netdev - Re: [RFC PATCH] Regression in linux 2.6.32 virtio

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Message-ID: <OF30FBF3A7.542644C5-ON6525768F.004B51CE-6525768F.004B9690@in.ibm.com>
Date:	Thu, 17 Dec 2009 19:26:50 +0530
From:	Krishna Kumar2 <krkumar2@...ibm.com>
To:	Herbert Xu <herbert@...dor.apana.org.au>
Cc:	davem@...emloft.net, mst@...hat.com, netdev@...r.kernel.org,
	Rusty Russell <rusty@...tcorp.com.au>,
	Sridhar Samudrala <sri@...ibm.com>
Subject: Re: [RFC PATCH] Regression in linux 2.6.32 virtio_net seen with	vhost-net

Herbert Xu <herbert@...dor.apana.org.au> wrote on 12/17/2009 07:10:53 PM:

> > Since dequeue_skb finds the first skb that was cached in gso_skb,
> > handles the stopped txq and returns NULL for all subsequent skbs.
> > And qdisc_restart bails out immediately. Only the first skb will
> > get requeue'd, and no more skbs are taken out of the queue -
> > either dequeue or gso_skb, till the driver restarts the queue.
>
> But dev_requeue_skb immediately reschedules a new TX softirq
> so this will keep going until the hardware wakes up the queue.
>
> That is, we have an infinite loop consuming CPU that could otherwise
> be spent doing useful work, such as generating new packets.

Except that dev_requeue_skb is never called again till the device is
restarted and stopped. It is called once per stop, and after this,
dequeue_skb never returns a skb, hence no more requeue's.

Thanks,

- KK

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