Subject: Re: [PATCH v12 08/26] nvme-tcp: Add DDP data-path

From: Aurelien Aptel <[email-masked]>
Date: Mon, 14 Aug 2023 19:12:47 +0300

Sagi Grimberg <[email-masked]> writes:
> On 7/12/23 19:14, Aurelien Aptel wrote:
>> +static int nvme_tcp_req_map_ddp_sg(struct nvme_tcp_request *req, struct request *rq)
>
> Why do you pass both req and rq? You can derive each from the other.

Thanks, we will remove the redundant parameter.

>> +{
>> +     int ret;
>> +
>> +     req->ddp.sg_table.sgl = req->ddp.first_sgl;
>> +     ret = sg_alloc_table_chained(&req->ddp.sg_table,
>> +                                  blk_rq_nr_phys_segments(rq),
>> +                                  req->ddp.sg_table.sgl, SG_CHUNK_SIZE);
>> +     if (ret)
>> +             return -ENOMEM;
>> +     req->ddp.nents = blk_rq_map_sg(rq->q, rq, req->ddp.sg_table.sgl);
>
> General question, I'm assuming that the hca knows how to deal with
> a controller that sends c2hdata in parts?

Yes, the hardware supports the offloading of multiple c2hdata PDUs per IO.

>> +static int nvme_tcp_setup_ddp(struct nvme_tcp_queue *queue, u16 command_id,
>> +                           struct request *rq)
>
> I think you can use nvme_cid(rq) instead of passing the command_id.

Thanks, we will use it.

>> +{
>> +     struct net_device *netdev = queue->ctrl->offloading_netdev;
>> +     struct nvme_tcp_request *req = blk_mq_rq_to_pdu(rq);
>> +     int ret;
>> +
>> +     if (rq_data_dir(rq) != READ ||
>> +         queue->ctrl->offload_io_threshold > blk_rq_payload_bytes(rq))
>> +             return 0;
>> +
>> +     req->ddp.command_id = command_id;
>> +     ret = nvme_tcp_req_map_ddp_sg(req, rq);
>
> Don't see why map_ddp_sg is not open-coded here, its the only call-site,
> and its pretty much does exactly what its called.

Sure, we will open code it.

>> @@ -1308,6 +1407,15 @@ static int nvme_tcp_try_send_cmd_pdu(struct nvme_tcp_request *req)
>>       else
>>               msg.msg_flags |= MSG_EOR;
>>
>> +     if (test_bit(NVME_TCP_Q_OFF_DDP, &queue->flags)) {
>> +             ret = nvme_tcp_setup_ddp(queue, pdu->cmd.common.command_id,
>> +                                      blk_mq_rq_from_pdu(req));
>> +             WARN_ONCE(ret, "ddp setup failed (queue 0x%x, cid 0x%x, ret=%d)",
>> +                       nvme_tcp_queue_id(queue),
>> +                       pdu->cmd.common.command_id,
>> +                       ret);
>> +     }
>
> Any reason why this is done here when sending the command pdu and not
> in setup time?

We wish to interact with the HW from the same CPU per queue, hence we
are calling setup_ddp() after queue->io_cpu == raw_smp_processor_id()
was checked in nvme_tcp_queue_request().