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

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Message-ID: <20090729004428.GA765@gondor.apana.org.au>
Date:	Wed, 29 Jul 2009 08:44:28 +0800
From:	Herbert Xu <herbert@...dor.apana.org.au>
To:	David Miller <davem@...emloft.net>
Cc:	krkumar2@...ibm.com, jarkao2@...il.com, netdev@...r.kernel.org
Subject: 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 :)

Cheers,
-- 
Visit Openswan at http://www.openswan.org/
Email: Herbert Xu ~{PmV>HI~} <herbert@...dor.apana.org.au>
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