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

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Date:	Thu, 13 Oct 2011 18:24:50 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Ben Hutchings <bhutchings@...arflare.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

Le jeudi 13 octobre 2011 à 17:05 +0100, Ben Hutchings a écrit :
> 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).
> 

Same behavior after my patch : skb_shared_info starts at a cache-line
boundary, like before, unless kmalloc() gives us unaligned memory (it
can in certain debugging situations)

So previous part (before skb_shared_info) will also be a multiple of
SMP_CACHE_BYTES because of kmalloc() behavior.

> 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)).

If you take a closer look, you'll see that my patch addresses your
concerns, but at minimal cpu cost.

kmalloc(size + SKB_DATA_ALIGN(sizeof(struct skb_shared_info)))

will give same result than :

kmalloc(SKB_DATA_ALIGN(size) + SKB_DATA_ALIGN(sizeof(struct
skb_shared_info)))

But my version is a bit faster (a single add of a compiler known
constant)



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