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Date: Fri, 25 Jan 2013 14:42:30 +0800
From: Wanlong Gao <gaowanlong@...fujitsu.com>
To: Jason Wang <jasowang@...hat.com>
CC: linux-kernel@...r.kernel.org,
Rusty Russell <rusty@...tcorp.com.au>,
"Michael S. Tsirkin" <mst@...hat.com>,
Eric Dumazet <erdnetdev@...il.com>,
virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
Wanlong Gao <gaowanlong@...fujitsu.com>
Subject: Re: [PATCH V6 2/3] virtio-net: split out clean affinity function
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 <rusty@...tcorp.com.au>
>>>>>> Cc: "Michael S. Tsirkin" <mst@...hat.com>
>>>>>> Cc: Jason Wang <jasowang@...hat.com>
>>>>>> Cc: Eric Dumazet <erdnetdev@...il.com>
>>>>>> Cc: virtualization@...ts.linux-foundation.org
>>>>>> Cc: netdev@...r.kernel.org
>>>>>> Signed-off-by: Wanlong Gao <gaowanlong@...fujitsu.com>
>>>>>> ---
>>>>>> 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;
>>>
>> --
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