Based on kernel version 4.16.1. Page generated on 2018-04-09 11:52 EST.
1 RDMA Controller 2 ---------------- 3 4 Contents 5 -------- 6 7 1. Overview 8 1-1. What is RDMA controller? 9 1-2. Why RDMA controller needed? 10 1-3. How is RDMA controller implemented? 11 2. Usage Examples 12 13 1. Overview 14 15 1-1. What is RDMA controller? 16 ----------------------------- 17 18 RDMA controller allows user to limit RDMA/IB specific resources that a given 19 set of processes can use. These processes are grouped using RDMA controller. 20 21 RDMA controller defines two resources which can be limited for processes of a 22 cgroup. 23 24 1-2. Why RDMA controller needed? 25 -------------------------------- 26 27 Currently user space applications can easily take away all the rdma verb 28 specific resources such as AH, CQ, QP, MR etc. Due to which other applications 29 in other cgroup or kernel space ULPs may not even get chance to allocate any 30 rdma resources. This can leads to service unavailability. 31 32 Therefore RDMA controller is needed through which resource consumption 33 of processes can be limited. Through this controller different rdma 34 resources can be accounted. 35 36 1-3. How is RDMA controller implemented? 37 ---------------------------------------- 38 39 RDMA cgroup allows limit configuration of resources. Rdma cgroup maintains 40 resource accounting per cgroup, per device using resource pool structure. 41 Each such resource pool is limited up to 64 resources in given resource pool 42 by rdma cgroup, which can be extended later if required. 43 44 This resource pool object is linked to the cgroup css. Typically there 45 are 0 to 4 resource pool instances per cgroup, per device in most use cases. 46 But nothing limits to have it more. At present hundreds of RDMA devices per 47 single cgroup may not be handled optimally, however there is no 48 known use case or requirement for such configuration either. 49 50 Since RDMA resources can be allocated from any process and can be freed by any 51 of the child processes which shares the address space, rdma resources are 52 always owned by the creator cgroup css. This allows process migration from one 53 to other cgroup without major complexity of transferring resource ownership; 54 because such ownership is not really present due to shared nature of 55 rdma resources. Linking resources around css also ensures that cgroups can be 56 deleted after processes migrated. This allow progress migration as well with 57 active resources, even though that is not a primary use case. 58 59 Whenever RDMA resource charging occurs, owner rdma cgroup is returned to 60 the caller. Same rdma cgroup should be passed while uncharging the resource. 61 This also allows process migrated with active RDMA resource to charge 62 to new owner cgroup for new resource. It also allows to uncharge resource of 63 a process from previously charged cgroup which is migrated to new cgroup, 64 even though that is not a primary use case. 65 66 Resource pool object is created in following situations. 67 (a) User sets the limit and no previous resource pool exist for the device 68 of interest for the cgroup. 69 (b) No resource limits were configured, but IB/RDMA stack tries to 70 charge the resource. So that it correctly uncharge them when applications are 71 running without limits and later on when limits are enforced during uncharging, 72 otherwise usage count will drop to negative. 73 74 Resource pool is destroyed if all the resource limits are set to max and 75 it is the last resource getting deallocated. 76 77 User should set all the limit to max value if it intents to remove/unconfigure 78 the resource pool for a particular device. 79 80 IB stack honors limits enforced by the rdma controller. When application 81 query about maximum resource limits of IB device, it returns minimum of 82 what is configured by user for a given cgroup and what is supported by 83 IB device. 84 85 Following resources can be accounted by rdma controller. 86 hca_handle Maximum number of HCA Handles 87 hca_object Maximum number of HCA Objects 88 89 2. Usage Examples 90 ----------------- 91 92 (a) Configure resource limit: 93 echo mlx4_0 hca_handle=2 hca_object=2000 > /sys/fs/cgroup/rdma/1/rdma.max 94 echo ocrdma1 hca_handle=3 > /sys/fs/cgroup/rdma/2/rdma.max 95 96 (b) Query resource limit: 97 cat /sys/fs/cgroup/rdma/2/rdma.max 98 #Output: 99 mlx4_0 hca_handle=2 hca_object=2000 100 ocrdma1 hca_handle=3 hca_object=max 101 102 (c) Query current usage: 103 cat /sys/fs/cgroup/rdma/2/rdma.current 104 #Output: 105 mlx4_0 hca_handle=1 hca_object=20 106 ocrdma1 hca_handle=1 hca_object=23 107 108 (d) Delete resource limit: 109 echo echo mlx4_0 hca_handle=max hca_object=max > /sys/fs/cgroup/rdma/1/rdma.max