netdev - Re: [PATCH 2/2] ixgbe: fix select

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Date:	Fri, 20 Mar 2009 23:21:38 -0700 (Pacific Daylight Time)
From:	"Waskiewicz Jr, Peter P" <peter.p.waskiewicz.jr@...el.com>
To:	Stephen Hemminger <shemminger@...tta.com>
cc:	"Waskiewicz Jr, Peter P" <peter.p.waskiewicz.jr@...el.com>,
	"Kirsher, Jeffrey T" <jeffrey.t.kirsher@...el.com>,
	David Miller <davem@...emloft.net>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"e1000-devel@...ts.sourceforge.net" 
	<e1000-devel@...ts.sourceforge.net>
Subject: Re: [PATCH 2/2] ixgbe: fix select_queue management (v2)

On Fri, 20 Mar 2009, Stephen Hemminger wrote:

> The default select queue function in the kernel is:
> 
> static struct netdev_queue *dev_pick_tx(struct net_device *dev,
> 					struct sk_buff *skb)
> {
> 	const struct net_device_ops *ops = dev->netdev_ops;
> 	u16 queue_index = 0;
> 
> 	if (ops->ndo_select_queue)
> 		queue_index = ops->ndo_select_queue(dev, skb);
> 	else if (dev->real_num_tx_queues > 1)
> 		queue_index = skb_tx_hash(dev, skb);
> 
> 	skb_set_queue_mapping(skb, queue_index);
> 	return netdev_get_tx_queue(dev, queue_index);
> }
> 
> So if driver (re)sets real_num_tx_queues to 1 then queue_index will always
> 0 and all traffic will go to one queue. This is the same as having your
> own select_queue function.

I see your point, but it is a hack in my opinion.  The device will have 8 
real Tx queues, not 1.  I'd much rather go with the original proposal, 
since if the code in dev_pick_tx() changed, it could silently break ixgbe.

-PJ Waskiewicz
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