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    SubjectRe: [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@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;



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