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

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Message-ID: <aNiXQ_UfG9k-f9-n@linux.alibaba.com>
Date: Sun, 28 Sep 2025 10:02:43 +0800
From: Dust Li <dust.li@...ux.alibaba.com>
To: Halil Pasic <pasic@...ux.ibm.com>, Paolo Abeni <pabeni@...hat.com>
Cc: Jakub Kicinski <kuba@...nel.org>, Simon Horman <horms@...nel.org>,
	"D. Wythe" <alibuda@...ux.alibaba.com>,
	Sidraya Jayagond <sidraya@...ux.ibm.com>,
	Wenjia Zhang <wenjia@...ux.ibm.com>,
	Mahanta Jambigi <mjambigi@...ux.ibm.com>,
	Tony Lu <tonylu@...ux.alibaba.com>,
	Wen Gu <guwen@...ux.alibaba.com>, netdev@...r.kernel.org,
	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-rdma@...r.kernel.org, linux-s390@...r.kernel.org
Subject: Re: [PATCH net-next v3 1/2] net/smc: make wr buffer count
 configurable

On 2025-09-28 00:55:15, Halil Pasic wrote:
>On Thu, 25 Sep 2025 13:25:40 +0200
>Halil Pasic <pasic@...ux.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