Date:   Fri, 16 Aug 2019 15:32:35 +0200
From:   Björn Töpel <bjorn.topel@...il.com>
To:     "Samudrala, Sridhar" <sridhar.samudrala@...el.com>
Cc:     Björn Töpel <bjorn.topel@...el.com>,
        "Karlsson, Magnus" <magnus.karlsson@...el.com>,
        Netdev <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        intel-wired-lan <intel-wired-lan@...ts.osuosl.org>,
        maciej.fijalkowski@...el.com, tom.herbert@...el.com
Subject: Re: [Intel-wired-lan] [PATCH bpf-next 0/5] Add support for SKIP_BPF
 flag for AF_XDP sockets

On Thu, 15 Aug 2019 at 18:46, Samudrala, Sridhar
<sridhar.samudrala@...el.com> wrote:
>
> On 8/15/2019 5:51 AM, Björn Töpel wrote:
> > On 2019-08-15 05:46, Sridhar Samudrala wrote:
> >> This patch series introduces XDP_SKIP_BPF flag that can be specified
> >> during the bind() call of an AF_XDP socket to skip calling the BPF
> >> program in the receive path and pass the buffer directly to the socket.
> >>
> >> When a single AF_XDP socket is associated with a queue and a HW
> >> filter is used to redirect the packets and the app is interested in
> >> receiving all the packets on that queue, we don't need an additional
> >> BPF program to do further filtering or lookup/redirect to a socket.
> >>
> >> Here are some performance numbers collected on
> >>    - 2 socket 28 core Intel(R) Xeon(R) Platinum 8180 CPU @ 2.50GHz
> >>    - Intel 40Gb Ethernet NIC (i40e)
> >>
> >> All tests use 2 cores and the results are in Mpps.
> >>
> >> turbo on (default)
> >> ---------------------------------------------
> >>                        no-skip-bpf    skip-bpf
> >> ---------------------------------------------
> >> rxdrop zerocopy           21.9         38.5
> >> l2fwd  zerocopy           17.0         20.5
> >> rxdrop copy               11.1         13.3
> >> l2fwd  copy                1.9          2.0
> >>
> >> no turbo :  echo 1 > /sys/devices/system/cpu/intel_pstate/no_turbo
> >> ---------------------------------------------
> >>                        no-skip-bpf    skip-bpf
> >> ---------------------------------------------
> >> rxdrop zerocopy           15.4         29.0
> >> l2fwd  zerocopy           11.8         18.2
> >> rxdrop copy                8.2         10.5
> >> l2fwd  copy                1.7          1.7
> >> ---------------------------------------------
> >>
> >
> > This work is somewhat similar to the XDP_ATTACH work [1]. Avoiding the
> > retpoline in the XDP program call is a nice performance boost! I like
> > the numbers! :-) I also like the idea of adding a flag that just does
> > what most AF_XDP Rx users want -- just getting all packets of a
> > certain queue into the XDP sockets.
> >
> > In addition to Toke's mail, I have some more concerns with the series:
> >
> > * AFAIU the SKIP_BPF only works for zero-copy enabled sockets. IMO, it
> >    should work for all modes (including XDP_SKB).
>
> This patch enables SKIP_BPF for AF_XDP sockets where an XDP program is
> attached at driver level (both zerocopy and copy modes)
> I tried a quick hack to see the perf benefit with generic XDP mode, but
> i didn't see any significant improvement in performance in that
> scenario. so i didn't include that mode.
>
> >
> > * In order to work, a user still needs an XDP program running. That's
> >    clunky. I'd like the behavior that if no XDP program is attached,
> >    and the option is set, the packets for a that queue end up in the
> >    socket. If there's an XDP program attached, the program has
> >    precedence.
>
> I think this would require more changes in the drivers to take XDP
> datapath even when there is no XDP program loaded.
>

Today, from a driver perspective, to enable XDP you pass a struct
bpf_prog pointer via the ndo_bpf. The program get executed in
BPF_PROG_RUN (via bpf_prog_run_xdp) from include/linux/filter.h.

I think it's possible to achieve what you're doing w/o *any* driver
modification. Pass a special, invalid, pointer to the driver (say
(void *)0x1 or smth more elegant), which has a special handling in
BPF_RUN_PROG e.g. setting a per-cpu state and return XDP_REDIRECT. The
per-cpu state is picked up in xdp_do_redirect and xdp_flush.

An approach like this would be general, and apply to all modes
automatically.

Thoughts?


> >
> > * It requires changes in all drivers. Not nice, and scales badly. Try
> >    making it generic (xdp_do_redirect/xdp_flush), so it Just Works for
> >    all XDP capable drivers.
>
> I tried to make this as generic as possible and make the changes to the
> driver very minimal, but could not find a way to avoid any changes at
> all to the driver. xdp_do_direct() gets called based after the call to
> bpf_prog_run_xdp() in the drivers.
>
> >
> > Thanks for working on this!
> >
> >
> > Björn
> >
> > [1]
> > https://lore.kernel.org/netdev/20181207114431.18038-1-bjorn.topel@gmail.com/
> >
> >
> >
> >> Sridhar Samudrala (5):
> >>    xsk: Convert bool 'zc' field in struct xdp_umem to a u32 bitmap
> >>    xsk: Introduce XDP_SKIP_BPF bind option
> >>    i40e: Enable XDP_SKIP_BPF option for AF_XDP sockets
> >>    ixgbe: Enable XDP_SKIP_BPF option for AF_XDP sockets
> >>    xdpsock_user: Add skip_bpf option
> >>
> >>   drivers/net/ethernet/intel/i40e/i40e_txrx.c   | 22 +++++++++-
> >>   drivers/net/ethernet/intel/i40e/i40e_xsk.c    |  6 +++
> >>   drivers/net/ethernet/intel/ixgbe/ixgbe_main.c | 20 ++++++++-
> >>   drivers/net/ethernet/intel/ixgbe/ixgbe_xsk.c  | 16 ++++++-
> >>   include/net/xdp_sock.h                        | 21 ++++++++-
> >>   include/uapi/linux/if_xdp.h                   |  1 +
> >>   include/uapi/linux/xdp_diag.h                 |  1 +
> >>   net/xdp/xdp_umem.c                            |  9 ++--
> >>   net/xdp/xsk.c                                 | 43 ++++++++++++++++---
> >>   net/xdp/xsk_diag.c                            |  5 ++-
> >>   samples/bpf/xdpsock_user.c                    |  8 ++++
> >>   11 files changed, 135 insertions(+), 17 deletions(-)
> >>
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