix issue in this scenario:

```
in terminal 1:
~# cd /sys/fs/cgroup/cpuset
~# mkdir test
~# cd test
~# cat cpuset.cpus
0-3
~# echo 1 > cpuset.cpu_exclusive (make sure you don't have other cgroups under root)

in terminal 2:
~# echo $$ > /sys/fs/cgroup/cpuset/test/tasks
// set resources.cpu.cpus="0-2" in config.json
~# runc run test1

back to terminal 1:
~# cd test1
~# cat cpuset.cpus
0-2
~# echo 1 > cpuset.cpu_exclusive

in terminal 3:
~# echo $$ > /sys/fs/cgroup/test/tasks
// set resources.cpu.cpus="3" in config.json
~# runc run test2
container_linux.go:247: starting container process caused "process_linux.go:258:
applying cgroup configuration for process caused \"failed to write 0-3\\n to
cpuset.cpus: write /sys/fs/cgroup/cpuset/test2/cpuset.cpus: invalid argument\""
```

Signed-off-by: Qiang Huang <[email protected]>

libcontainer/cgroups/fs/cpuset.go

@@ -61,9 +61,26 @@ func (s *CpusetGroup) ApplyDir(dir string, cgroup *configs.Cgroup, pid int) erro
 	if err != nil {
 		return err
 	}
-	if err := s.ensureParent(dir, root); err != nil {
+	// 'ensureParent' start with parent because we don't want to
+	// explicitly inherit from parent, it could conflict with
+	// 'cpuset.cpu_exclusive'.
+	if err := s.ensureParent(filepath.Dir(dir), root); err != nil {
 		return err
 	}
+	if err := os.MkdirAll(dir, 0755); err != nil {
+		return err
+	}
+	// We didn't inherit cpuset configs from parent, but we have
+	// to ensure cpuset configs are set before moving task into the
+	// cgroup.
+	// The logic is, if user specified cpuset configs, use these
+	// specified configs, otherwise, inherit from parent. This makes
+	// cpuset configs work correctly with 'cpuset.cpu_exclusive', and
+	// keep backward compatbility.
+	if err := s.ensureCpusAndMems(dir, cgroup); err != nil {
+		return err
+	}
+
 	// because we are not using d.join we need to place the pid into the procs file
 	// unlike the other subsystems
 	if err := cgroups.WriteCgroupProc(dir, pid); err != nil {
@@ -136,3 +153,10 @@ func (s *CpusetGroup) copyIfNeeded(current, parent string) error {
 func (s *CpusetGroup) isEmpty(b []byte) bool {
 	return len(bytes.Trim(b, "\n")) == 0
 }
+
+func (s *CpusetGroup) ensureCpusAndMems(path string, cgroup *configs.Cgroup) error {
+	if err := s.Set(path, cgroup); err != nil {
+		return err
+	}
+	return s.copyIfNeeded(path, filepath.Dir(path))
+}