Re: [RFC PATCH v3 10/12] tcp: RX path for devmem TCP

[Willem de Bruijn]

> > IMHO, we need a better UAPI to receive the tokens and give them back to
> > the kernel. CMSG + setsockopt(SO_DEVMEM_DONTNEED) get the job done,
> > but look dated and hacky :-(
> >
> > We should either do some kind of user/kernel shared memory queue to
> > receive/return the tokens (similar to what Jonathan was doing in his
> > proposal?)
>
> I'll take a look at Jonathan's proposal, sorry, I'm not immediately
> familiar but I wanted to respond :-) But is the suggestion here to
> build a new kernel-user communication channel primitive for the
> purpose of passing the information in the devmem cmsg? IMHO that seems
> like an overkill. Why add 100-200 lines of code to the kernel to add
> something that can already be done with existing primitives? I don't
> see anything concretely wrong with cmsg & setsockopt approach, and if
> we switch to something I'd prefer to switch to an existing primitive
> for simplicity?
>
> The only other existing primitive to pass data outside of the linear
> buffer is the MSG_ERRQUEUE that is used for zerocopy. Is that
> preferred? Any other suggestions or existing primitives I'm not aware
> of?
>
> > or bite the bullet and switch to io_uring.
> >
>
> IMO io_uring & socket support are orthogonal, and one doesn't preclude
> the other. As you know we like to use sockets and I believe there are
> issues with io_uring adoption at Google that I'm not familiar with
> (and could be wrong). I'm interested in exploring io_uring support as
> a follow up but I think David Wei will be interested in io_uring
> support as well anyway.

I also disagree that we need to replace a standard socket interface
with something "faster", in quotes.

This interface is not the bottleneck to the target workload.

Replacing the synchronous sockets interface with something more
performant for workloads where it is, is an orthogonal challenge.
However we do that, I think that traditional sockets should continue
to be supported.

The feature may already even work with io_uring, as both recvmsg with
cmsg and setsockopt have io_uring support now.