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

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Message-ID: <20250928005515.61a57542.pasic@linux.ibm.com>
Date: Sun, 28 Sep 2025 00:55:15 +0200
From: Halil Pasic <pasic@...ux.ibm.com>
To: Paolo Abeni <pabeni@...hat.com>
Cc: Jakub Kicinski <kuba@...nel.org>, Simon Horman <horms@...nel.org>,
        "D.
 Wythe" <alibuda@...ux.alibaba.com>,
        Dust Li <dust.li@...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 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.
I assume that qp_attr.cap.max_send_wr == qp_attr.cap.max_recv_wr
is not something we would want to preserve.

Regards,
Halil