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

Christoph Hellwig
2022-03-10T08:38:20Z

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@lst.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.