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

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Message-ID: <875zfrufem.fsf@toke.dk>
Date:   Fri, 28 Feb 2020 12:20:33 +0100
From:   Toke Høiland-Jørgensen <toke@...hat.com>
To:     Luigi Rizzo <lrizzo@...gle.com>, netdev@...r.kernel.org,
        davem@...emloft.net, hawk@...nel.org, sameehj@...zon.com
Cc:     linux-kernel@...r.kernel.org, Luigi Rizzo <lrizzo@...gle.com>
Subject: Re: [PATCH v4] netdev attribute to control xdpgeneric skb linearization

Luigi Rizzo <lrizzo@...gle.com> writes:

> 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>

Acked-by: Toke Høiland-Jørgensen <toke@...hat.com>