Date:   Fri, 02 Oct 2020 08:25:49 -0700
From:   John Fastabend <john.fastabend@...il.com>
To:     Lorenzo Bianconi <lorenzo@...nel.org>, bpf@...r.kernel.org,
        netdev@...r.kernel.org
Cc:     davem@...emloft.net, kuba@...nel.org, ast@...nel.org,
        daniel@...earbox.net, shayagr@...zon.com, sameehj@...zon.com,
        john.fastabend@...il.com, dsahern@...nel.org, brouer@...hat.com,
        lorenzo.bianconi@...hat.com, echaudro@...hat.com
Subject: RE: [PATCH v4 bpf-next 00/13] mvneta: introduce XDP multi-buffer
 support

Lorenzo Bianconi wrote:
> This series introduce XDP multi-buffer support. The mvneta driver is
> the first to support these new "non-linear" xdp_{buff,frame}. Reviewers
> please focus on how these new types of xdp_{buff,frame} packets
> traverse the different layers and the layout design. It is on purpose
> that BPF-helpers are kept simple, as we don't want to expose the
> internal layout to allow later changes.
> 
> For now, to keep the design simple and to maintain performance, the XDP
> BPF-prog (still) only have access to the first-buffer. It is left for
> later (another patchset) to add payload access across multiple buffers.
> This patchset should still allow for these future extensions. The goal
> is to lift the XDP MTU restriction that comes with XDP, but maintain
> same performance as before.
> 
> The main idea for the new multi-buffer layout is to reuse the same
> layout used for non-linear SKB. This rely on the "skb_shared_info"
> struct at the end of the first buffer to link together subsequent
> buffers. Keeping the layout compatible with SKBs is also done to ease
> and speedup creating an SKB from an xdp_{buff,frame}. Converting
> xdp_frame to SKB and deliver it to the network stack is shown in cpumap
> code (patch 13/13).

Using the end of the buffer for the skb_shared_info struct is going to
become driver API so unwinding it if it proves to be a performance issue
is going to be ugly. So same question as before, for the use case where
we receive packet and do XDP_TX with it how do we avoid cache miss
overhead? This is not just a hypothetical use case, the Facebook
load balancer is doing this as well as Cilium and allowing this with
multi-buffer packets >1500B would be useful.

Can we write the skb_shared_info lazily? It should only be needed once
we know the packet is going up the stack to some place that needs the
info. Which we could learn from the return code of the XDP program.

> 
> A multi-buffer bit (mb) has been introduced in xdp_{buff,frame} structure
> to notify the bpf/network layer if this is a xdp multi-buffer frame (mb = 1)
> or not (mb = 0).
> The mb bit will be set by a xdp multi-buffer capable driver only for
> non-linear frames maintaining the capability to receive linear frames
> without any extra cost since the skb_shared_info structure at the end
> of the first buffer will be initialized only if mb is set.

Thanks above is clearer.

> 
> In order to provide to userspace some metdata about the non-linear
> xdp_{buff,frame}, we introduced 2 bpf helpers:
> - bpf_xdp_get_frags_count:
>   get the number of fragments for a given xdp multi-buffer.
> - bpf_xdp_get_frags_total_size:
>   get the total size of fragments for a given xdp multi-buffer.

Whats the use case for these? Do you have an example where knowing
the frags count is going to be something a BPF program will use?
Having total size seems interesting but perhaps we should push that
into the metadata so its pulled into the cache if users are going to
be reading it on every packet or something.

> 
> Typical use cases for this series are:
> - Jumbo-frames
> - Packet header split (please see Google���s use-case @ NetDevConf 0x14, [0])
> - TSO
> 
> More info about the main idea behind this approach can be found here [1][2].
> 
> We carried out some throughput tests in a standard linear frame scenario in order
> to verify we did not introduced any performance regression adding xdp multi-buff
> support to mvneta:
> 
> offered load is ~ 1000Kpps, packet size is 64B, mvneta descriptor size is one PAGE
> 
> commit: 879456bedbe5 ("net: mvneta: avoid possible cache misses in mvneta_rx_swbm")
> - xdp-pass:      ~162Kpps
> - xdp-drop:      ~701Kpps
> - xdp-tx:        ~185Kpps
> - xdp-redirect:  ~202Kpps
> 
> mvneta xdp multi-buff:
> - xdp-pass:      ~163Kpps
> - xdp-drop:      ~739Kpps
> - xdp-tx:        ~182Kpps
> - xdp-redirect:  ~202Kpps
> 
> Changes since v3:
> - rebase ontop of bpf-next
> - add patch 10/13 to copy back paged data from a xdp multi-buff frame to
>   userspace buffer for xdp multi-buff selftests
> 
> Changes since v2:
> - add throughput measurements
> - drop bpf_xdp_adjust_mb_header bpf helper
> - introduce selftest for xdp multibuffer
> - addressed comments on bpf_xdp_get_frags_count
> - introduce xdp multi-buff support to cpumaps
> 
> Changes since v1:
> - Fix use-after-free in xdp_return_{buff/frame}
> - Introduce bpf helpers
> - Introduce xdp_mb sample program
> - access skb_shared_info->nr_frags only on the last fragment
> 
> Changes since RFC:
> - squash multi-buffer bit initialization in a single patch
> - add mvneta non-linear XDP buff support for tx side
> 
> [0] https://netdevconf.info/0x14/session.html?talk-the-path-to-tcp-4k-mtu-and-rx-zerocopy
> [1] https://github.com/xdp-project/xdp-project/blob/master/areas/core/xdp-multi-buffer01-design.org
> [2] https://netdevconf.info/0x14/session.html?tutorial-add-XDP-support-to-a-NIC-driver (XDPmulti-buffers section)
> 
> Lorenzo Bianconi (11):
>   xdp: introduce mb in xdp_buff/xdp_frame
>   xdp: initialize xdp_buff mb bit to 0 in all XDP drivers
>   net: mvneta: update mb bit before passing the xdp buffer to eBPF layer
>   xdp: add multi-buff support to xdp_return_{buff/frame}
>   net: mvneta: add multi buffer support to XDP_TX
>   bpf: move user_size out of bpf_test_init
>   bpf: introduce multibuff support to bpf_prog_test_run_xdp()
>   bpf: test_run: add skb_shared_info pointer in bpf_test_finish
>     signature
>   bpf: add xdp multi-buffer selftest
>   net: mvneta: enable jumbo frames for XDP
>   bpf: cpumap: introduce xdp multi-buff support
> 
> Sameeh Jubran (2):
>   bpf: introduce bpf_xdp_get_frags_{count, total_size} helpers
>   samples/bpf: add bpf program that uses xdp mb helpers
> 
>  drivers/net/ethernet/amazon/ena/ena_netdev.c  |   1 +
>  drivers/net/ethernet/broadcom/bnxt/bnxt_xdp.c |   1 +
>  .../net/ethernet/cavium/thunder/nicvf_main.c  |   1 +
>  .../net/ethernet/freescale/dpaa2/dpaa2-eth.c  |   1 +
>  drivers/net/ethernet/intel/i40e/i40e_txrx.c   |   1 +
>  drivers/net/ethernet/intel/ice/ice_txrx.c     |   1 +
>  drivers/net/ethernet/intel/ixgbe/ixgbe_main.c |   1 +
>  .../net/ethernet/intel/ixgbevf/ixgbevf_main.c |   1 +
>  drivers/net/ethernet/marvell/mvneta.c         | 131 +++++++------
>  .../net/ethernet/marvell/mvpp2/mvpp2_main.c   |   1 +
>  drivers/net/ethernet/mellanox/mlx4/en_rx.c    |   1 +
>  .../net/ethernet/mellanox/mlx5/core/en_rx.c   |   1 +
>  .../ethernet/netronome/nfp/nfp_net_common.c   |   1 +
>  drivers/net/ethernet/qlogic/qede/qede_fp.c    |   1 +
>  drivers/net/ethernet/sfc/rx.c                 |   1 +
>  drivers/net/ethernet/socionext/netsec.c       |   1 +
>  drivers/net/ethernet/ti/cpsw.c                |   1 +
>  drivers/net/ethernet/ti/cpsw_new.c            |   1 +
>  drivers/net/hyperv/netvsc_bpf.c               |   1 +
>  drivers/net/tun.c                             |   2 +
>  drivers/net/veth.c                            |   1 +
>  drivers/net/virtio_net.c                      |   2 +
>  drivers/net/xen-netfront.c                    |   1 +
>  include/net/xdp.h                             |  31 ++-
>  include/uapi/linux/bpf.h                      |  14 ++
>  kernel/bpf/cpumap.c                           |  45 +----
>  net/bpf/test_run.c                            | 118 ++++++++++--
>  net/core/dev.c                                |   1 +
>  net/core/filter.c                             |  42 ++++
>  net/core/xdp.c                                | 104 ++++++++++
>  samples/bpf/Makefile                          |   3 +
>  samples/bpf/xdp_mb_kern.c                     |  68 +++++++
>  samples/bpf/xdp_mb_user.c                     | 182 ++++++++++++++++++
>  tools/include/uapi/linux/bpf.h                |  14 ++
>  .../testing/selftests/bpf/prog_tests/xdp_mb.c |  79 ++++++++
>  .../selftests/bpf/progs/test_xdp_multi_buff.c |  24 +++
>  36 files changed, 757 insertions(+), 123 deletions(-)
>  create mode 100644 samples/bpf/xdp_mb_kern.c
>  create mode 100644 samples/bpf/xdp_mb_user.c
>  create mode 100644 tools/testing/selftests/bpf/prog_tests/xdp_mb.c
>  create mode 100644 tools/testing/selftests/bpf/progs/test_xdp_multi_buff.c
> 
> -- 
> 2.26.2
> 

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