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

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Message-ID: <1327496953.24561.323.camel@zakaz.uk.xensource.com>
Date:	Wed, 25 Jan 2012 13:09:13 +0000
From:	Ian Campbell <Ian.Campbell@...rix.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
CC:	"netdev@...r.kernel.org" <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

On Wed, 2012-01-25 at 12:51 +0000, Eric Dumazet wrote:
> 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()

Presumably build_skb should be using SKB_WITH_OVERHEAD instead of open
coding it? (which I think is how I managed to miss it)

>  and all places we use
> SKB_DATA_ALIGN(sizeof(struct skb_shared_info))

> (for example in some drivers)

I'm not sure how I missed these with my grep. Obviously it is necessary
to catch them all in order to avoid the performance drop you mention
below. Fortunately there is fewer than a dozen instances in half a dozen
drivers to check.

> 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

Can you elaborate on the specific benchmark you used there?

Thanks,
Ian

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