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