Subject: Re: [PATCH v4 bpf-next 00/13] mvneta: introduce XDP multi-buffer support

From: Lorenzo Bianconi <[email-masked]>
Date: Fri, 2 Oct 2020 18:06:23 +0200

> 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.

Hi John,

I agree, I think for XDP_TX use-case it is not strictly necessary to fill the
skb_hared_info. The driver can just keep this info on the stack and use it
inserting the packet back to the DMA ring.
For mvneta I implemented it in this way to keep the code aligned with ndo_xdp_xmit
path since it is a low-end device. I guess we are not introducing any API constraint
for XDP_TX. A high-end device can implement multi-buff for XDP_TX in a different way
in order to avoid the cache miss.

We need to fill the skb_shared info only when we want to pass the frame to the
network stack (build_skb() can directly reuse skb_shared_info->frags[]) or for
XDP_REDIRECT use-case.

> 
> > 
> > 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.

At the moment we do not have any use-case for these helpers (not considering
the sample in the series :)). We introduced them to provide some basic metadata
about the non-linear xdp_frame.
IIRC we decided to introduce some helpers instead of adding this info in xdp_frame
in order to save space on it (for xdp it is essential xdp_frame to fit in a single
cache-line).

Regards,
Lorenzo

> 
> > 
> > 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
> > 
> 
>