netdev - Re: [patch net-next-2.6 V3] net: convert bonding to use rx

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Date:	Sat, 19 Feb 2011 11:56:23 +0100
From:	Nicolas de Pesloüan 
	<nicolas.2p.debian@...il.com>
To:	Jiri Pirko <jpirko@...hat.com>
CC:	Jay Vosburgh <fubar@...ibm.com>,
	David Miller <davem@...emloft.net>, kaber@...sh.net,
	eric.dumazet@...il.com, netdev@...r.kernel.org,
	shemminger@...ux-foundation.org, andy@...yhouse.net
Subject: Re: [patch net-next-2.6 V3] net: convert bonding to use rx_handler

Le 19/02/2011 09:05, Jiri Pirko a écrit :
> This patch converts bonding to use rx_handler. Results in cleaner
> __netif_receive_skb() with much less exceptions needed. Also
> bond-specific work is moved into bond code.
>
> Signed-off-by: Jiri Pirko<jpirko@...hat.com>
>
> v1->v2:
>          using skb_iif instead of new input_dev to remember original
> 	device
> v2->v3:
> 	set orig_dev = skb->dev if skb_iif is set
>

Why do we need to let the rx_handlers call netif_rx() or __netif_receive_skb()?

Bonding used to be handled with very few overhead, simply replacing skb->dev with skb->dev->master. 
Time has passed and we eventually added many special processing for bonding into 
__netif_receive_skb(), but the overhead remained very light.

Calling netif_rx() (or __netif_receive_skb()) to allow nesting would probably lead to some overhead.

Can't we, instead, loop inside __netif_receive_skb(), and deliver whatever need to be delivered, to 
whoever need, inside the loop ?

rx_handler = rcu_dereference(skb->dev->rx_handler);
while (rx_handler) {
	/* ...  */
	orig_dev = skb->dev;
	skb = rx_handler(skb);
	/* ... */
	rx_handler = (skb->dev != orig_dev) ? rcu_dereference(skb->dev->rx_handler) : NULL;
}

This would reduce the overhead, while still allowing nesting: vlan on top on bonding, bridge on top 
on bonding, ...

That way, we can probably keep the list of crossed devices inside a local array, and call 
deliver_skb() with the current "orig_dev" when appropriate. No need to overload sk_buff nor to use a 
global variable.

Of course, this might be a very simplistic view.

Any comments?

	Nicolas.
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