netdev - Re: [PATCH net-next v3] MPLS: Add limited GSO support

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Message-ID: <1368811574.3301.109.camel@edumazet-glaptop>
Date:	Fri, 17 May 2013 10:26:14 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Simon Horman <horms@...ge.net.au>
Cc:	dev@...nvswitch.org, netdev@...r.kernel.org,
	Jesse Gross <jesse@...ira.com>,
	Pravin B Shelar <pshelar@...ira.com>,
	jarno.rajahalme@....com,
	Maciej Żenczykowski <maze@...gle.com>,
	Ben Hutchings <bhutchings@...arflare.com>
Subject: Re: [PATCH net-next v3] MPLS: Add limited GSO support

On Fri, 2013-05-17 at 15:50 +0900, Simon Horman wrote:

> @@ -509,6 +511,8 @@ struct sk_buff {
>  		__u32		reserved_tailroom;
>  	};
>  
> +	__be16			inner_protocol;
> +	/* 16/48 bit hole */
>  	sk_buff_data_t		inner_transport_header;
>  	sk_buff_data_t		inner_network_header;
>  	sk_buff_data_t		inner_mac_header;

We are reaching the point where sk_buff is so big that per packet cost
is killing us, and guys want linux kernel bypass.

sizeof(sk_buff) = 0xf8 -> 0x100

sizeof(skbuff_fclone_cache) = 0x200 -> 0x240 

So TCP stack performance is going to be hurt by this change, as the
atomic_t containing fclone_ref will be in a separate cache line.

__copy_skb_header() needs to be smarter and perform a bulk copy using
long words

Maybe we could use 16 bits instead of 32 for the inner_*_headers ?



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