linux-kernel - Re: [PATCH v4] netdev attribute to control xdpgeneric skb linearization

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Message-ID: <20200228110043.2771fddb@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
Date:   Fri, 28 Feb 2020 11:00:43 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Luigi Rizzo <lrizzo@...gle.com>
Cc:     netdev@...r.kernel.org, toke@...hat.com, davem@...emloft.net,
        hawk@...nel.org, sameehj@...zon.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4] netdev attribute to control xdpgeneric skb
 linearization

On Fri, 28 Feb 2020 02:54:35 -0800 Luigi Rizzo wrote:
> Add a netdevice flag to control skb linearization in generic xdp mode.
> 
> The attribute can be modified through
> 	/sys/class/net/<DEVICE>/xdpgeneric_linearize
> The default is 1 (on)
> 
> Motivation: xdp expects linear skbs with some minimum headroom, and
> generic xdp calls skb_linearize() if needed. The linearization is
> expensive, and may be unnecessary e.g. when the xdp program does
> not need access to the whole payload.
> This sysfs entry allows users to opt out of linearization on a
> per-device basis (linearization is still performed on cloned skbs).
> 
> On a kernel instrumented to grab timestamps around the linearization
> code in netif_receive_generic_xdp, and heavy netperf traffic with 1500b
> mtu, I see the following times (nanoseconds/pkt)
> 
> The receiver generally sees larger packets so the difference is more
> significant.
> 
> ns/pkt                   RECEIVER                 SENDER
> 
>                     p50     p90     p99       p50   p90    p99
> 
> LINEARIZATION:    600ns  1090ns  4900ns     149ns 249ns  460ns
> NO LINEARIZATION:  40ns    59ns    90ns      40ns  50ns  100ns
> 
> v1 --> v2 : added Documentation
> v2 --> v3 : adjusted for skb_cloned
> v3 --> v4 : renamed to xdpgeneric_linearize, documentation
> 
> Signed-off-by: Luigi Rizzo <lrizzo@...gle.com>

Just load your program in cls_bpf. No extensions or knobs needed.

Making xdpgeneric-only extensions without touching native XDP makes 
no sense to me. Is this part of some greater vision?