+5-4. PID
+
+The process number controller is used to allow a cgroup to stop any
+new tasks from being fork()'d or clone()'d after a specified limit is
+reached.
+
+The number of tasks in a cgroup can be exhausted in ways which other
+controllers cannot prevent, thus warranting its own controller. For
+example, a fork bomb is likely to exhaust the number of tasks before
+hitting memory restrictions.
+
+Note that PIDs used in this controller refer to TIDs, process IDs as
+used by the kernel.
+
+
+5-4-1. PID Interface Files
+
+ pids.max
+
+ A read-write single value file which exists on non-root cgroups. The
+ default is "max".
+
+ Hard limit of number of processes.
+
+ pids.current
+
+ A read-only single value file which exists on all cgroups.
+
+ The number of processes currently in the cgroup and its descendants.
+
+Organisational operations are not blocked by cgroup policies, so it is
+possible to have pids.current > pids.max. This can be done by either
+setting the limit to be smaller than pids.current, or attaching enough
+processes to the cgroup such that pids.current is larger than
+pids.max. However, it is not possible to violate a cgroup PID policy
+through fork() or clone(). These will return -EAGAIN if the creation
+of a new process would cause a cgroup policy to be violated.
+
+
+5-5. RDMA
+
+The "rdma" controller regulates the distribution and accounting of
+of RDMA resources.
+
+5-5-1. RDMA Interface Files
+
+ rdma.max
+ A readwrite nested-keyed file that exists for all the cgroups
+ except root that describes current configured resource limit
+ for a RDMA/IB device.
+
+ Lines are keyed by device name and are not ordered.
+ Each line contains space separated resource name and its configured
+ limit that can be distributed.
+
+ The following nested keys are defined.
+
+ hca_handle Maximum number of HCA Handles
+ hca_object Maximum number of HCA Objects
+
+ An example for mlx4 and ocrdma device follows.
+
+ mlx4_0 hca_handle=2 hca_object=2000
+ ocrdma1 hca_handle=3 hca_object=max
+
+ rdma.current
+ A read-only file that describes current resource usage.
+ It exists for all the cgroup except root.
+
+ An example for mlx4 and ocrdma device follows.
+
+ mlx4_0 hca_handle=1 hca_object=20
+ ocrdma1 hca_handle=1 hca_object=23
+
+
+5-6. Misc
+
+5-6-1. perf_event
+
+perf_event controller, if not mounted on a legacy hierarchy, is
+automatically enabled on the v2 hierarchy so that perf events can
+always be filtered by cgroup v2 path. The controller can still be
+moved to a legacy hierarchy after v2 hierarchy is populated.
+
+