netdev - Re: [PATCH net-next] net: netdev_alloc_skb() use build

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Message-ID: <1338818958.2760.1824.camel@edumazet-glaptop>
Date:	Mon, 04 Jun 2012 16:09:18 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	Willy Tarreau <w@....eu>, David Miller <davem@...emloft.net>,
	netdev@...r.kernel.org
Subject: Re: [PATCH net-next] net: netdev_alloc_skb() use build_skb()

On Mon, 2012-06-04 at 16:01 +0200, Eric Dumazet wrote:

> Not sure 12 bytes of headroom would be enough (instead of the
> NET_SKB_PAD reserved in netdev_alloc_skb_ip_align(), but what could be
> done indeed is to use the first page as the skb->head, so using
> build_skb() indeed, removing one fragment, one (small) copy and one
> {put|get}_page() pair.

It would also avoid 'pulling' tcp data payload in linear part.

page_to_skb() does :

copy = len;
if (copy > skb_tailroom(skb))
	copy = skb_tailroom(skb);
memcpy(skb_put(skb, copy), p, copy);

This means GRO or TCP coalescing (or splice()) has to handle two
segments to fetch data.




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