linux-kernel - Re: [PATCH v2 2/2] gro: optimise redundant parsing of packets

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Message-ID: <CANn89iK03mcdu=dn+kj-St27Y2OvSzQ5G=VzqwutR0Khn1cSUg@mail.gmail.com>
Date:   Wed, 22 Feb 2023 16:41:51 +0100
From:   Eric Dumazet <edumazet@...gle.com>
To:     Richard Gobert <richardbgobert@...il.com>
Cc:     davem@...emloft.net, kuba@...nel.org, pabeni@...hat.com,
        dsahern@...nel.org, alexanderduyck@...com, lixiaoyan@...gle.com,
        steffen.klassert@...unet.com, lucien.xin@...il.com,
        ye.xingchen@....com.cn, iwienand@...hat.com, leon@...nel.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/2] gro: optimise redundant parsing of packets

On Wed, Feb 22, 2023 at 4:13 PM Richard Gobert <richardbgobert@...il.com> wrote:
>
> Currently the IPv6 extension headers are parsed twice: first in
> ipv6_gro_receive, and then again in ipv6_gro_complete.
>
> By using the new ->transport_proto field, and also storing the size of the
> network header, we can avoid parsing extension headers a second time in
> ipv6_gro_complete (which saves multiple memory dereferences and conditional
> checks inside ipv6_exthdrs_len for a varying amount of extension headers in IPv6
> packets).
>
> The implementation had to handle both inner and outer layers in case of
> encapsulation (as they can't use the same field).
>
> Performance tests for TCP stream over IPv6 with a varying amount of extension
> headers demonstrate throughput improvement of ~0.7%.
>
> In addition, I fixed a potential existing problem:
>  - The call to skb_set_inner_network_header at the beginning of
>    ipv6_gro_complete calculates inner_network_header based on skb->data by
>    calling skb_set_inner_network_header, and setting it to point to the beginning
>    of the ip header.
>  - If a packet is going to be handled by BIG TCP, the following code block is
>    going to shift the packet header, and skb->data is going to be changed as
>    well.
>
> When the two flows are combined, inner_network_header will point to the wrong
> place.

net-next is closed.

If you think a fix is needed, please send a stand-alone and minimal
patch so that we can discuss its merit.

Note :

BIG TCP only supports native IPv6, not encapsulated traffic,
so we should not bother with inner_network_header yet.