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

From: Wanlong Gao <[email-masked]>
Date: Fri, 25 Jan 2013 14:42:30 +0800

On 01/25/2013 02:12 PM, Jason Wang wrote:
> 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?

IMHO, serialize every time will take lock and may slow down this path,
but the hot unplug path will be more cold than it. So I prefer reset the
preferable queues in DOWN_PREPARE but not serialize them. Agree?

Thanks,
Wanlong Gao

> 
> 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;
>>>
>> --
>> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
>> the body of a message to [email-masked]
>> More majordomo info at  http://vger.kernel.org/majordomo-info.html
>> Please read the FAQ at  http://www.tux.org/lkml/
> 
>