netdev - Re: [PATCH net-next] bnx2x: reduce skb truesize by 50%

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Date:	Sun, 13 Nov 2011 22:08:15 +0200
From:	"Eilon Greenstein" <eilong@...adcom.com>
To:	"Eric Dumazet" <eric.dumazet@...il.com>
cc:	"David Miller" <davem@...emloft.net>,
	"bhutchings@...arflare.com" <bhutchings@...arflare.com>,
	"pstaszewski@...are.pl" <pstaszewski@...are.pl>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next] bnx2x: reduce skb truesize by 50%

On Sun, 2011-11-13 at 11:42 -0800, Eric Dumazet wrote:
> Le dimanche 13 novembre 2011 à 20:53 +0200, Eilon Greenstein a écrit :
> 
> > I’m not sure I’m following the math over here. Assuming L1 is 64 bytes,
> > we need up to 63 bytes to align the start address (assuming SLOB is
> > being used) and additional (up to) 63 bytes at the end. That can sum up
> > to 126 bytes  am I missing something?
> > 
> 
> What do you really mean by aligning the end ?

I mean padding it to full cache line.

> How can both start and end of a frame can be aligned ?

The packet will start at aligned address and (using padding) will end at
cache line boundaries.

> If hardware needs extra room after the end of frame, then we already
> have it (since we store struct skb_shared_info here)

We have some space in there, but as far as I can tell it's not up to 63
bytes, right? We will overrun the dataref.

Thanks,
Eilon


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