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From:   Stanislav Fomichev <sdf@google.com>
Date:   Mon, 6 Nov 2023 15:32:22 -0800
Message-ID: <CAKH8qBueYgpxQTvTwngOs6RNjy9yvLF92s1p5nFrobw_UprNMQ@mail.gmail.com>
Subject: Re: [RFC PATCH v3 10/12] tcp: RX path for devmem TCP
To:     Willem de Bruijn <willemdebruijn.kernel@gmail.com>
Cc:     Mina Almasry <almasrymina@google.com>, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org,
        linux-kselftest@vger.kernel.org, linux-media@vger.kernel.org,
        dri-devel@lists.freedesktop.org, linaro-mm-sig@lists.linaro.org,
        "David S. Miller" <davem@davemloft.net>,
        Eric Dumazet <edumazet@google.com>,
        Jakub Kicinski <kuba@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>,
        Jesper Dangaard Brouer <hawk@kernel.org>,
        Ilias Apalodimas <ilias.apalodimas@linaro.org>,
        Arnd Bergmann <arnd@arndb.de>,
        David Ahern <dsahern@kernel.org>,
        Shuah Khan <shuah@kernel.org>,
        Sumit Semwal <sumit.semwal@linaro.org>,
        =?UTF-8?Q?Christian_K=C3=B6nig?= <christian.koenig@amd.com>,
        Shakeel Butt <shakeelb@google.com>,
        Jeroen de Borst <jeroendb@google.com>,
        Praveen Kaligineedi <pkaligineedi@google.com>,
        Willem de Bruijn <willemb@google.com>,
        Kaiyuan Zhang <kaiyuanz@google.com>
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On Mon, Nov 6, 2023 at 2:56=E2=80=AFPM Willem de Bruijn
<willemdebruijn.kernel@gmail.com> wrote:
>
> On Mon, Nov 6, 2023 at 2:34=E2=80=AFPM Stanislav Fomichev <sdf@google.com=
> wrote:
> >
> > On 11/06, Willem de Bruijn wrote:
> > > > > IMHO, we need a better UAPI to receive the tokens and give them b=
ack to
> > > > > the kernel. CMSG + setsockopt(SO_DEVMEM_DONTNEED) get the job don=
e,
> > > > > 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 se=
ems
> > > > like an overkill. Why add 100-200 lines of code to the kernel to ad=
d
> > > > 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 primitiv=
e
> > > > for simplicity?
> > > >
> > > > The only other existing primitive to pass data outside of the linea=
r
> > > > buffer is the MSG_ERRQUEUE that is used for zerocopy. Is that
> > > > preferred? Any other suggestions or existing primitives I'm not awa=
re
> > > > of?
> > > >
> > > > > or bite the bullet and switch to io_uring.
> > > > >
> > > >
> > > > IMO io_uring & socket support are orthogonal, and one doesn't precl=
ude
> > > > the other. As you know we like to use sockets and I believe there a=
re
> > > > 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.

Yeah, I might be overly sensitive to the apis that use get/setsockopt
for something more involved than setting a flag.
Probably because I know that bpf will (unnecessarily) trigger on these :-D
I had to implement that bpf "bypass" (or fastpath) for
TCP_ZEROCOPY_RECEIVE and it looks like this token recycle might also
benefit from something similar.

> 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.

Tangential: should tokens be u64? Otherwise we can't have more than
4gb unacknowledged. Or that's a reasonable constraint?


> 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
> 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.