netdev - Re: [bpf-next V5 PATCH 0/8] xdp: introduce bulking for ndo_xdp

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Message-ID: <20180525014241.upvogkgchjyttsej@ast-mbp>
Date:   Thu, 24 May 2018 18:42:43 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Jesper Dangaard Brouer <brouer@...hat.com>
Cc:     netdev@...r.kernel.org, Daniel Borkmann <borkmann@...earbox.net>,
        Christoph Hellwig <hch@...radead.org>,
        BjörnTöpel <bjorn.topel@...el.com>,
        John Fastabend <john.fastabend@...il.com>,
        Magnus Karlsson <magnus.karlsson@...el.com>,
        makita.toshiaki@....ntt.co.jp
Subject: Re: [bpf-next V5 PATCH 0/8] xdp: introduce bulking for ndo_xdp_xmit
 API

On Thu, May 24, 2018 at 04:45:41PM +0200, Jesper Dangaard Brouer wrote:
> This patchset change ndo_xdp_xmit API to take a bulk of xdp frames.
> 
> 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).
> 
> ---
> V5: Fixed up issues spotted by Daniel and John
> 
> V4: Splitout 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.

The patch 6/8 would have benefited from a review from Intel folks,
but the series have been pending for too long already, hence
Applied to bpf-next.
Please address any follow up reviews if/when they come.
Thanks Jesper.