Subject: Re: [PATCH V6 2/3] virtio-net: split out clean affinity function

From: Jason Wang <[email-masked]>
Date: Fri, 25 Jan 2013 14:12:28 +0800

On 01/25/2013 01:40 PM, Wanlong Gao wrote:
> On 01/25/2013 01:13 PM, Jason Wang wrote:
>> On 01/25/2013 12:20 PM, Wanlong Gao wrote:
>>> On 01/25/2013 11:28 AM, Jason Wang wrote:
>>>> On 01/21/2013 07:25 PM, Wanlong Gao wrote:
>>>>> Split out the clean affinity function to virtnet_clean_affinity().
>>>>>
>>>>> 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]>
>>>>> ---
>>>>> V5->V6: NEW
>>>>>
>>>>>  drivers/net/virtio_net.c | 67 +++++++++++++++++++++++++++---------------------
>>>>>  1 file changed, 38 insertions(+), 29 deletions(-)
>>>>>
>>>>> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
>>>>> index 70cd957..1a35a8c 100644
>>>>> --- a/drivers/net/virtio_net.c
>>>>> +++ b/drivers/net/virtio_net.c
>>>>> @@ -1016,48 +1016,57 @@ static int virtnet_vlan_rx_kill_vid(struct net_device *dev, u16 vid)
>>>>>  	return 0;
>>>>>  }
>>>>>  
>>>>> -static void virtnet_set_affinity(struct virtnet_info *vi, bool set)
>>>>> +static void virtnet_clean_affinity(struct virtnet_info *vi, long hcpu)
>>>>>  {
>>>>>  	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
>>>>> -	 * setting the affinity hint to eliminate the contention.
>>>>> -	 */
>>>>> -	if ((vi->curr_queue_pairs == 1 ||
>>>>> -	     vi->max_queue_pairs != num_online_cpus()) && set) {
>>>>> -		if (vi->affinity_hint_set)
>>>>> -			set = false;
>>>>> -		else
>>>>> -			return;
>>>>> -	}
>>>>> -
>>>>> -	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_ptr(vi->vq_index, cpu) = i;
>>>>> -			i++;
>>>>> -		}
>>>>> -
>>>>> -		vi->affinity_hint_set = true;
>>>>> -	} else {
>>>>> -		for(i = 0; i < vi->max_queue_pairs; i++) {
>>>>> +	if (vi->affinity_hint_set) {
>>>>> +		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);
>>>>>  		}
>>>>>  
>>>>>  		i = 0;
>>>>> -		for_each_online_cpu(cpu)
>>>>> +		for_each_online_cpu(cpu) {
>>>>> +			if (cpu == hcpu)
>>>>> +				continue;
>>>>>  			*per_cpu_ptr(vi->vq_index, cpu) =
>>>>>  				++i % vi->curr_queue_pairs;
>>>>> +		}
>>>>>  
>>>> Some questions here:
>>>>
>>>> - Did we need reset the affinity of the queue here like the this?
>>>>
>>>> virtqueue_set_affinity(vi->sq[*per_cpu_ptr(vi->vq_index, hcpu)], -1);
>>>> virtqueue_set_affinity(vi->rq[*per_cpu_ptr(vi->vq_index, hcpu)], -1);
>>> I think no, we are going to unset the affinity of all the set queues,
>>> include hcpu.
>>>
>>>> - Looks like we need also reset the percpu index when
>>>> vi->affinity_hint_set is false.
>>> Yes, follow this and the comment on [1/3].
>>>
>>>> - Does this really need this reset? Consider we're going to reset the
>>>> percpu in CPU_DEAD?
>>> I think resetting when CPU_DOWN_PREPARE can avoid selecting the wrong queue
>>> on the dying CPU.
>> Didn't understand this. What does 'wrong queue' here mean? Looks like
>> you didn't change the preferable queue of the dying CPU and just change
>> all others.
> How about setting the vq index to -1 on hcpu when doing DOWN_PREPARE?
> So that let it select txq to 0 when the CPU is dying.

Looks safe, so look like what you're going to solve here is the the race
between cpu hotplug and virtnet_set_channels(). A possible better
solution is to serialize them by protecting virtnet_set_queues() by
get_online_cpus() also. After this, we can make sure the number of
channels were not changed during cpu hotplug, and looks like there's no
need to reset the preferable queues in DOWN_PREPARE.

What's your opinion?

Thanks
>
> Thanks,
> Wanlong Gao
>
>>> Thanks,
>>> Wanlong Gao
>>>
>>>> Thanks
>>>>>  		vi->affinity_hint_set = false;
>>>>>  	}
>>>>>  }
>>>>>  
>>>>> +static void virtnet_set_affinity(struct virtnet_info *vi)
>>>>> +{
>>>>> +	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
>>>>> +	 * setting the affinity hint to eliminate the contention.
>>>>> +	 */
>>>>> +	if (vi->curr_queue_pairs == 1 ||
>>>>> +	    vi->max_queue_pairs != num_online_cpus()) {
>>>>> +		if (vi->affinity_hint_set)
>>>>> +			virtnet_clean_affinity(vi, -1);
>>>>> +		else
>>>>> +			return;
>>>>> +	}
>>>>> +
>>>>> +	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_ptr(vi->vq_index, cpu) = i;
>>>>> +		i++;
>>>>> +	}
>>>>> +
>>>>> +	vi->affinity_hint_set = true;
>>>>> +}
>>>>> +
>>>>>  static void virtnet_get_ringparam(struct net_device *dev,
>>>>>  				struct ethtool_ringparam *ring)
>>>>>  {
>>>>> @@ -1105,7 +1114,7 @@ static int virtnet_set_channels(struct net_device *dev,
>>>>>  		netif_set_real_num_rx_queues(dev, queue_pairs);
>>>>>  
>>>>>  		get_online_cpus();
>>>>> -		virtnet_set_affinity(vi, true);
>>>>> +		virtnet_set_affinity(vi);
>>>>>  		put_online_cpus();
>>>>>  	}
>>>>>  
>>>>> @@ -1274,7 +1283,7 @@ static void virtnet_del_vqs(struct virtnet_info *vi)
>>>>>  {
>>>>>  	struct virtio_device *vdev = vi->vdev;
>>>>>  
>>>>> -	virtnet_set_affinity(vi, false);
>>>>> +	virtnet_clean_affinity(vi, -1);
>>>>>  
>>>>>  	vdev->config->del_vqs(vdev);
>>>>>  
>>>>> @@ -1398,7 +1407,7 @@ static int init_vqs(struct virtnet_info *vi)
>>>>>  		goto err_free;
>>>>>  
>>>>>  	get_online_cpus();
>>>>> -	virtnet_set_affinity(vi, true);
>>>>> +	virtnet_set_affinity(vi);
>>>>>  	put_online_cpus();
>>>>>  
>>>>>  	return 0;
>>
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