Subject: Re: [PATCH V5 2/2] virtio-net: reset virtqueue affinity when doing cpu hotplug

From: Jason Wang <[email-masked]>
Date: Fri, 18 Jan 2013 14:23:29 +0800

On 01/18/2013 01:32 PM, 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: Eric Dumazet <[email-masked]>
> Cc: [email-masked]
> Cc: [email-masked]
> Signed-off-by: Wanlong Gao <[email-masked]>
> ---
> V4->V5:
> 	New method to deal with the cpu hotplug actions (Rusty)
>
>  drivers/net/virtio_net.c | 50 ++++++++++++++++++++++++++++++++++++++++++------
>  1 file changed, 44 insertions(+), 6 deletions(-)
>
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 440b0eb..061f2c5 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);
> @@ -126,6 +127,9 @@ struct virtnet_info {
>  
>  	/* Per-cpu variable to show the mapping from CPU to virtqueue */
>  	int __percpu *vq_index;
> +
> +	/* CPU hot plug notifier */
> +	struct notifier_block nb;
>  };
>  
>  struct skb_vnet_hdr {
> @@ -1016,7 +1020,7 @@ 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_set_affinity(struct virtnet_info *vi, bool set, long hcpu)
>  {
>  	int i;
>  	int cpu;
> @@ -1026,7 +1030,8 @@ static void virtnet_set_affinity(struct virtnet_info *vi, bool set)
>  	 * setting the affinity hint to eliminate the contention.
>  	 */
>  	if ((vi->curr_queue_pairs == 1 ||
> -	     vi->max_queue_pairs != num_online_cpus()) && set) {
> +	     vi->max_queue_pairs != num_online_cpus() - ((hcpu == -1) ? 0 : 1))
> +	    && set) {
>  		if (vi->affinity_hint_set)
>  			set = false;
>  		else

This make complex the this function. How about introduce another
function that just clean the affinity of cpu during CPU_DOWN_PREPARE.
Then we could get a simpler virtnet_set_affinity() called in CPU_DEAD
like you previous version?
> @@ -1036,6 +1041,8 @@ static void virtnet_set_affinity(struct virtnet_info *vi, bool set)
>  	if (set) {
>  		i = 0;
>  		for_each_online_cpu(cpu) {
> +			if (cpu == hcpu)
> +				continue;
>  			virtqueue_set_affinity(vi->rq[i].vq, cpu);
>  			virtqueue_set_affinity(vi->sq[i].vq, cpu);
>  			*per_cpu_ptr(vi->vq_index, cpu) = i;
> @@ -1050,14 +1057,36 @@ static void virtnet_set_affinity(struct virtnet_info *vi, bool set)
>  		}
>  
>  		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;
> +		}
>  
>  		vi->affinity_hint_set = false;
>  	}
>  }
>  
> +static int virtnet_cpu_callback(struct notifier_block *nfb,
> +			        unsigned long action, void *hcpu)
> +{
> +	struct virtnet_info *vi = container_of(nfb, struct virtnet_info, nb);
> +
> +	switch(action & ~CPU_TASKS_FROZEN) {
> +	case CPU_ONLINE:
> +	case CPU_DOWN_FAILED:
> +		virtnet_set_affinity(vi, true, -1);
> +		break;
> +	case CPU_DOWN_PREPARE:
> +		virtnet_set_affinity(vi, true, (long)hcpu);
> +		break;
> +	default:
> +		break;
> +	}
> +	return NOTIFY_OK;
> +}
> +
>  static void virtnet_get_ringparam(struct net_device *dev,
>  				struct ethtool_ringparam *ring)
>  {
> @@ -1105,7 +1134,7 @@ 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, true, -1);
>  		put_online_cpus();
>  	}
>  
> @@ -1275,7 +1304,7 @@ static void virtnet_del_vqs(struct virtnet_info *vi)
>  	struct virtio_device *vdev = vi->vdev;
>  
>  	get_online_cpus();
> -	virtnet_set_affinity(vi, false);
> +	virtnet_set_affinity(vi, false, -1);
>  	put_online_cpus();
>  
>  	vdev->config->del_vqs(vdev);
> @@ -1400,7 +1429,7 @@ static int init_vqs(struct virtnet_info *vi)
>  		goto err_free;
>  
>  	get_online_cpus();
> -	virtnet_set_affinity(vi, true);
> +	virtnet_set_affinity(vi, true, -1);
>  	put_online_cpus();
>  
>  	return 0;
> @@ -1534,6 +1563,13 @@ static int virtnet_probe(struct virtio_device *vdev)
>  		}
>  	}
>  
> +	vi->nb.notifier_call = &virtnet_cpu_callback;
> +	err = register_hotcpu_notifier(&vi->nb);
> +	if (err) {
> +		pr_debug("virtio_net: registering cpu notifier failed\n");
> +		goto free_recv_bufs;
> +	}
> +
>  	/* Assume link up if device can't report link status,
>  	   otherwise get link status from config. */
>  	if (virtio_has_feature(vi->vdev, VIRTIO_NET_F_STATUS)) {
> @@ -1580,6 +1616,8 @@ static void virtnet_remove(struct virtio_device *vdev)
>  {
>  	struct virtnet_info *vi = vdev->priv;
>  
> +	unregister_hotcpu_notifier(&vi->nb);
> +
>  	/* Prevent config work handler from accessing the device. */
>  	mutex_lock(&vi->config_lock);
>  	vi->config_enable = false;