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

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Message-Id: <20090728.180647.241258705.davem@davemloft.net>
Date:	Tue, 28 Jul 2009 18:06:47 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	herbert@...dor.apana.org.au
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

From: Herbert Xu <herbert@...dor.apana.org.au>
Date: Wed, 29 Jul 2009 08:44:28 +0800

> 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 :)

More interesting to me is the case where the queue is not
filled up, or is very nearly so. :-)

But yes I do see your point.
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