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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) -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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