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

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Date:	Thu, 13 Oct 2011 22:51:55 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Andi Kleen <ak@...ux.intel.com>
Cc:	David Miller <davem@...emloft.net>, netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next] net: more accurate skb truesize

Le jeudi 13 octobre 2011 à 13:33 -0700, Andi Kleen a écrit :
> On Thu, Oct 13, 2011 at 05:26:21PM +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.
> 
> It's still quite inaccurate, especially for the kmalloced data area if it's not
> paged. It would be better to ask slab how much memory was really 
> allocated. But at least this could be done more easily now with the new 
> macro, so it's definitely a step in the right direction.

Note : in skb_alloc() function, SKB_TRUESIZE(size) delivers the exact
value : I do the ksize(data) call to ask how many byte kmalloc()
provided me.

So skb->truesize is quite accurate (unless KMEMCHECK or other debug
stuff is used of course)

For the SKB_TRUESIZE() macro, we dont want to do a dummy call to
kmalloc()/kfree(), since its basically used to roughly set a queue
limit.

Thanks !


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