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

From: Wanlong Gao <[email-masked]>
Date: Wed, 09 Jan 2013 09:52:54 +0800

On 01/08/2013 06:26 PM, Jason Wang wrote:
> On 01/08/2013 06:07 PM, Wanlong Gao wrote:
>> As Michael mentioned, set affinity and select queue will not work very
>> well when CPU IDs are not consecutive, this can happen with hot unplug.
>> Fix this bug by traversal the online CPUs, and create a per cpu variable
>> to find the mapping from CPU to the preferable virtual-queue.
>>
>> Cc: Rusty Russell <[email-masked]>
>> Cc: "Michael S. Tsirkin" <[email-masked]>
>> Cc: Jason Wang <[email-masked]>
>> Cc: Eric Dumazet <[email-masked]>
>> Cc: [email-masked]
>> Cc: [email-masked]
>> Signed-off-by: Wanlong Gao <[email-masked]>
>> ---
>>  drivers/net/virtio_net.c | 39 +++++++++++++++++++++++++++++----------
>>  1 file changed, 29 insertions(+), 10 deletions(-)
>>
>> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
>> index a6fcf15..a77f86c 100644
>> --- a/drivers/net/virtio_net.c
>> +++ b/drivers/net/virtio_net.c
>> @@ -41,6 +41,8 @@ module_param(gso, bool, 0444);
>>  #define VIRTNET_SEND_COMMAND_SG_MAX    2
>>  #define VIRTNET_DRIVER_VERSION "1.0.0"
>>  
>> +DEFINE_PER_CPU(int, vq_index) = -1;
>> +
> 
> I think this should not be a global one, consider we may have more than
> one virtio-net cards with different max queues.

Yes, would you move this into virtio_info?

>>  struct virtnet_stats {
>>  	struct u64_stats_sync tx_syncp;
>>  	struct u64_stats_sync rx_syncp;
>> @@ -1016,6 +1018,7 @@ static int virtnet_vlan_rx_kill_vid(struct net_device *dev, u16 vid)
>>  static void virtnet_set_affinity(struct virtnet_info *vi, bool set)
>>  {
>>  	int i;
>> +	int cpu;
>>  
>>  	/* In multiqueue mode, when the number of cpu is equal to the number of
>>  	 * queue pairs, we let the queue pairs to be private to one cpu by
>> @@ -1029,16 +1032,29 @@ static void virtnet_set_affinity(struct virtnet_info *vi, bool set)
>>  			return;
>>  	}
>>  
>> -	for (i = 0; i < vi->max_queue_pairs; i++) {
>> -		int cpu = set ? i : -1;
>> -		virtqueue_set_affinity(vi->rq[i].vq, cpu);
>> -		virtqueue_set_affinity(vi->sq[i].vq, cpu);
>> -	}
>> +	if (set) {
>> +		i = 0;
>> +		for_each_online_cpu(cpu) {
>> +			virtqueue_set_affinity(vi->rq[i].vq, cpu);
>> +			virtqueue_set_affinity(vi->sq[i].vq, cpu);
>> +			per_cpu(vq_index, cpu) = i;
>> +			i++;
>> +			if (i >= vi->max_queue_pairs)
>> +				break;
> 
> Can this happen? we check only set when the number are equal.

will remove.

>> +		}
>>  
>> -	if (set)
>>  		vi->affinity_hint_set = true;
>> -	else
>> +	} else {
>> +		for(i = 0; i < vi->max_queue_pairs; i++) {
>> +			virtqueue_set_affinity(vi->rq[i].vq, -1);
>> +			virtqueue_set_affinity(vi->sq[i].vq, -1);
>> +		}
>> +
>> +		for_each_online_cpu(cpu)
>> +			per_cpu(vq_index, cpu) = -1;
>> +
> 
> This looks suboptimal since it may leads only txq zero is used.

So, which value is best for txq when we don't set affinity?
just remain to smp_processor_id()?

Thanks,
Wanlong Gao

>>  		vi->affinity_hint_set = false;
>> +	}
>>  }
>>  
>>  static void virtnet_get_ringparam(struct net_device *dev,
>> @@ -1127,12 +1143,15 @@ static int virtnet_change_mtu(struct net_device *dev, int new_mtu)
>>  
>>  /* To avoid contending a lock hold by a vcpu who would exit to host, select the
>>   * txq based on the processor id.
>> - * TODO: handle cpu hotplug.
>>   */
>>  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;
>>  
>>  	while (unlikely(txq >= dev->real_num_tx_queues))
>>  		txq -= dev->real_num_tx_queues;
> 
>