[PATCHv7 3/3] rdmacg: Added documentation for rdmacg

[Parav Pandit]

Added documentation for v1 and v2 version describing high
level design and usage examples on using rdma controller.

Signed-off-by: Parav Pandit <pandit.parav@xxxxxxxxx>
---
 Documentation/cgroup-v1/rdma.txt | 111 +++++++++++++++++++++++++++++++++++++++
 Documentation/cgroup-v2.txt      |  33 ++++++++++++
 2 files changed, 144 insertions(+)
 create mode 100644 Documentation/cgroup-v1/rdma.txt

diff --git a/Documentation/cgroup-v1/rdma.txt b/Documentation/cgroup-v1/rdma.txt
new file mode 100644
index 0000000..1973502
--- /dev/null
+++ b/Documentation/cgroup-v1/rdma.txt
@@ -0,0 +1,111 @@
+				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 allows operating on resources defined by the IB stack
+which are mainly IB verb resources and in future hardware specific
+well defined resources.
+
+1-2. Why RDMA controller needed?
+--------------------------------
+
+Currently user space applications can easily take away all the rdma device
+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 leads to service unavailability.
+
+Therefore RDMA controller is needed through which resource consumption
+of processes can be limited. Through this controller various different rdma
+resources described by IB stack can be accounted.
+
+1-3. How is RDMA controller implemented?
+----------------------------------------
+
+RDMA cgroup allows limit configuration of resources. These resources are not
+defined by the rdma controller. Instead they are defined by the IB stack.
+This provides great flexibility to allow IB stack to define new resources,
+without any changes to rdma cgroup.
+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 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 thatâs not the primary use case.
+
+Whenever RDMA resource charing 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 it 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.
+
+2. Usage Examples
+-----------------
+
+(a) Configure resource limit:
+echo mlx4_0 mr=100 qp=10 ah=2 > /sys/fs/cgroup/rdma/1/rdma.max
+echo ocrdma1 mr=120 qp=20 cq=10 > /sys/fs/cgroup/rdma/2/rdma.max
+
+(b) Query resource limit:
+cat /sys/fs/cgroup/rdma/2/rdma.max
+#Output:
+mlx4_0 mr=100 qp=10 ah=2 pd=max
+ocrdma1 mr=120 qp=20 cq=10 pd=max ah=max
+
+(c) Query current usage:
+cat /sys/fs/cgroup/rdma/2/rdma.current
+#Output:
+mlx4_0 mr=95 qp=8 ah=2
+ocrdma1 mr=0 qp=20 cq=10
+
+(d) Delete resource limit:
+echo mlx4_0 mr=max qp=max ah=max > /sys/fs/cgroup/rdma/1/rdma.max
diff --git a/Documentation/cgroup-v2.txt b/Documentation/cgroup-v2.txt
index ff49cf9..0ec4605 100644
--- a/Documentation/cgroup-v2.txt
+++ b/Documentation/cgroup-v2.txt
@@ -47,6 +47,8 @@ CONTENTS
   5-3. IO
     5-3-1. IO Interface Files
     5-3-2. Writeback
+  5-4. RDMA
+    5-4-1. RDMA Interface Files
 P. Information on Kernel Programming
   P-1. Filesystem Support for Writeback
 D. Deprecated v1 Core Features
@@ -1088,6 +1090,37 @@ writeback as follows.
 	total available memory and applied the same way as
 	vm.dirty[_background]_ratio.
 
+5-4. RDMA
+
+The "rdma" controller regulates the distribution of RDMA resources.
+This controller implements resource accounting of resources defined
+by IB stack.
+
+5-4-1. RDMA Interface Files
+
+  rdma.max
+	A readwrite 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.
+
+	An example for mlx4 and ocrdma device follows.
+
+	  mlx4_0 ah=2 mr=1000 qp=104
+	  ocrdma1 cq=10 mr=900 qp=89
+	  mlx4_1 uctx=max ah=max pd=max cq=max qp=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 mr=1000 qp=102 ah=2 flow=10 srq=0
+	  ocrdma1 mr=900 qp=79 cq=10 flow=0 srq=0
+	  mlx4_1 uctx=max ah=max pd=max cq=max qp=max
 
 P. Information on Kernel Programming
 
-- 
1.8.3.1