netdev - Re: [PATCH net-next] net: more accurate skb truesize

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Date:	Thu, 13 Oct 2011 17:05:01 +0100
From:	Ben Hutchings <bhutchings@...arflare.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	David Miller <davem@...emloft.net>,
	netdev <netdev@...r.kernel.org>, Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH net-next] net: more accurate skb truesize

On Thu, 2011-10-13 at 17:26 +0200, Eric Dumazet wrote:
> skb truesize currently accounts for sk_buff struct and part of skb head.
> 
> Considering that skb_shared_info is larger than sk_buff, its time to
> take it into account for better memory accounting.
> 
> This patch introduces SKB_TRUESIZE(X) macro to centralize various
> assumptions into a single place.
> 
> At skb alloc phase, we put skb_shared_info struct at the exact end of
> skb head, to allow a better use of memory (lowering number of
> reallocations), since kmalloc() gives us power-of-two memory blocks.
[...]
> index 5b2c5f1..be66154 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -184,11 +184,15 @@ struct sk_buff *__alloc_skb(unsigned int size, gfp_t gfp_mask,
>  		goto out;
>  	prefetchw(skb);
>  
> -	size = SKB_DATA_ALIGN(size);
> -	data = kmalloc_node_track_caller(size + sizeof(struct skb_shared_info),
> -			gfp_mask, node);
> +	size += SKB_DATA_ALIGN(sizeof(struct skb_shared_info));
[...]

If we want to put the data and skb_shared_info on separate cache-lines
then we should use:
	size = SKB_DATA_ALIGN(size) + sizeof(struct skb_shared_info);
(which is effectively what we're doing now).

If that's not important, and we just want to be sure that the allocation
occupies at least a whole cache line, then it should be:
	size = SKB_DATA_ALIGN(size + sizeof(struct skb_shared_info));

But I don't think it makes sense to use SKB_DATA_ALIGN(sizeof(struct
skb_shared_info)).

Ben.

-- 
Ben Hutchings, Staff Engineer, Solarflare
Not speaking for my employer; that's the marketing department's job.
They asked us to note that Solarflare product names are trademarked.

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