Break down groups into subsystems

Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@crosbymichael.com> (github: crosbymichael)

cgroups/fs/perf_event.go

@@ -0,0 +1,16 @@
+package fs
+
+import (
+	"github.com/dotcloud/docker/pkg/cgroups"
+)
+
+type perfEventGroup struct {
+}
+
+func (s *perfEventGroup) Set(d *data) error {
+	// we just want to join this group even though we don't set anything
+	if _, err := d.join("perf_event"); err != nil && err != cgroups.ErrNotFound {
+		return err
+	}
+	return nil
+}