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

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Date:   Wed, 22 Feb 2023 10:27:54 -0500
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Richard Gobert <richardbgobert@...il.com>, davem@...emloft.net,
        edumazet@...gle.com, 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

Richard Gobert 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.
> 
> The fix is to place the whole encapsulation branch after the BIG TCP code block.

This should be a separate fix patch?

> This way, inner_network_header is calculated with a correct value of skb->data.
> Also, by arranging the code that way, the optimisation does not add an additional
> branch.
> 
> Signed-off-by: Richard Gobert <richardbgobert@...il.com>
> ---
>  include/net/gro.h      |  9 +++++++++
>  net/ethernet/eth.c     | 14 +++++++++++---
>  net/ipv6/ip6_offload.c | 20 +++++++++++++++-----
>  3 files changed, 35 insertions(+), 8 deletions(-)
> 
> diff --git a/include/net/gro.h b/include/net/gro.h
> index 7b47dd6ce94f..35f60ea99f6c 100644
> --- a/include/net/gro.h
> +++ b/include/net/gro.h
> @@ -86,6 +86,15 @@ struct napi_gro_cb {
>  
>  	/* used to support CHECKSUM_COMPLETE for tunneling protocols */
>  	__wsum	csum;
> +
> +	/* Used in ipv6_gro_receive() */
> +	u16	network_len;
> +
> +	/* Used in eth_gro_receive() */
> +	__be16	network_proto;
> +

Why also cache eth->h_proto? That is not mentioned in the commit message.

> +	/* Used in ipv6_gro_receive() */
> +	u8	transport_proto;