netdev - Re: [RFC] multiqueue changes

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Date:	Thu, 29 Oct 2009 17:37:23 +0100
From:	Patrick McHardy <kaber@...sh.net>
To:	Jarek Poplawski <jarkao2@...il.com>
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

Jarek Poplawski wrote:
> On Wed, Oct 28, 2009 at 06:27:10PM +0100, Patrick McHardy wrote:
>> 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.

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.

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.

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