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

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Message-ID: <20250928203202.7d31a9bb.pasic@linux.ibm.com>
Date: Sun, 28 Sep 2025 20:32:02 +0200
From: Halil Pasic <pasic@...ux.ibm.com>
To: Dust Li <dust.li@...ux.alibaba.com>
Cc: Paolo Abeni <pabeni@...hat.com>, 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, Halil Pasic <pasic@...ux.ibm.com>
Subject: Re: [PATCH net-next v3 1/2] net/smc: make wr buffer count
 configurable

On Sun, 28 Sep 2025 19:42:54 +0800
Dust Li <dust.li@...ux.alibaba.com> wrote:

> >> 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
> >>                  */  
> >
> >But what are "the unsolicited" rdma_writes? I have heard of
> >unsolicited receive, where the data is received without
> >consuming a WR previously put on the RQ on the receiving end, but
> >the concept of unsolicited rdma_writes eludes me completely.  
> 
> unsolicited RDMA Writes means those RDMA Writes won't generate
> CQEs on the local side. You can refer to:
> https://www.rdmamojo.com/2014/05/27/solicited-event/
> 
> >
> >I guess what you are trying to say, and what I understand is
> >that we first put the payload into the RMB of the remote, which
> >may require up 2 RDMA_WRITE operations, probably because we may
> >cross the end (and start) of the array that hosts the circular
> >buffer, and then we send a CDC message to update the cursor.
> >
> >For the latter a  ib_post_send() is used in smc_wr_tx_send()
> >and AFAICT it consumes a WR from wr_tx_bufs. For the former
> >we consume a single wr_tx_rdmas which and each wr_tx_rdmas
> >has 2 WR allocated.  
> 
> Right.

Thank you Dust Li! Unfortunately I have already spinned a v4. Let
me add back that comment, as for people knowledgeable enough it does
not appear to be confusing at all. I can try to improve that comment
and maybe add a new one on the reason why we do need more WRs on the
receive end than on the send end, after this series has been merged. Or
if you want to do it yourself, I'm happy with it as well. In the end
it is you who me helped get a better understanding of this :)

Thank you again!

Regards,
Halil