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

Wanlong Gao
2013-01-10T09:26:11Z

On 01/10/2013 08:49 AM, Rusty Russell wrote:
> Wanlong Gao <gaowanlong@cn.fujitsu.com> writes:
>> On 01/09/2013 07:31 AM, Rusty Russell wrote:
>>> Wanlong Gao <gaowanlong@cn.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?

How about just remove this loop?

Eric, can you give a help here?

Thanks,
Wanlong Gao

>
> Thanks,
> Rusty.
>