Re: [PATCH v2 bpf-next 0/2] xsk: introduce generic almost-zerocopy xmit

[Magnus Karlsson <magnus.karlsson@gmail.com>]

On Tue, Apr 13, 2021 at 3:49 AM Xuan Zhuo <xuanzhuo@linux.alibaba.com> wrote:
>
> On Mon, 12 Apr 2021 16:13:12 +0200, Magnus Karlsson <magnus.karlsson@gmail.com> wrote:
> > On Wed, Mar 31, 2021 at 2:27 PM Alexander Lobakin <alobakin@pm.me> wrote:
> > >
> > > This series is based on the exceptional generic zerocopy xmit logics
> > > initially introduced by Xuan Zhuo. It extends it the way that it
> > > could cover all the sane drivers, not only the ones that are capable
> > > of xmitting skbs with no linear space.
> > >
> > > The first patch is a random while-we-are-here improvement over
> > > full-copy path, and the second is the main course. See the individual
> > > commit messages for the details.
> > >
> > > The original (full-zerocopy) path is still here and still generally
> > > faster, but for now it seems like virtio_net will remain the only
> > > user of it, at least for a considerable period of time.
> > >
> > > From v1 [0]:
> > >  - don't add a whole SMP_CACHE_BYTES because of only two bytes
> > >    (NET_IP_ALIGN);
> > >  - switch to zerocopy if the frame is 129 bytes or longer, not 128.
> > >    128 still fit to kmalloc-512, while a zerocopy skb is always
> > >    kmalloc-1024 -> can potentially be slower on this frame size.
> > >
> > > [0] https://lore.kernel.org/netdev/20210330231528.546284-1-alobakin@pm.me
> > >
> > > Alexander Lobakin (2):
> > >   xsk: speed-up generic full-copy xmit
> >
> > I took both your patches for a spin on my machine and for the first
> > one I do see a small but consistent drop in performance. I thought it
> > would go the other way, but it does not so let us put this one on the
> > shelf for now.
> >
> > >   xsk: introduce generic almost-zerocopy xmit
> >
> > This one wreaked havoc on my machine ;-). The performance dropped with
> > 75% for packets larger than 128 bytes when the new scheme kicks in.
> > Checking with perf top, it seems that we spend much more time
> > executing the sendmsg syscall. Analyzing some more:
> >
> > $ sudo bpftrace -e 'kprobe:__sys_sendto { @calls = @calls + 1; }
> > interval:s:1 {printf("calls/sec: %d\n", @calls); @calls = 0;}'
> > Attaching 2 probes...
> > calls/sec: 1539509 with your patch compared to
> >
> > calls/sec: 105796 without your patch
> >
> > The application spends a lot of more time trying to get the kernel to
> > send new packets, but the kernel replies with "have not completed the
> > outstanding ones, so come back later" = EAGAIN. Seems like the
> > transmission takes longer when the skbs have fragments, but I have not
> > examined this any further. Did you get a speed-up?
>
> Regarding this solution, I actually tested it on my mlx5 network card, but the
> performance was severely degraded, so I did not continue this solution later. I
> guess it might have something to do with the physical network card. We can try
> other network cards.

I tried it on a third card and got a 40% degradation, so let us scrap
this idea. It should stay optional as it is today as the (software)
drivers that benefit from this can turn it on explicitly.

> links: https://www.spinics.net/lists/netdev/msg710918.html
>
> Thanks.
>
> >
> > >  net/xdp/xsk.c | 32 ++++++++++++++++++++++----------
> > >  1 file changed, 22 insertions(+), 10 deletions(-)
> > >
> > > --
> > > Well, this is untested. I currently don't have an access to my setup
> > > and is bound by moving to another country, but as I don't know for
> > > sure at the moment when I'll get back to work on the kernel next time,
> > > I found it worthy to publish this now -- if any further changes will
> > > be required when I already will be out-of-sight, maybe someone could
> > > carry on to make a another revision and so on (I'm still here for any
> > > questions, comments, reviews and improvements till the end of this
> > > week).
> > > But this *should* work with all the sane drivers. If a particular
> > > one won't handle this, it's likely ill. Any tests are highly
> > > appreciated. Thanks!
> > > --
> > > 2.31.1
> > >
> > >