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

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Message-ID: <Pine.WNT.4.64.0903211843140.17444@ppwaskie-MOBL2.amr.corp.intel.com>
Date:	Sat, 21 Mar 2009 18:48:18 -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>,
	David Miller <davem@...emloft.net>,
	"Kirsher, Jeffrey T" <jeffrey.t.kirsher@...el.com>,
	"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 Sat, 21 Mar 2009, Stephen Hemminger wrote:

> On Sat, 21 Mar 2009 00:43:40 -0700 (Pacific Daylight Time)
> "Waskiewicz Jr, Peter P" <peter.p.waskiewicz.jr@...el.com> wrote:
> 
> > On Sat, 21 Mar 2009, David Miller wrote:
> > 
> > > From: "Waskiewicz Jr, Peter P" <peter.p.waskiewicz.jr@...el.com>
> > > Date: Fri, 20 Mar 2009 23:21:38 -0700 (Pacific Daylight Time)
> > > 
> > > > 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.
> > > 
> > > It can't, if you only advertise one transmit queue the kernel
> > > can never ever choose anything other than queue zero.  It's
> > > impossible.
> > > 
> > > Stephen's right, you guys don't need your select queue override.
> > > 
> > > And if you recall I suspected this from the very beginning.
> > > 
> > > You guys never ever think out of the box, ever...  if it's
> > > not straightforward, you guys won't got for it.  That makes
> > > it very frustrating to get anything done.
> > 
> > This patch will break DCB in ixgbe.  We need all 8 queues, because the 
> > user will be assigning tc filters to the sch_multiq qdisc to get traffic 
> > into priority queues.  If we take Stephen's patch and tell the stack we 
> > have 1 real_num_tx_queues, then we get 1 band in sch_multiq, which makes 
> > it impossible to assign traffic to priorities 1 through 8:
> > 
> 
> How does it make sense to say you have 8 bands, but only use one.

That's not how DCB works.  We use sch_multiq to identify the traffic we 
want to put into the 8 bands.  So in other words, the user will add tc 
filters to move the traffic around.  We override select queue to filter 
the rest of the traffic into a single queue, so we don't randomly put 
traffic into the other hardware priority queues.

Either way, thanks for cleaning this up Stephen.  It was something I 
needed to do, and haven't done yet.  So I very much appreciate it.

Cheers,
-PJ Waskiewicz
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