Re: [bpf-next V4 PATCH 0/8] xdp: introduce bulking for ndo_xdp_xmit API

Daniel Borkmann
2018-05-23T09:24:16Z

On 05/18/2018 03:34 PM, Jesper Dangaard Brouer wrote:
> This patchset change ndo_xdp_xmit API to take a bulk of xdp frames.
>
> In this V4 patchset, I've split-out the patches from 4 to 8 patches.
> I cannot split the driver changes from the NDO change, but I've tried
> to isolated the NDO change together with the driver change as much as
> possible.
>
> When kernel is compiled with CONFIG_RETPOLINE, every indirect function
> pointer (branch) call hurts performance. For XDP this have a huge
> negative performance impact.
>
> This patchset reduce the needed (indirect) calls to ndo_xdp_xmit, but
> also prepares for further optimizations. The DMA APIs use of indirect
> function pointer calls is the primary source the regression. It is
> left for a followup patchset, to use bulking calls towards the DMA API
> (via the scatter-gatter calls).
>
> The other advantage of this API change is that drivers can easier
> amortize the cost of any sync/locking scheme, over the bulk of
> packets. The assumption of the current API is that the driver
> implemementing the NDO will also allocate a dedicated XDP TX queue for
> every CPU in the system. Which is not always possible or practical to
> configure. E.g. ixgbe cannot load an XDP program on a machine with
> more than 96 CPUs, due to limited hardware TX queues. E.g. virtio_net
> is hard to configure as it requires manually increasing the
> queues. E.g. tun driver chooses to use a per XDP frame producer lock
> modulo smp_processor_id over avail queues.
>
> I'm considered adding 'flags' to ndo_xdp_xmit, but it's not part of
> this patchset. This will be a followup patchset, once we know if this
> will be needed (e.g. for non-map xdp_redirect flush-flag, and if
> AF_XDP chooses to use ndo_xdp_xmit for TX).
>
> ---
>
> Jesper Dangaard Brouer (8):
> bpf: devmap introduce dev_map_enqueue
> bpf: devmap prepare xdp frames for bulking
> xdp: add tracepoint for devmap like cpumap have
> samples/bpf: xdp_monitor use tracepoint xdp:xdp_devmap_xmit
> xdp: introduce xdp_return_frame_rx_napi
> xdp: change ndo_xdp_xmit API to support bulking
> xdp/trace: extend tracepoint in devmap with an err
> samples/bpf: xdp_monitor use err code from tracepoint xdp:xdp_devmap_xmit

Series applied to bpf-next, thanks Jesper. (Some minor comments in the patches.)