linux-kernel - Re: [PATCH v2 1/2] nvmet-tcp: support specifying the congestion-control

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Date:   Thu, 10 Mar 2022 09:38:11 +0100
From:   Christoph Hellwig <hch@....de>
To:     Mingbao Sun <sunmingbao@....com>
Cc:     Christoph Hellwig <hch@....de>, Keith Busch <kbusch@...nel.org>,
        Jens Axboe <axboe@...com>, Sagi Grimberg <sagi@...mberg.me>,
        Chaitanya Kulkarni <kch@...dia.com>,
        linux-nvme@...ts.infradead.org, linux-kernel@...r.kernel.org,
        tyler.sun@...l.com, ping.gan@...l.com, yanxiu.cai@...l.com,
        libin.zhang@...l.com, ao.sun@...l.com
Subject: Re: [PATCH v2 1/2] nvmet-tcp: support specifying the
 congestion-control

On Wed, Mar 09, 2022 at 05:52:03PM +0800, Mingbao Sun wrote:
> On Wed, 9 Mar 2022 07:15:41 +0100
> Christoph Hellwig <hch@....de> wrote:
> 
> > On Wed, Mar 09, 2022 at 01:37:11PM +0800, Mingbao Sun wrote:
> > > +		if (port->nport->tcp_congestion) {
> > > +			icsk_new = inet_csk(newsock->sk);
> > > +			if (icsk_new->icsk_ca_ops != icsk->icsk_ca_ops) {
> > > +				pr_warn("congestion abnormal: expected %s, actual %s.\n",
> > > +					icsk->icsk_ca_ops->name,
> > > +					icsk_new->icsk_ca_ops->name);
> > > +			}
> > > +		}  
> > 
> > What is the point of having this code?
> 
> Well, this could happen in certain circumstances.
> Take the result from my test as an example:
> 
>     - The congestion of the listening socket of the target was set to
>       ‘dctcp’.
> 
>     - But the congestion of the socket of the host side was set to
>       ‘cubic’.
> 
>     - Then the congestion of the socket of the new connection at the
>       target side would automatically be altered to ‘dctcp-reno’.
> 
> In case tcp_congestion was explicitly set for the target, it can be
> supposed that the user attaches great importance to performance.
> So we’d better make the users aware that the system is not working
> in the way they expect. 

A warning message really seems very severe for a condition like this.
Maybe the better interface is a way to figure out which congestion
control algorithm is in use by reading a sysfs file.