Re: [PATCH] RDMA/mlx4: Spread completion vectors for proxy CQs

Chuck Lever
2019-02-19T17:39:15Z

> On Feb 19, 2019, at 12:32 PM, Håkon Bugge <haakon.bugge@oracle.com> wrote:
>
>
>
>> On 19 Feb 2019, at 15:58, Chuck Lever <chuck.lever@oracle.com> wrote:
>>
>> Hey Håkon-
>>
>>> On Feb 18, 2019, at 1:33 PM, Håkon Bugge <haakon.bugge@oracle.com> wrote:
>>>
>>> MAD packet sending/receiving is not properly virtualized in
>>> CX-3. Hence, these are proxied through the PF driver. The proxying
>>> uses UD QPs. The associated CQs are created with completion vector
>>> zero.
>>>
>>> This leads to great imbalance in CPU processing, in particular during
>>> heavy RDMA CM traffic.
>>>
>>> Solved by selecting the completion vector on a round-robin base.
>>
>> I've got a similar patch for NFS and NFSD. I'm wondering if this
>> should be turned into a core helper, simple as it is. Perhaps
>> it would be beneficial if all participating ULPs used the same
>> global counter?
>
>
> A global counter works for this commit, because the QPs and associated CQs are (pretty) persistent. That is, VMs doesn't come and go that often.
>
> In the more general ULP case, the usage model is probably a lot more intermittent. Hence, a least-load approach is probably better. That can be implemented in ib core. I've seen in the past an enum IB_CQ_USE_LEAST_LOAD_VECTOR for signalling this behaviour and define that to e.g. -1, that is, outside of 0..(num_comp_vectors-1).

Indeed, passing such a value to either ib_create_cq or ib_alloc_cq
could allow the compvec to be selected automatically. Using a
round-robin would be the first step towards something smarter, and
the ULPs need be none the wiser when more smart-i-tude eventually
comes along.

> But this mechanism doesn't know which CQs that delivers the most interrupts. We lack an ib_modify_cq() that may change the CQ to EQ association, to _really_ spread the interrupts, not the CQ to EQ association.
>
> Anyway, Jason mentioned in a private email that maybe we could use the new completion API or something? I am not familiar with that one (yet).
>
> Well, I can volunteer to do the least load approach in ib core and change all (plain stupid) zero comp_vectors in ULPs and core, if that seems like an interim approach.

Please update net/sunrpc/xprtrdma/{svc_rdma_,}transport.c as well.
It should be straightforward, and I'm happy to review and test as
needed.

> Thxs, Håkon
>
>
>
>
>>
>>
>>> The imbalance can be demonstrated in a bare-metal environment, where
>>> two nodes have instantiated 8 VFs each. This using dual ported HCAs,
>>> so we have 16 vPorts per physical server.
>>>
>>> 64 processes are associated with each vPort and creates and destroys
>>> one QP for each of the remote 64 processes. That is, 1024 QPs per
>>> vPort, all in all 16K QPs. The QPs are created/destroyed using the
>>> CM.
>>>
>>> Before this commit, we have (excluding all completion IRQs with zero
>>> interrupts):
>>>
>>> 396: mlx4-1@0000:94:00.0 199126
>>> 397: mlx4-2@0000:94:00.0 1
>>>
>>> With this commit:
>>>
>>> 396: mlx4-1@0000:94:00.0 12568
>>> 397: mlx4-2@0000:94:00.0 50772
>>> 398: mlx4-3@0000:94:00.0 10063
>>> 399: mlx4-4@0000:94:00.0 50753
>>> 400: mlx4-5@0000:94:00.0 6127
>>> 401: mlx4-6@0000:94:00.0 6114
>>> []
>>> 414: mlx4-19@0000:94:00.0 6122
>>> 415: mlx4-20@0000:94:00.0 6117
>>>
>>> The added pr_info shows:
>>>
>>> create_pv_resources: slave:0 port:1, vector:0, num_comp_vectors:62
>>> create_pv_resources: slave:0 port:1, vector:1, num_comp_vectors:62
>>> create_pv_resources: slave:0 port:2, vector:2, num_comp_vectors:62
>>> create_pv_resources: slave:0 port:2, vector:3, num_comp_vectors:62
>>> create_pv_resources: slave:1 port:1, vector:4, num_comp_vectors:62
>>> create_pv_resources: slave:1 port:2, vector:5, num_comp_vectors:62
>>> []
>>> create_pv_resources: slave:8 port:2, vector:18, num_comp_vectors:62
>>> create_pv_resources: slave:8 port:1, vector:19, num_comp_vectors:62
>>>
>>> Signed-off-by: Håkon Bugge <haakon.bugge@oracle.com>
>>> ---
>>> drivers/infiniband/hw/mlx4/mad.c | 4 ++++
>>> 1 file changed, 4 insertions(+)
>>>
>>> diff --git a/drivers/infiniband/hw/mlx4/mad.c b/drivers/infiniband/hw/mlx4/mad.c
>>> index 936ee1314bcd..300839e7f519 100644
>>> --- a/drivers/infiniband/hw/mlx4/mad.c
>>> +++ b/drivers/infiniband/hw/mlx4/mad.c
>>> @@ -1973,6 +1973,7 @@ static int create_pv_resources(struct ib_device *ibdev, int slave, int port,
>>> {
>>> int ret, cq_size;
>>> struct ib_cq_init_attr cq_attr = {};
>>> + static atomic_t comp_vect = ATOMIC_INIT(-1);
>>>
>>> if (ctx->state != DEMUX_PV_STATE_DOWN)
>>> return -EEXIST;
>>> @@ -2002,6 +2003,9 @@ static int create_pv_resources(struct ib_device *ibdev, int slave, int port,
>>> cq_size *= 2;
>>>
>>> cq_attr.cqe = cq_size;
>>> + cq_attr.comp_vector = atomic_inc_return(&comp_vect) % ibdev->num_comp_vectors;
>>> + pr_info("slave:%d port:%d, vector:%d, num_comp_vectors:%d\n",
>>> + slave, port, cq_attr.comp_vector, ibdev->num_comp_vectors);
>>> ctx->cq = ib_create_cq(ctx->ib_dev, mlx4_ib_tunnel_comp_handler,
>>> NULL, ctx, &cq_attr);
>>> if (IS_ERR(ctx->cq)) {
>>> --
>>> 2.20.1
>>>
>>
>> --
>> Chuck Lever

--
Chuck Lever