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Date:   Mon, 23 Sep 2019 08:38:56 -0400
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Steffen Klassert <steffen.klassert@...unet.com>
Cc:     Network Development <netdev@...r.kernel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Subash Abhinov Kasiviswanathan <subashab@...eaurora.org>,
        Marcelo Ricardo Leitner <marcelo.leitner@...il.com>
Subject: Re: [PATCH RFC 3/5] net: Add a netdev software feature set that
 defaults to off.

On Fri, Sep 20, 2019 at 12:49 AM Steffen Klassert
<steffen.klassert@...unet.com> wrote:
>
> The previous patch added the NETIF_F_GRO_FRAGLIST feature.
> This is a software feature that should default to off.
> Current software features default to on, so add a new
> feature set that defaults to off.
>
> Signed-off-by: Steffen Klassert <steffen.klassert@...unet.com>
> ---
>  include/linux/netdev_features.h | 3 +++
>  net/core/dev.c                  | 2 +-
>  2 files changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/netdev_features.h b/include/linux/netdev_features.h
> index b239507da2a0..34d050bb1ae6 100644
> --- a/include/linux/netdev_features.h
> +++ b/include/linux/netdev_features.h
> @@ -230,6 +230,9 @@ static inline int find_next_netdev_feature(u64 feature, unsigned long start)
>  /* changeable features with no special hardware requirements */
>  #define NETIF_F_SOFT_FEATURES  (NETIF_F_GSO | NETIF_F_GRO)
>
> +/* Changeable features with no special hardware requirements that defaults to off. */
> +#define NETIF_F_SOFT_FEATURES_OFF      NETIF_F_GRO_FRAGLIST
> +

NETIF_F_GRO_FRAGLIST is not really a device feature, but a way to
configure which form of UDP GRO to apply.

The UDP GRO benchmarks were largely positive, but not a strict win if
I read Paolo's previous results correctly. Even if enabling to by
default, it probably should come with a sysctl to disable for specific
workloads.

If so, how about a ternary per-netns sysctl {off, on without gro-list,
on with gro-list} instead of configuring through ethtool?

Alternative, the choice between gro-list or not could perhaps be
informed by whether ip_forward is set. I think we discussed that and
it was rejected, but I cannot remember why.

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