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+===============
+RDMA Controller
+===============
+
+.. Contents
+
+   1. Overview
+     1-1. What is RDMA controller?
+     1-2. Why RDMA controller needed?
+     1-3. How is RDMA controller implemented?
+   2. Usage Examples
+
+1. Overview
+===========
+
+1-1. What is RDMA controller?
+-----------------------------
+
+RDMA controller allows user to limit RDMA/IB specific resources that a given
+set of processes can use. These processes are grouped using RDMA controller.
+
+RDMA controller defines two resources which can be limited for processes of a
+cgroup.
+
+1-2. Why RDMA controller needed?
+--------------------------------
+
+Currently user space applications can easily take away all the rdma verb
+specific resources such as AH, CQ, QP, MR etc. Due to which other applications
+in other cgroup or kernel space ULPs may not even get chance to allocate any
+rdma resources. This can lead to service unavailability.
+
+Therefore RDMA controller is needed through which resource consumption
+of processes can be limited. Through this controller different rdma
+resources can be accounted.
+
+1-3. How is RDMA controller implemented?
+----------------------------------------
+
+RDMA cgroup allows limit configuration of resources. Rdma cgroup maintains
+resource accounting per cgroup, per device using resource pool structure.
+Each such resource pool is limited up to 64 resources in given resource pool
+by rdma cgroup, which can be extended later if required.
+
+This resource pool object is linked to the cgroup css. Typically there
+are 0 to 4 resource pool instances per cgroup, per device in most use cases.
+But nothing limits to have it more. At present hundreds of RDMA devices per
+single cgroup may not be handled optimally, however there is no
+known use case or requirement for such configuration either.
+
+Since RDMA resources can be allocated from any process and can be freed by any
+of the child processes which shares the address space, rdma resources are
+always owned by the creator cgroup css. This allows process migration from one
+to other cgroup without major complexity of transferring resource ownership;
+because such ownership is not really present due to shared nature of
+rdma resources. Linking resources around css also ensures that cgroups can be
+deleted after processes migrated. This allow progress migration as well with
+active resources, even though that is not a primary use case.
+
+Whenever RDMA resource charging occurs, owner rdma cgroup is returned to
+the caller. Same rdma cgroup should be passed while uncharging the resource.
+This also allows process migrated with active RDMA resource to charge
+to new owner cgroup for new resource. It also allows to uncharge resource of
+a process from previously charged cgroup which is migrated to new cgroup,
+even though that is not a primary use case.
+
+Resource pool object is created in following situations.
+(a) User sets the limit and no previous resource pool exist for the device
+of interest for the cgroup.
+(b) No resource limits were configured, but IB/RDMA stack tries to
+charge the resource. So that it correctly uncharge them when applications are
+running without limits and later on when limits are enforced during uncharging,
+otherwise usage count will drop to negative.
+
+Resource pool is destroyed if all the resource limits are set to max and
+it is the last resource getting deallocated.
+
+User should set all the limit to max value if it intents to remove/unconfigure
+the resource pool for a particular device.
+
+IB stack honors limits enforced by the rdma controller. When application
+query about maximum resource limits of IB device, it returns minimum of
+what is configured by user for a given cgroup and what is supported by
+IB device.
+
+Following resources can be accounted by rdma controller.
+
+  ==========    =============================
+  hca_handle    Maximum number of HCA Handles
+  hca_object    Maximum number of HCA Objects
+  ==========    =============================
+
+2. Usage Examples
+=================
+
+(a) Configure resource limit::
+
+        echo mlx4_0 hca_handle=2 hca_object=2000 > /sys/fs/cgroup/rdma/1/rdma.max
+        echo ocrdma1 hca_handle=3 > /sys/fs/cgroup/rdma/2/rdma.max
+
+(b) Query resource limit::
+
+        cat /sys/fs/cgroup/rdma/2/rdma.max
+        #Output:
+        mlx4_0 hca_handle=2 hca_object=2000
+        ocrdma1 hca_handle=3 hca_object=max
+
+(c) Query current usage::
+
+        cat /sys/fs/cgroup/rdma/2/rdma.current
+        #Output:
+        mlx4_0 hca_handle=1 hca_object=20
+        ocrdma1 hca_handle=1 hca_object=23
+
+(d) Delete resource limit::
+
+        echo echo mlx4_0 hca_handle=max hca_object=max > /sys/fs/cgroup/rdma/1/rdma.max