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

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Date:	Mon, 20 May 2013 12:18:17 +0900
From:	Simon Horman <horms@...ge.net.au>
To:	Eric Dumazet <eric.dumazet@...il.com>
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, May 17, 2013 at 10:26:14AM -0700, Eric Dumazet wrote:
> 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 ?

Thanks. I have made a patch to implement that and it seems to
both reduce the size of struct sk_buff and leave a hole for inner_protocol.

I will also update this patch to guard inner_protocol with
#ifdef CONFIG_NET_MPLS_GSO in struct sk_buff as it is otherwise unused.


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