netdev - Re: [PATCH 02/11] net: add function to allocate skbuff head without data area

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Message-ID: <1345444774.5158.241.camel@edumazet-glaptop>
Date:	Mon, 20 Aug 2012 08:39:34 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Patrick McHardy <kaber@...sh.net>
Cc:	Florian.Westphal@...hos.com, netdev@...r.kernel.org,
	netfilter-devel@...r.kernel.org
Subject: Re: [PATCH 02/11] net: add function to allocate skbuff head
 without data area

On Mon, 2012-08-20 at 08:18 +0200, Patrick McHardy wrote:
> Add a function to allocate a skbuff head without any data. This will be
> used by memory mapped netlink to attached data from the mmaped area to
> the skb.
> 
> Additionally change skb_release_all() to check whether the skb has a
> data area to allow the skb destructor to clear the data pointer in
> case only a head has been allocated.

      * ...

>  
> +struct sk_buff *__alloc_skb_head(gfp_t gfp_mask, int node)
> +{
> +	struct sk_buff *skb;
> +
> +	/* Get the HEAD */
> +	skb = kmem_cache_alloc_node(skbuff_head_cache,
> +				    gfp_mask & ~__GFP_DMA, node);
> +	if (!skb)
> +		goto out;
> +	prefetchw(skb);
> +

This prefetch is probably not worth it, since you write on skb on the
following memset(). Cpu wont have enough 'time' to take benefit from
this hint.

> +	/*
> +	 * Only clear those fields we need to clear, not those that we will
> +	 * actually initialise below. Hence, don't put any more fields after
> +	 * the tail pointer in struct sk_buff!
> +	 */
> +	memset(skb, 0, offsetof(struct sk_buff, tail));


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