Re: [PATCH net-next v3 1/2] net/smc: make wr buffer count configurable

Dust Li
2025-09-28T02:02:47Z

On 2025-09-28 00:55:15, Halil Pasic wrote:
>On Thu, 25 Sep 2025 13:25:40 +0200
>Halil Pasic <pasic@linux.ibm.com> wrote:
>
>> > [...]
>> > > @@ -683,6 +678,8 @@ int smc_ib_create_queue_pair(struct smc_link *lnk)
>> > > };
>> > > int rc;
>> > >
>> > > + qp_attr.cap.max_send_wr = 3 * lnk->lgr->max_send_wr;
>> > > + qp_attr.cap.max_recv_wr = lnk->lgr->max_recv_wr;
>> >
>> > Possibly:
>> >
>> > cap = max(3 * lnk->lgr->max_send_wr, lnk->lgr->max_recv_wr);
>> > qp_attr.cap.max_send_wr = cap;
>> > qp_attr.cap.max_recv_wr = cap
>> >
>> > to avoid assumption on `max_send_wr`, `max_recv_wr` relative values.
>>
>> Can you explain a little more. I'm happy to do the change, but I would
>> prefer to understand why is keeping qp_attr.cap.max_send_wr ==
>> qp_attr.cap.max_recv_wr better? But if you tell: "Just trust me!" I will.
>
>Due to a little accident we ended up having a private conversation
>on this, which I'm going to sum up quickly.
>
>Paolo stated that he has no strong preference and that I should at
>least add a comment, which I will do for v4.
>
>Unfortunately I don't quite understand why qp_attr.cap.max_send_wr is 3
>times the number of send WR buffers we allocate. My understanding
>is that qp_attr.cap.max_send_wr is about the number of send WQEs.

We have at most 2 RDMA Write for 1 RDMA send. So 3 times is necessary.
That is explained in the original comments. Maybe it's better to keep it.

```
.cap = {
/* include unsolicited rdma_writes as well,
* there are max. 2 RDMA_WRITE per 1 WR_SEND
*/
.max_send_wr = SMC_WR_BUF_CNT * 3,
.max_recv_wr = SMC_WR_BUF_CNT * 3,
.max_send_sge = SMC_IB_MAX_SEND_SGE,
.max_recv_sge = lnk->wr_rx_sge_cnt,
.max_inline_data = 0,
},
```

>I assume that qp_attr.cap.max_send_wr == qp_attr.cap.max_recv_wr
>is not something we would want to preserve.

IIUC, RDMA Write won't consume any RX wqe on the receive side, so I think
the .max_recv_wr can be SMC_WR_BUF_CNT if we don't use RDMA_WRITE_IMM.

Best regards,
Dust