netdev - Re: [PATCH take 2] pkt_sched: Fix qdisc_watchdog() vs. dev

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Date:	Mon, 15 Sep 2008 07:20:08 +0000
From:	Jarek Poplawski <jarkao2@...il.com>
To:	Herbert Xu <herbert@...dor.apana.org.au>
Cc:	Alexander Duyck <alexander.duyck@...il.com>,
	David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
	kaber@...sh.net
Subject: Re: [PATCH take 2] pkt_sched: Fix qdisc_watchdog() vs.
	dev_deactivate() race

On Sun, Sep 14, 2008 at 11:19:22PM -0700, Herbert Xu wrote:
> On Mon, Sep 15, 2008 at 06:07:58AM +0000, Jarek Poplawski wrote:
> >
> > Well, it was only wondering, and probably you are right this is wrong.
> > On the other hand, simple_tx_hash() choices are "probabilistic": user
> > doesn't care if it goes through tx_queue #1 or #11. And here, in some
> > cases, some tx_queues could be always full while other always empty,
> > so some dynamic rehashing could be thought of, but I understand it's
> > not trivial.
> 
> No that would be totally wrong.  One of the important constraints
> on a TX hashing mechanism is to preserve packet ordering within
> a flow.  If simple_tx_hash started placing the same packet in
> different queues then it would do the same thing to flows which
> is unacceptable.

Of course preserving a flow consistency is must-be here, but I think
there are rehashing algorithms used in similar cases (sch_sfq) which
take care for this. As a matter of fact, I've thought of requeuing as
a best place to detect possible problems, but now I see that
Alexander's proposal let's to do this simply by observing this
TCQ_F_STOPPED flag, so I withdraw my objection.

Cheers,
Jarek P.
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