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

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Message-ID: <20091217112754.GA7755@ff.dom.local>
Date:	Thu, 17 Dec 2009 11:27:55 +0000
From:	Jarek Poplawski <jarkao2@...il.com>
To:	Krishna Kumar2 <krkumar2@...ibm.com>
Cc:	Sridhar Samudrala <sri@...ibm.com>,
	Herbert Xu <herbert@...dor.apana.org.au>, mst@...hat.com,
	netdev@...r.kernel.org, Rusty Russell <rusty@...tcorp.com.au>
Subject: Re: [RFC PATCH] Regression in linux 2.6.32 virtio_net seen with
	vhost-net

On 17-12-2009 11:03, Krishna Kumar2 wrote:
>> Sridhar Samudrala <sri@...ibm.com>
>>
>> Re: [RFC PATCH] Regression in linux 2.6.32 virtio_net seen with vhost-net
>>
>> Herbert Xu wrote:
>>> On Wed, Dec 16, 2009 at 09:05:32PM -0800, Sridhar Samudrala wrote:
>>>
>>>> I think sch_direct_xmit() is not even calling dev_hard_start_xmit() as
> 
>>>> the tx queue is stopped
>>>> and does a dev_requeue_skb() and returns NETDEV_TX_BUSY.
>>>>
>>> Yes but if the queue was stopped then we shouldn't even get into
>>> sch_direct_xmit.
>> I don't see any checks for txq_stopped in the callers of
> sch_direct_xmit() :
>> __dev_xmit_skb() and qdisc_restart().  Both these routines get the txq
>> and call
>> sch_direct_xmit() which checks if tx queue is stopped or frozen.
>>
>> Am i missing something?
> 
> Yes - dequeue_skb.
> 
> The final skb, before the queue was stopped, is transmitted by
> the driver. The next time sch_direct_xmit is called, it gets a
> skb and finds the device is stopped and requeue's the skb.

So we _should_ get into sch_direct_xmit when the queue was stopped...
I guess Herbert might forget the multiqueue change, and Sridhar isn't
missing much. ;-)

Thanks,
Jarek P.
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