netdev - Re: [PATCH net] gro: Make GRO aware of lightweight tunnels.

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Date:	Wed, 20 Jan 2016 16:48:28 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Jesse Gross <jesse@...nel.org>
Cc:	David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
	Thomas Graf <tgraf@...g.ch>, John <john.phillips5@....com>
Subject: Re: [PATCH net] gro: Make GRO aware of lightweight tunnels.

On Wed, 2016-01-20 at 16:27 -0800, Jesse Gross wrote:
> GRO is currently not aware of tunnel metadata generated by lightweight
> tunnels and stored in the dst. This leads to two possible problems:
>  * Incorrectly merging two frames that have different metadata.
>  * Leaking of allocated metadata from merged frames.
> 
> This avoids those problems by comparing the tunnel information before
> merging, similar to how we handle other metadata (such as vlan tags),
> and releasing any state when we are done.
> 
> Reported-by: John <john.phillips5@....com>
> Fixes: 2e15ea39 ("ip_gre: Add support to collect tunnel metadata.")
> Signed-off-by: Jesse Gross <jesse@...nel.org>
> ---
>  include/net/dst_metadata.h | 23 +++++++++++++++++++++++
>  net/core/dev.c             |  9 +++++++--
>  2 files changed, 30 insertions(+), 2 deletions(-)
> 
> diff --git a/include/net/dst_metadata.h b/include/net/dst_metadata.h
> index 6816f0f..c3de935 100644
> --- a/include/net/dst_metadata.h
> +++ b/include/net/dst_metadata.h
> @@ -44,6 +44,29 @@ static inline bool skb_valid_dst(const struct sk_buff *skb)
>  	return dst && !(dst->flags & DST_METADATA);
>  }
>  
> +static inline int skb_metadata_dst_cmp(struct sk_buff *skb_a,
> +				       struct sk_buff *skb_b)
> +{
> +	const struct metadata_dst *a = skb_metadata_dst(skb_a);
> +	const struct metadata_dst *b = skb_metadata_dst(skb_b);
> +
> +	if (!a != !b)
> +		return 1;
> +
> +	if (!a)
> +		return 0;
> +

It is adding 4 conditional test per flow in GRO engine for the fast
path... 

With up to 8 flows in GRO (per RX queue), it is a total of 32 added
conditional tests.

You should shortcut to one test :

if (!(skb_a->_skb_refdst | skb_b->_skb_refdst)
	return 0;