Cgroups: check for cpuset.cpus before set the value

Assume we put some CPUs offline and then bring them online again, since
there is not a mechanism that to help docker to refresh the state of CPUs,
all the following containers will never use those off-and-on CPUs again.

So we should always check whether there has already a change of CPUs before
we set it to the container.

Signed-off-by: Hu Keping <hukeping@huawei.com>

libcontainer/cgroups/fs/cpuset.go

@@ -26,6 +26,11 @@ func (s *CpusetGroup) Apply(d *data) error {
 
 func (s *CpusetGroup) Set(path string, cgroup *configs.Cgroup) error {
 	if cgroup.CpusetCpus != "" {
+		// We should check whether there has already a change of CPUs before we specify it
+		if err := s.amend(path, cgroup); err != nil {
+			return err
+		}
+
 		if err := writeFile(path, "cpuset.cpus", cgroup.CpusetCpus); err != nil {
 			return err
 		}
@@ -133,3 +138,25 @@ 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) amend(path string, cgroup *configs.Cgroup) error {
+
+	var newCPUs string
+	var err error
+
+	parentDir := filepath.Dir(path)
+	grandParentDir := filepath.Dir(parentDir)
+
+	// Generally it should be /sys/fs/cgroup/cpuset
+	if newCPUs, err = readFile(grandParentDir, "cpuset.cpus"); err != nil {
+		return err
+	}
+	if newCPUs != cgroup.CpusetCpus {
+		// Generally it should be /sys/fs/cgroup/cpuset/docker
+		if err := writeFile(parentDir, "cpuset.cpus", newCPUs); err != nil {
+			return err
+		}
+	}
+
+	return nil
+}