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

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Date:	Wed, 29 Jul 2009 18:58:33 +0530
From:	Krishna Kumar2 <krkumar2@...ibm.com>
To:	Herbert Xu <herbert@...dor.apana.org.au>
Cc:	David Miller <davem@...emloft.net>, jarkao2@...il.com,
	netdev@...r.kernel.org
Subject: Re: [RFC] [PATCH] Don't run __qdisc_run() on a stopped TX queue

Hi Herbert,

Herbert Xu <herbert@...dor.apana.org.au> wrote on 07/29/2009 06:14:28 AM:
>
> Re: [RFC] [PATCH] Don't run __qdisc_run() on a stopped TX queue
>
> On Tue, Jul 28, 2009 at 12:59:19PM -0700, David Miller wrote:
> >
> > The premise is that there'd be only one.  The qdisc lock.
> >
> > If the traffic is distributed, flow wise, the driver XMIT
> > lock would spread due to multiqueue.
>
> Suppose that we have a single large flow going through that has
> filled up the hardware queue and is now backlogged in the qdisc
> with qdisc_run on CPU A.   Now some other flow comes along and
> sends a packet on CPU B.
>
> So now CPU A and B will both be processing packets for the first
> flow causing loads of lock contention.
>
> But worse yet, we have introduced packet reordering.  So are you
> convinced now :)

I am probably misunderstanding you, but ... For different flows, is
this an issue? The same TCP connection will always use a single TXQ,
even if it runs on different CPUs since dev_pick_tx will select the
same txq, so how will reordering happen?

thanks,

- KK

(I am off tomorrow, but will be around for the next 3 hours)

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