netdev - Re: [PATCH v3 1/6] net: pad skb data and shinfo as a whole rather than individually

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Message-ID: <1327495879.2425.29.camel@edumazet-HP-Compaq-6005-Pro-SFF-PC>
Date:	Wed, 25 Jan 2012 13:51:19 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Ian Campbell <ian.campbell@...rix.com>
Cc:	netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH v3 1/6] net: pad skb data and shinfo as a whole rather
 than individually

Le mercredi 25 janvier 2012 à 12:27 +0000, Ian Campbell a écrit :
> This reduces the minimum overhead required for this allocation such that the
> shinfo can be grown in the following patch without overflowing 2048 bytes for a
> 1500 byte frame.
> 
> Reducing this overhead while also growing the shinfo means that sometimes the
> tail end of the data can end up in the same cache line as the beginning of the
> shinfo. Specifically in the case of the 64 byte cache lines on a 64 bit system
> the first 8 bytes of shinfo can overlap the tail cacheline of the data. In many
> cases the allocation slop means that there is no overlap.
> 
> Signed-off-by: Ian Campbell <ian.campbell@...rix.com>
> Cc: "David S. Miller" <davem@...emloft.net>
> Cc: Eric Dumazet <eric.dumazet@...il.com>
> ---

Hmm... you missed build_skb() and all places we use
SKB_DATA_ALIGN(sizeof(struct skb_shared_info))

(for example in some drivers)

If you want to see possible performance impact of your changes, see
commit e52fcb2462ac (bnx2x: uses build_skb() in receive path), and
expect a drop from 820.000 pps to 720.000 pps



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