netdev - Re: [PATCH net-next 3/3] net: gro: register gso and gro offload on separate lists

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Date:   Thu, 3 Feb 2022 08:11:43 -0800
From:   Eric Dumazet <edumazet@...gle.com>
To:     Paolo Abeni <pabeni@...hat.com>
Cc:     netdev <netdev@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Alexander Lobakin <alexandr.lobakin@...el.com>
Subject: Re: [PATCH net-next 3/3] net: gro: register gso and gro offload on
 separate lists

On Thu, Feb 3, 2022 at 7:48 AM Paolo Abeni <pabeni@...hat.com> wrote:
>
> So that we know each element in gro_list has valid gro callbacks
> (and the same for gso). This allows dropping a bunch of conditional
> in fastpath.
>
> Before:
> objdump -t net/core/gro.o | grep " F .text"
> 0000000000000bb0 l     F .text  000000000000033c dev_gro_receive
>
> After:
> 0000000000000bb0 l     F .text  0000000000000325 dev_gro_receive
>
> Signed-off-by: Paolo Abeni <pabeni@...hat.com>
> ---
>  include/linux/netdevice.h |  3 +-
>  net/core/gro.c            | 90 +++++++++++++++++++++++----------------
>  2 files changed, 56 insertions(+), 37 deletions(-)
>
> diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
> index 3213c7227b59..406cb457d788 100644
> --- a/include/linux/netdevice.h
> +++ b/include/linux/netdevice.h
> @@ -2564,7 +2564,8 @@ struct packet_offload {
>         __be16                   type;  /* This is really htons(ether_type). */
>         u16                      priority;
>         struct offload_callbacks callbacks;
> -       struct list_head         list;
> +       struct list_head         gro_list;
> +       struct list_head         gso_list;
>  };
>

On the other hand, this makes this object bigger, increasing the risk
of spanning cache lines.

It would be nice to group all struct packet_offload together in the
same section to increase data locality.

I played in the past with a similar idea, but splitting struct
packet_offload in two structures, one for GRO, one for GSO.
(Note that GSO is hardly ever use with modern NIC)

But the gains were really marginal.