Subject: Re: [RFC PATCH] virtio-net: reset virtqueue affinity when doing cpu hotplug

From: Jason Wang <[email-masked]>
Date: Thu, 27 Dec 2012 11:34:16 +0800

On 12/26/2012 06:46 PM, Michael S. Tsirkin wrote:
> On Wed, Dec 26, 2012 at 03:06:54PM +0800, Wanlong Gao wrote:
>> Add a cpu notifier to virtio-net, so that we can reset the
>> virtqueue affinity if the cpu hotplug happens. It improve
>> the performance through enabling or disabling the virtqueue
>> affinity after doing cpu hotplug.
>>
>> Cc: Rusty Russell <[email-masked]>
>> Cc: "Michael S. Tsirkin" <[email-masked]>
>> Cc: Jason Wang <[email-masked]>
>> Cc: [email-masked]
>> Cc: [email-masked]
>> Signed-off-by: Wanlong Gao <[email-masked]>
> Thanks for looking into this.
> Some comments:
>
> 1. Looks like the logic in
> virtnet_set_affinity (and in virtnet_select_queue)
> will not work very well when CPU IDs are not
> consequitive. This can happen with hot unplug.
>
> Maybe we should add a VQ allocator, and defining
> a per-cpu variable specifying the VQ instead
> of using CPU ID.

Yes, and generate the affinity hint based on the mapping. Btw, what does
VQ allocator means here?
>
>
> 2. The below code seems racy e.g. when CPU is added
> 	during device init.
>
> 3. using a global cpu_hotplug seems inelegant.
> In any case we should document what is the
> meaning of this variable.
>
>> ---
>>  drivers/net/virtio_net.c | 39 ++++++++++++++++++++++++++++++++++++++-
>>  1 file changed, 38 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
>> index a6fcf15..9710cf4 100644
>> --- a/drivers/net/virtio_net.c
>> +++ b/drivers/net/virtio_net.c
>> @@ -26,6 +26,7 @@
>>  #include <linux/scatterlist.h>
>>  #include <linux/if_vlan.h>
>>  #include <linux/slab.h>
>> +#include <linux/cpu.h>
>>  
>>  static int napi_weight = 128;
>>  module_param(napi_weight, int, 0444);
>> @@ -34,6 +35,8 @@ static bool csum = true, gso = true;
>>  module_param(csum, bool, 0444);
>>  module_param(gso, bool, 0444);
>>  
>> +static bool cpu_hotplug = false;
>> +
>>  /* FIXME: MTU in config. */
>>  #define MAX_PACKET_LEN (ETH_HLEN + VLAN_HLEN + ETH_DATA_LEN)
>>  #define GOOD_COPY_LEN	128
>> @@ -1041,6 +1044,26 @@ static void virtnet_set_affinity(struct virtnet_info *vi, bool set)
>>  		vi->affinity_hint_set = false;
>>  }
>>  
>> +static int virtnet_cpu_callback(struct notifier_block *nfb,
>> +			       unsigned long action, void *hcpu)
>> +{
>> +	switch(action) {
>> +	case CPU_ONLINE:
>> +	case CPU_ONLINE_FROZEN:
>> +	case CPU_DEAD:
>> +	case CPU_DEAD_FROZEN:
>> +		cpu_hotplug = true;
>> +		break;
>> +	default:
>> +		break;
>> +	}
>> +	return NOTIFY_OK;
>> +}
>> +
>> +static struct notifier_block virtnet_cpu_notifier = {
>> +	.notifier_call = virtnet_cpu_callback,
>> +};
>> +
>>  static void virtnet_get_ringparam(struct net_device *dev,
>>  				struct ethtool_ringparam *ring)
>>  {
>> @@ -1131,7 +1154,14 @@ static int virtnet_change_mtu(struct net_device *dev, int new_mtu)
>>   */
>>  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) :
>> +	int txq;
>> +
>> +	if (unlikely(cpu_hotplug == true)) {
>> +		virtnet_set_affinity(netdev_priv(dev), true);
>> +		cpu_hotplug = false;
>> +	}
>> +
>> +	txq = skb_rx_queue_recorded(skb) ? skb_get_rx_queue(skb) :
>>  		  smp_processor_id();
>>  
>>  	while (unlikely(txq >= dev->real_num_tx_queues))
>> @@ -1248,6 +1278,8 @@ static void virtnet_del_vqs(struct virtnet_info *vi)
>>  {
>>  	struct virtio_device *vdev = vi->vdev;
>>  
>> +	unregister_hotcpu_notifier(&virtnet_cpu_notifier);
>> +
>>  	virtnet_set_affinity(vi, false);
>>  
>>  	vdev->config->del_vqs(vdev);
>> @@ -1372,6 +1404,11 @@ static int init_vqs(struct virtnet_info *vi)
>>  		goto err_free;
>>  
>>  	virtnet_set_affinity(vi, true);
>> +
>> +	ret = register_hotcpu_notifier(&virtnet_cpu_notifier);
>> +	if (ret)
>> +		goto err_free;
>> +
>>  	return 0;
>>  
>>  err_free:
>> -- 
>> 1.8.0