Subject: Re: [PATCH V6 1/3] virtio-net: fix the set affinity bug when CPU IDs are not consecutive

From: Jason Wang <[email-masked]>
Date: Fri, 25 Jan 2013 11:26:29 +0800

On 01/21/2013 07:25 PM, Wanlong Gao wrote:
> As Michael mentioned, set affinity and select queue will not work very
> well when CPU IDs are not consecutive, this can happen with hot unplug.
> Fix this bug by traversal the online CPUs, and create a per cpu variable
> to find the mapping from CPU to the preferable virtual-queue.
>
> 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:
> 	remove {get|put}_online_cpus from virtnet_del_vqs (Jason)
> V4->V5:
> 	Add get/put_online_cpus to avoid CPUs go up and down during operations (Rusty)
>
> V3->V4:
> 	move vq_index into virtnet_info	(Jason)
> 	change the mapping value when not setting affinity (Jason)
> 	address the comments about select_queue (Rusty)
>
>  drivers/net/virtio_net.c | 58 +++++++++++++++++++++++++++++++++++++++---------
>  1 file changed, 47 insertions(+), 11 deletions(-)
>
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index a6fcf15..70cd957 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -123,6 +123,9 @@ struct virtnet_info {
>  
>  	/* Does the affinity hint is set for virtqueues? */
>  	bool affinity_hint_set;
> +
> +	/* Per-cpu variable to show the mapping from CPU to virtqueue */
> +	int __percpu *vq_index;
>  };
>  
>  struct skb_vnet_hdr {
> @@ -1016,6 +1019,7 @@ static int virtnet_vlan_rx_kill_vid(struct net_device *dev, u16 vid)
>  static void virtnet_set_affinity(struct virtnet_info *vi, bool set)
>  {
>  	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
> @@ -1029,16 +1033,29 @@ static void virtnet_set_affinity(struct virtnet_info *vi, bool set)
>  			return;
>  	}
>  
> -	for (i = 0; i < vi->max_queue_pairs; i++) {
> -		int cpu = set ? i : -1;
> -		virtqueue_set_affinity(vi->rq[i].vq, cpu);
> -		virtqueue_set_affinity(vi->sq[i].vq, cpu);
> -	}
> +	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++;
> +		}
>  
> -	if (set)
>  		vi->affinity_hint_set = true;
> -	else
> +	} else {
> +		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)
> +			*per_cpu_ptr(vi->vq_index, cpu) =
> +				++i % vi->curr_queue_pairs;
> +
>  		vi->affinity_hint_set = false;
> +	}
>  }

This looks wrong, since you always choose txq based on the per-cpu, I
think the per-cpu index should be set unconditionally even if cpus !=
queues. Consider you may boot a guest with 4 vcpus and 2 queues, you
need initialize it also in this case. Otherwise, you may always get txq
0 to be selected.
>  
>  static void virtnet_get_ringparam(struct net_device *dev,
> @@ -1087,7 +1104,9 @@ static int virtnet_set_channels(struct net_device *dev,
>  		netif_set_real_num_tx_queues(dev, queue_pairs);
>  		netif_set_real_num_rx_queues(dev, queue_pairs);
>  
> +		get_online_cpus();
>  		virtnet_set_affinity(vi, true);
> +		put_online_cpus();
>  	}
>  
>  	return err;
> @@ -1127,12 +1146,19 @@ static int virtnet_change_mtu(struct net_device *dev, int new_mtu)
>  
>  /* To avoid contending a lock hold by a vcpu who would exit to host, select the
>   * txq based on the processor id.
> - * TODO: handle cpu hotplug.
>   */
>  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) :
> -		  smp_processor_id();
> +	int txq;
> +	struct virtnet_info *vi = netdev_priv(dev);
> +
> +	if (skb_rx_queue_recorded(skb)) {
> +		txq = skb_get_rx_queue(skb);
> +	} else {
> +		txq = *__this_cpu_ptr(vi->vq_index);
> +		if (txq == -1)
> +			txq = 0;
> +	}
>  
>  	while (unlikely(txq >= dev->real_num_tx_queues))
>  		txq -= dev->real_num_tx_queues;
> @@ -1371,7 +1397,10 @@ static int init_vqs(struct virtnet_info *vi)
>  	if (ret)
>  		goto err_free;
>  
> +	get_online_cpus();
>  	virtnet_set_affinity(vi, true);
> +	put_online_cpus();
> +
>  	return 0;
>  
>  err_free:
> @@ -1453,6 +1482,10 @@ static int virtnet_probe(struct virtio_device *vdev)
>  	if (vi->stats == NULL)
>  		goto free;
>  
> +	vi->vq_index = alloc_percpu(int);
> +	if (vi->vq_index == NULL)
> +		goto free_stats;
> +
>  	mutex_init(&vi->config_lock);
>  	vi->config_enable = true;
>  	INIT_WORK(&vi->config_work, virtnet_config_changed_work);
> @@ -1476,7 +1509,7 @@ static int virtnet_probe(struct virtio_device *vdev)
>  	/* Allocate/initialize the rx/tx queues, and invoke find_vqs */
>  	err = init_vqs(vi);
>  	if (err)
> -		goto free_stats;
> +		goto free_index;
>  
>  	netif_set_real_num_tx_queues(dev, 1);
>  	netif_set_real_num_rx_queues(dev, 1);
> @@ -1520,6 +1553,8 @@ free_recv_bufs:
>  free_vqs:
>  	cancel_delayed_work_sync(&vi->refill);
>  	virtnet_del_vqs(vi);
> +free_index:
> +	free_percpu(vi->vq_index);
>  free_stats:
>  	free_percpu(vi->stats);
>  free:
> @@ -1554,6 +1589,7 @@ static void virtnet_remove(struct virtio_device *vdev)
>  
>  	flush_work(&vi->config_work);
>  
> +	free_percpu(vi->vq_index);
>  	free_percpu(vi->stats);
>  	free_netdev(vi->dev);
>  }