netdev - Re: [RFC] multiqueue changes

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Date:	Wed, 28 Oct 2009 22:23:37 +0100
From:	Jarek Poplawski <jarkao2@...il.com>
To:	Patrick McHardy <kaber@...sh.net>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	"David S. Miller" <davem@...emloft.net>,
	Linux Netdev List <netdev@...r.kernel.org>
Subject: Re: [RFC] multiqueue changes

On Wed, Oct 28, 2009 at 06:27:10PM +0100, Patrick McHardy wrote:
> Jarek Poplawski wrote:
> > On Thu, Oct 08, 2009 at 09:03:44AM +0000, Jarek Poplawski wrote:
> > ...
> >> num_tx_queues. But it seems we should rather use more often
> >> real_num_tx_queue in schedulers code like dumps and maybe more
> >> (like e.g. sch_multiq does).
> > 
> > ...i.e. probably everywhere between dev_activate and dev_deactivate
> > all qdisc operations could use real_num_tx_queues (including a test
> > like: netif_is_real_multique), unless I miss something.
> 
> We don't seem to be supporting changing real_num_tx_queues for
> registered devices currently (at least I couldn't find it).
> So I guess it depends on how this would be implemented.
> 
> Simply changing the dev->real_num_tx_queues value while the
> device is down would require qdisc operations to operate on
> all possible queues since the amount of queues in use could
> be changed after the qdisc is created/configured, but before
> the device is set up. This approach has more complications
> like switching between mq and non-mq root qdiscs, taking care
> of non-default root qdisc (cloning them to the new queues), etc.
> 
> A simpler alternative would be to destroy the existing root
> qdisc on any change to real_num_tx_queues and have dev_activate()
> set it up from scratch. In this case, we could (as you suggested)
> use real_num_tx_queues, which should fix the problem Eric reported.

Actually, I changed my mind after Eric's and especially David's
explanations. Probably there will be needed some changes in handling
the real_num_tx_queues, but there is no reason to misuse them for
masking a totally useless num_tx_queues value, like in this case. So,
IMHO, its mainly about the driver(s) (and maybe a bit of API change)
here.

Jarek P.
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