Fix system container detection in kubelet on systemd

pkg/kubelet/cm/container_manager_linux.go

@@ -511,19 +511,49 @@ func ensureDockerInContainer(cadvisor cadvisor.Interface, oomScoreAdj int, manag
 	return utilerrors.NewAggregate(errs)
 }
 
-// Gets the (CPU) container the specified pid is in.
+// getContainer returns the cgroup associated with the specified pid.
+// It enforces a unified hierarchy for memory and cpu cgroups.
+// On systemd environments, it uses the name=systemd cgroup for the specified pid.
 func getContainer(pid int) (string, error) {
 	cgs, err := cgroups.ParseCgroupFile(fmt.Sprintf("/proc/%d/cgroup", pid))
 	if err != nil {
 		return "", err
 	}
 
-	cg, ok := cgs["cpu"]
-	if ok {
-		return cg, nil
+	cpu, found := cgs["cpu"]
+	if !found {
+		return "", cgroups.NewNotFoundError("cpu")
+	}
+	memory, found := cgs["memory"]
+	if !found {
+		return "", cgroups.NewNotFoundError("memory")
+	}
+
+	// since we use this container for accounting, we need to ensure its a unified hierarchy.
+	if cpu != memory {
+		return "", fmt.Errorf("cpu and memory cgroup hierarchy not unified.  cpu: %s, memory: %s", cpu, memory)
+	}
+
+	// on systemd, every pid is in a unified cgroup hierarchy (name=systemd as seen in systemd-cgls)
+	// cpu and memory accounting is off by default, users may choose to enable it per unit or globally.
+	// users could enable CPU and memory accounting globally via /etc/systemd/system.conf (DefaultCPUAccounting=true DefaultMemoryAccounting=true).
+	// users could also enable CPU and memory accounting per unit via CPUAccounting=true and MemoryAccounting=true
+	// we only warn if accounting is not enabled for CPU or memory so as to not break local development flows where kubelet is launched in a terminal.
+	// for example, the cgroup for the user session will be something like /user.slice/user-X.slice/session-X.scope, but the cpu and memory
+	// cgroup will be the closest ancestor where accounting is performed (most likely /) on systems that launch docker containers.
+	// as a result, on those systems, you will not get cpu or memory accounting statistics for kubelet.
+	// in addition, you would not get memory or cpu accounting for the runtime unless accounting was enabled on its unit (or globally).
+	if systemd, found := cgs["name=systemd"]; found {
+		if systemd != cpu {
+			glog.Warningf("CPUAccounting not enabled for pid: %d", pid)
+		}
+		if systemd != memory {
+			glog.Warningf("MemoryAccounting not enabled for pid: %d", pid)
+		}
+		return systemd, nil
 	}
 
-	return "", cgroups.NewNotFoundError("cpu")
+	return cpu, nil
 }
 
 // Ensures the system container is created and all non-kernel threads and process 1