netdev - [RFC] allow skb->head to point/alias to first skb frag

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Date:	Thu, 26 Apr 2012 10:10:54 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Ilpo Järvinen <ilpo.jarvinen@...sinki.fi>
Cc:	David Miller <davem@...emloft.net>, rick.jones2@...com,
	Netdev <netdev@...r.kernel.org>, therbert@...gle.com,
	ncardwell@...gle.com, maze@...gle.com,
	Yuchung Cheng <ycheng@...gle.com>
Subject: [RFC] allow skb->head to point/alias to first skb frag

On Tue, 2012-04-24 at 10:56 +0200, Eric Dumazet wrote:

> Really I have many doubts about GRO today.

Particularly if a NIC driver provides linear skbs :
Resulting gro skb is a list of skbs, no memory savings, and many cache
line misses when copying to userland.

Maybe we could have a new kind of skb head allocation/setup, pointing to
a frag of 2048 bytes instead of a kmalloc() blob.

Right now, a fragged skb used 3 blocks of memory :

1) struct sk_buff
2) a bloc of 512 or 1024 or 2048 bytes of memory (skb->head)
3) a frag of 2048 (or PAGE_SIZE/2 or PAGE_SIZE)

While a linear skb has :

1) struct sk_buff
2) a bloc of 512 or 1024 or 2048 bytes of memory (skb->head) from
kmalloc()


Idea would have :

1) struct sk_buff
2) skb->head points to frag (aliasing, no memory allocation)
3) frag of 2048 (or PAGE_SIZE/2 or PAGE_SIZE)

Or the reverse (no frag so that skb is considered as linea), but special
code to 'allow' this skb head be considered as a frag when needed
(splice() code, or GRO merge, or TCP coalescing)


That would make GRO (and TCP coalescing) much more efficient, since the
resulting aggregated skb would be :
1) struct sk_buff
2) skb->head points to 1st frag (aliasing, no memory allocation)
3) array of [1..16] frags



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