netdev - Re: [PATCH V3 1/2] virtio-net: fix the set affinity bug when CPU IDs are not consecutive

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Date:	Tue, 08 Jan 2013 18:26:20 +0800
From:	Jason Wang <jasowang@...hat.com>
To:	Wanlong Gao <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 V3 1/2] virtio-net: fix the set affinity bug when CPU
 IDs are not consecutive

On 01/08/2013 06:07 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 <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>
> ---
>  drivers/net/virtio_net.c | 39 +++++++++++++++++++++++++++++----------
>  1 file changed, 29 insertions(+), 10 deletions(-)
>
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index a6fcf15..a77f86c 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -41,6 +41,8 @@ module_param(gso, bool, 0444);
>  #define VIRTNET_SEND_COMMAND_SG_MAX    2
>  #define VIRTNET_DRIVER_VERSION "1.0.0"
>  
> +DEFINE_PER_CPU(int, vq_index) = -1;
> +

I think this should not be a global one, consider we may have more than
one virtio-net cards with different max queues.
>  struct virtnet_stats {
>  	struct u64_stats_sync tx_syncp;
>  	struct u64_stats_sync rx_syncp;
> @@ -1016,6 +1018,7 @@ 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 +1032,29 @@ 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(vq_index, cpu) = i;
> +			i++;
> +			if (i >= vi->max_queue_pairs)
> +				break;

Can this happen? we check only set when the number are equal.
> +		}
>  
> -	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);
> +		}
> +
> +		for_each_online_cpu(cpu)
> +			per_cpu(vq_index, cpu) = -1;
> +

This looks suboptimal since it may leads only txq zero is used.
>  		vi->affinity_hint_set = false;
> +	}
>  }
>  
>  static void virtnet_get_ringparam(struct net_device *dev,
> @@ -1127,12 +1143,15 @@ 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 = 0;
> +
> +	if (skb_rx_queue_recorded(skb))
> +		txq = skb_get_rx_queue(skb);
> +	else if ((txq = per_cpu(vq_index, smp_processor_id())) == -1)
> +		txq = 0;
>  
>  	while (unlikely(txq >= dev->real_num_tx_queues))
>  		txq -= dev->real_num_tx_queues;

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