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

[Pavel Begunkov]

On 11/6/23 22:55, Willem de Bruijn wrote:
> On Mon, Nov 6, 2023 at 2:34 PM Stanislav Fomichev <sdf@xxxxxxxxxx> wrote:
> > On 11/06, Willem de Bruijn wrote:
> > > > > 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.
> >
> > I'm not really concerned with faster. I would prefer something cleaner :-)
> >
> > Or maybe we should just have it documented. With some kind of path
> > towards beautiful world where we can create dynamic queues..
>
> I suppose we just disagree on the elegance of the API.
>
> The concise notification API returns tokens as a range for
> compression, encoding as two 32-bit unsigned integers start + length.
> It allows for even further batching by returning multiple such ranges
> in a single call.

FWIW, nothing prevents io_uring from compressing ranges. The io_uring
zc RFC returns {offset, size} as well, though at the moment the would
lie in the same page.

> This is analogous to the MSG_ZEROCOPY notification mechanism from
> kernel to user.
>
> The synchronous socket syscall interface can be replaced by something
> asynchronous like io_uring. This already works today? Whatever

If you mean async io_uring recv, it does work. In short, internally
it polls the socket and then calls sock_recvmsg(). There is also a
feature that would make it return back to polling after sock_recvmsg()
and loop like this.

> asynchronous ring-based API would be selected, io_uring or otherwise,
> I think the concise notification encoding would remain as is.
>
> Since this is an operation on a socket, I find a setsockopt the
> fitting interface.

--
Pavel Begunkov