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Date: Fri, 25 Jan 2013 15:04:30 +0800
From: Jason Wang <jasowang@...hat.com>
To: gaowanlong@...fujitsu.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
Subject: Re: [PATCH V6 2/3] virtio-net: split out clean affinity function
On 01/25/2013 02:42 PM, Wanlong Gao wrote:
> 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?
I think it's ok since we're in control path. And the point is when
you're trying to reset the affinity / preferable queues during cpu
hotplug callback, there will be another request in
virtnet_set_channels() which changing the number of queues. So the the
result of cpus == queues may out of date. Anyway you need some
synchronization.
>
> 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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