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

Jason Wang
2013-01-08T10:26:41Z

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@rustcorp.com.au>
> Cc: "Michael S. Tsirkin" <mst@redhat.com>
> Cc: Jason Wang <jasowang@redhat.com>
> Cc: Eric Dumazet <erdnetdev@gmail.com>
> Cc: virtualization@lists.linux-foundation.org
> Cc: netdev@vger.kernel.org
> Signed-off-by: Wanlong Gao <gaowanlong@cn.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;