netdev - Re: [PATCH V3 1/2] virtio-net: fix the set affinity bug when CPU IDs are not consecutive

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Message-ID: <87a9sh3lru.fsf@rustcorp.com.au>
Date:	Thu, 10 Jan 2013 11:19:09 +1030
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Wanlong Gao <gaowanlong@...fujitsu.com>
Cc:	linux-kernel@...r.kernel.org,
	"Michael S. Tsirkin" <mst@...hat.com>,
	Jason Wang <jasowang@...hat.com>,
	Eric Dumazet <erdnetdev@...il.com>,
	virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
	Wanlong Gao <gaowanlong@...fujitsu.com>
Subject: Re: [PATCH V3 1/2] virtio-net: fix the set affinity bug when CPU IDs are not consecutive

Wanlong Gao <gaowanlong@...fujitsu.com> writes:
> On 01/09/2013 07:31 AM, Rusty Russell wrote:
>> Wanlong Gao <gaowanlong@...fujitsu.com> writes:
>>>   */
>>>  static u16 virtnet_select_queue(struct net_device *dev, struct sk_buff *skb)
>>>  {
>>> -	int txq = skb_rx_queue_recorded(skb) ? skb_get_rx_queue(skb) :
>>> -		  smp_processor_id();
>>> +	int txq = 0;
>>> +
>>> +	if (skb_rx_queue_recorded(skb))
>>> +		txq = skb_get_rx_queue(skb);
>>> +	else if ((txq = per_cpu(vq_index, smp_processor_id())) == -1)
>>> +		txq = 0;
>> 
>> You should use __get_cpu_var() instead of smp_processor_id() here, ie:
>> 
>>         else if ((txq = __get_cpu_var(vq_index)) == -1)
>> 
>> And AFAICT, no reason to initialize txq to 0 to start with.
>> 
>> So:
>> 
>>         int txq;
>> 
>>         if (skb_rx_queue_recorded(skb))
>> 		txq = skb_get_rx_queue(skb);
>>         else {
>>                 txq = __get_cpu_var(vq_index);
>>                 if (txq == -1)
>>                         txq = 0;
>>         }
>
> Got it, thank you.
>
>> 
>> Now, just to confirm, I assume this can happen even if we use vq_index,
>> right, because of races with virtnet_set_channels?
>
> I still can't understand this race, could you explain more? thank you.

I assume that someone can call virtnet_set_channels() while we are
inside virtnet_select_queue(), so they reduce dev->real_num_tx_queues,
causing virtnet_set_channels to do:

	while (unlikely(txq >= dev->real_num_tx_queues))
		txq -= dev->real_num_tx_queues;

Otherwise, when is this loop called?

Thanks,
Rusty.
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