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References: <20230808031913.46965-1-huangjie.albert@bytedance.com> <87v8dpbv5r.fsf@toke.dk>
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From:   =?UTF-8?B?6buE5p2w?= <huangjie.albert@bytedance.com>
Date:   Wed, 9 Aug 2023 15:13:47 +0800
Message-ID: <CABKxMyNrwSOrzpq6mhqtU_kEk5B9nKPODtmfjJO5_NmGpw_Oag@mail.gmail.com>
Subject: Re: Re: [RFC v3 Optimizing veth xsk performance 0/9]
To:     =?UTF-8?B?VG9rZSBIw7hpbGFuZC1Kw7hyZ2Vuc2Vu?= <toke@redhat.com>
Cc:     "David S. Miller" <davem@davemloft.net>,
        Eric Dumazet <edumazet@google.com>,
        Jakub Kicinski <kuba@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Jesper Dangaard Brouer <hawk@kernel.org>,
        John Fastabend <john.fastabend@gmail.com>,
        =?UTF-8?B?QmrDtnJuIFTDtnBlbA==?= <bjorn@kernel.org>,
        Magnus Karlsson <magnus.karlsson@intel.com>,
        Maciej Fijalkowski <maciej.fijalkowski@intel.com>,
        Jonathan Lemon <jonathan.lemon@gmail.com>,
        Pavel Begunkov <asml.silence@gmail.com>,
        Yunsheng Lin <linyunsheng@huawei.com>,
        Kees Cook <keescook@chromium.org>,
        Richard Gobert <richardbgobert@gmail.com>,
        "open list:NETWORKING DRIVERS" <netdev@vger.kernel.org>,
        open list <linux-kernel@vger.kernel.org>,
        "open list:XDP (eXpress Data Path)" <bpf@vger.kernel.org>
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Toke H=C3=B8iland-J=C3=B8rgensen <toke@redhat.com> =E4=BA=8E2023=E5=B9=B48=
=E6=9C=888=E6=97=A5=E5=91=A8=E4=BA=8C 20:01=E5=86=99=E9=81=93=EF=BC=9A
>
> Albert Huang <huangjie.albert@bytedance.com> writes:
>
> > AF_XDP is a kernel bypass technology that can greatly improve performan=
ce.
> > However,for virtual devices like veth,even with the use of AF_XDP socke=
ts,
> > there are still many additional software paths that consume CPU resourc=
es.
> > This patch series focuses on optimizing the performance of AF_XDP socke=
ts
> > for veth virtual devices. Patches 1 to 4 mainly involve preparatory wor=
k.
> > Patch 5 introduces tx queue and tx napi for packet transmission, while
> > patch 8 primarily implements batch sending for IPv4 UDP packets, and pa=
tch 9
> > add support for AF_XDP tx need_wakup feature. These optimizations signi=
ficantly
> > reduce the software path and support checksum offload.
> >
> > I tested those feature with
> > A typical topology is shown below:
> > client(send):                                        server:(recv)
> > veth<-->veth-peer                                    veth1-peer<--->vet=
h1
> >   1       |                                                  |   7
> >           |2                                                6|
> >           |                                                  |
> >         bridge<------->eth0(mlnx5)- switch -eth1(mlnx5)<--->bridge1
> >                   3                    4                 5
> >              (machine1)                              (machine2)
>
> I definitely applaud the effort to improve the performance of af_xdp
> over veth, this is something we have flagged as in need of improvement
> as well.
>
> However, looking through your patch series, I am less sure that the
> approach you're taking here is the right one.
>
> AFAIU (speaking about the TX side here), the main difference between
> AF_XDP ZC and the regular transmit mode is that in the regular TX mode
> the stack will allocate an skb to hold the frame and push that down the
> stack. Whereas in ZC mode, there's a driver NDO that gets called
> directly, bypassing the skb allocation entirely.
>
> In this series, you're implementing the ZC mode for veth, but the driver
> code ends up allocating an skb anyway. Which seems to be a bit of a
> weird midpoint between the two modes, and adds a lot of complexity to
> the driver that (at least conceptually) is mostly just a
> reimplementation of what the stack does in non-ZC mode (allocate an skb
> and push it through the stack).
>
> So my question is, why not optimise the non-zc path in the stack instead
> of implementing the zc logic for veth? It seems to me that it would be
> quite feasible to apply the same optimisations (bulking, and even GRO)
> to that path and achieve the same benefits, without having to add all
> this complexity to the veth driver?
>
> -Toke
>
thanks!
This idea is really good indeed. You've reminded me, and that's
something I overlooked. I will now consider implementing the solution
you've proposed and test the performance enhancement.

Albert.