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

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Date:	Wed, 29 Jul 2009 19:11:34 +0800
From:	Herbert Xu <herbert@...dor.apana.org.au>
To:	Jarek Poplawski <jarkao2@...il.com>
Cc:	David Miller <davem@...emloft.net>, krkumar2@...ibm.com,
	netdev@...r.kernel.org
Subject: Re: [RFC] [PATCH] Don't run __qdisc_run() on a stopped TX queue

On Wed, Jul 29, 2009 at 11:04:36AM +0000, Jarek Poplawski wrote:
>
> How about this: instead of the _RUNNING flag we take tx lock while
> holding qdisc lock and release qdisc lock just after (before xmit).
> This should prevent reordering, and probably could improve cache use:
> CPU B which takes qdisc lock only for enqueuing now, would use it for
> dequeuing too, plus if accidentally the next xmit goes to a different
> tx queue, it could start before CPU A finishes. Otherwise it would
> simply wait for CPU A (without tx lock contention). Of course it
> needs testing... 

Well reordering isn't the only problem, the lock contention brought
upon by two CPUs both trying to transmit the same flow from the
qdisc is just as bad.

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