netdev - Re: [PATCH ver2] Avoid enqueuing skb for default qdiscs

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Date:	Mon, 01 Feb 2010 10:52:20 +0100
From:	Maxime Bizon <mbizon@...ebox.fr>
To:	Krishna Kumar <krkumar2@...ibm.com>
Cc:	davem@...emloft.net, Jarek Poplawski <jarkao2@...il.com>,
	netdev@...r.kernel.org, herbert@...dor.apana.org.au,
	kaber@...sh.net
Subject: Re: [PATCH ver2] Avoid enqueuing skb for default qdiscs

On Thu, 2009-08-06 at 17:14 +0530, Krishna Kumar wrote:

Hello,

> From: Krishna Kumar <krkumar2@...ibm.com>
> 
> dev_queue_xmit enqueue's a skb and calls qdisc_run which
> dequeue's the skb and xmits it. In most cases, the skb that
> is enqueue'd is the same one that is dequeue'd (unless the
> queue gets stopped or multiple cpu's write to the same queue
> and ends in a race with qdisc_run). For default qdiscs, we
> can remove the redundant enqueue/dequeue and simply xmit the
> skb since the default qdisc is work-conserving.

Sorry old stuff, but I just noticed this.

One side effect of this patch is that tc now shows a lot of requeue for
slow interfaces (slow dsl link in my case), because sch_direct_xmit is
called even if device stopped its queue.

My first reflex was too investigate for a misbehaving driver (returning
TX_BUSY), but start_xmit is not even called, sch_direct_xmit notices
that queue is stopped, and just does a dequeue/requeue.

Since we're talking about slow interfaces, this has no impact on
performance, the requeue counter incrementing is just a bit scary.

Shouldn't we check for stopped queue earlier to avoid this ?

Regards,

-- 
Maxime

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