netdev - Re: [net-next-2.6 PATCH] net: netif_set_real_num_rx_queues may cap num_rx

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Date:	Tue, 05 Oct 2010 17:34:36 +0100
From:	Ben Hutchings <bhutchings@...arflare.com>
To:	John Fastabend <john.r.fastabend@...el.com>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"therbert@...gle.com" <therbert@...gle.com>
Subject: Re: [net-next-2.6 PATCH] net: netif_set_real_num_rx_queues may cap
 num_rx_queues at init time

On Tue, 2010-10-05 at 09:08 -0700, John Fastabend wrote:
> On 10/4/2010 10:35 PM, Eric Dumazet wrote:
> > Le lundi 04 octobre 2010 à 15:00 -0700, John Fastabend a écrit :
> >> The logic for netif_set_real_num_rx_queues is the following,
> >>
> >> netif_set_real_num_rx_queues(dev, rxq)
> >> {
> >> 	...
> >> 	if (dev->reg_state == NETREG_REGISTERED) {
> >> 		...
> >> 	} else {
> >> 		dev->num_rx_queues = rxq;
> >> 	}
> >>
> >> 	dev->real_num_rx_queues = rxq;
> >> 	return 0;
> >> }
> >>
> >> Some drivers init path looks like the following,
> >>
> >> alloc_etherdev_mq(priv_sz, max_num_queues_ever);
> >> ...
> >> netif_set_real_num_rx_queues(dev, queues_to_use_now);
> >> ...
> >> register_netdev(dev);
> >> ...
> >>
> >> Because netif_set_real_num_rx_queues sets num_rx_queues if the
> >> reg state is not NETREG_REGISTERED we end up with the incorrect
> >> max number of rx queues. This patch proposes to remove the else
> >> clause above so this does not occur.  Also just reading the
> >> function set_real_num it seems a bit unexpected that num_rx_queues
> >> gets set.
> >>
> > 
> > You dont tell why its "incorrect".
> > 
> 
> OK that is a poor description.
> 
> > Why should we keep num_rx_queues > real_num_rx_queues ?
> > 
> 
> If we do not ever need them then we should not keep them I agree.
> But having netif_set_real_num_rx_queues set something other then
> 'real_num_rx_queues' does not seem right to me at least. Also
> netif_set_real_num_tx_queues and netif_set_real_num_rx_queues have
> different behavior. It would be nice if this weren't the case but
> they allocate queues in two places.
[...]

I only did this to satisfy Eric's desire to reduce memory usage.
However, I believe that there are currently no drivers that dynamically
increase numbers of RX or TX queues.  Until there are, there is not much
point in removing this assignment to num_rx_queues.

Ben.

-- 
Ben Hutchings, Senior Software Engineer, Solarflare Communications
Not speaking for my employer; that's the marketing department's job.
They asked us to note that Solarflare product names are trademarked.

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