Re: [PATCH net-next] net/smc: introduce shadow sockets for fallback connections

[Paolo Abeni <pabeni@redhat.com>]

On Tue, 2023-03-21 at 07:19 +0000, Kai Shen wrote:
> SMC-R performs not so well on fallback situations right now,
> especially on short link server fallback occasions. We are planning
> to make SMC-R widely used and handling this fallback performance
> issue is really crucial to us. Here we introduce a shadow socket
> method to try to relief this problem.
> 
> Basicly, we use two more accept queues to hold incoming connections,
> one for fallback connections and the other for smc-r connections.
> We implement this method by using two more 'shadow' sockets and
> make the connection path of fallback connections almost the same as
> normal tcp connections.
> 
> Now the SMC-R accept path is like:
>   1. incoming connection
>   2. schedule work to smc sock alloc, tcp accept and push to smc
>      acceptq
>   3. wake up user to accept
> 
> When fallback happens on servers, the accepting path is the same
> which costs more than normal tcp accept path. In fallback
> situations, the step 2 above is not necessary and the smc sock is
> also not needed. So we use two more shadow sockets when one smc
> socket start listening. When new connection comes, we pop the req
> to the fallback socket acceptq or the non-fallback socket acceptq
> according to its syn_smc flag. As a result, when fallback happen we
> can graft the user socket with a normal tcp sock instead of a smc
> sock and get rid of the cost generated by step 2 and smc sock
> releasing.
> 
>                +-----> non-fallback socket acceptq
>                |
> incoming req --+
>                |
>                +-----> fallback socket acceptq
> 
> With the help of shadow socket, we gain similar performance as tcp
> connections on short link nginx server fallback occasions as what
> is illustrated below.

It looks like only the shadow sockets' receive queue is needed/used.

Have you considered instead adding 2 receive queues to smc_sock, and
implement a custom accept() variant fetching the accepted sockets from
there?

That will allow better encapsulating the changes into the smc code and
will avoid creating that 2 non-listening but almost listening sockets
which look quite strange.

Cheers,

Paolo