netdev - Re: [RFC] multiqueue changes

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Date:	Fri, 30 Oct 2009 10:00:33 +0000
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 Thu, Oct 29, 2009 at 11:12:39PM +0100, Patrick McHardy wrote:
> Jarek Poplawski wrote:
> > On Thu, Oct 29, 2009 at 05:37:23PM +0100, Patrick McHardy wrote:
> > ...
> >> Well, we do need both values for supporting changes to the actually
> >> used numbers of TX queues. If I understood Dave's explanation correctly,
> >> this is also what's intended. It also doesn't seem unreasonable
> >> what bnx2 is doing.
> > 
> > Exactly. With a growing number of cores, both available and powered
> > off, these values will be soon treated more carefully than now.
> > 
> >> But getting back to the problem Eric reported - so you're suggesting
> >> that bnx2.c should also adjust num_tx_queues in case the hardware
> >> doesn't support multiqueue? That seems reasonable as well.
> > 
> > Currently, declaring num_tx_queues with alloc_netdev_mq() looks like
> > too soon for some drivers. It seems they should be able to do it
> > separately later during the .probe.
> 
> The value passed into alloc_netdev_mq() is just used for allocation
> purposes, from what I can tell there's no downside in reducing it
> before the dev_activate() call.

Right, but IMHO this reducing (or reallocation) should be done with
some API. Simple overwriting of num_tx_queues proposed by Eric, even
if no downside now, seems to be asking for problems in the future.

> > There is a question if .ndo_open should be considered too.
> 
> I currently can't see any purpose in decreasing num_tx_queues after
> registration instead of real_num_tx_queues.

I agree, but since Eric's example shows some drivers do it (almost)
like this, I'd prefer authors/maintainers answer this question.

Jarek P.
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