netdev - Re: [RFC] [PATCH] Don't run __qdisc

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Message-ID: <20090728064319.GA5424@ff.dom.local>
Date:	Tue, 28 Jul 2009 06:43:19 +0000
From:	Jarek Poplawski <jarkao2@...il.com>
To:	David Miller <davem@...emloft.net>
Cc:	herbert@...dor.apana.org.au, krkumar2@...ibm.com,
	netdev@...r.kernel.org
Subject: Re: [RFC] [PATCH] Don't run __qdisc_run() on a stopped TX queue

On Mon, Jul 27, 2009 at 09:21:07PM -0700, David Miller wrote:
> From: Herbert Xu <herbert@...dor.apana.org.au>
> Date: Tue, 28 Jul 2009 10:48:13 +0800
> 
> > On Mon, Jul 27, 2009 at 07:28:44PM -0700, David Miller wrote:
> >>
> >> There is a locking benefit even for non-default qdiscs.
> >> 
> >> Instead of two choke points (qdisc lock and queue lock) there
> >> is now only one (qdisc lock) and consdiering the cost of
> >> things like setting up IOMMU mappings and hitting chip
> >> registers the qdisc lock is the shortest held of the two.
> > 
> > But only one CPU can process a given qdisc at one time so I don't
> > see why there is a second choke point if you use a single queue
> > with a non-default qdisc.
> 
> Good point, but this only suggests that we might want to undo that
> queue runner exclusivity state bit for this case especially when we
> know that we are feeding a multiqueue device.

I guess the main sacrifice of Herbert's idea is sch_multiq, and we
could probably consider some compromise like doing qdisc_run (with
qdisc_restart) a callback or only adding a "if default qdisc" check,
depending on costs/gains.

Jarek P.
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