Re: [RFC PATCH] virtio-net: reset virtqueue affinity when doing cpu hotplug

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Dec 27, 2012 at 11:34:16AM +0800, Jason Wang wrote:
> On 12/26/2012 06:46 PM, Michael S. Tsirkin wrote:
> > On Wed, Dec 26, 2012 at 03:06:54PM +0800, 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 <rusty@rustcorp.com.au>
> >> Cc: "Michael S. Tsirkin" <mst@redhat.com>
> >> Cc: Jason Wang <jasowang@redhat.com>
> >> Cc: virtualization@lists.linux-foundation.org
> >> Cc: netdev@vger.kernel.org
> >> Signed-off-by: Wanlong Gao <gaowanlong@cn.fujitsu.com>
> > Thanks for looking into this.
> > Some comments:
> >
> > 1. Looks like the logic in
> > virtnet_set_affinity (and in virtnet_select_queue)
> > will not work very well when CPU IDs are not
> > consequitive. This can happen with hot unplug.
> >
> > Maybe we should add a VQ allocator, and defining
> > a per-cpu variable specifying the VQ instead
> > of using CPU ID.
> 
> Yes, and generate the affinity hint based on the mapping. Btw, what does
> VQ allocator means here?

Some logic to generate CPU to VQ mapping.

> >
> >
> > 2. The below code seems racy e.g. when CPU is added
> > 	during device init.
> >
> > 3. using a global cpu_hotplug seems inelegant.
> > In any case we should document what is the
> > meaning of this variable.
> >
> >> ---
> >>  drivers/net/virtio_net.c | 39 ++++++++++++++++++++++++++++++++++++++-
> >>  1 file changed, 38 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> >> index a6fcf15..9710cf4 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);
> >> @@ -34,6 +35,8 @@ static bool csum = true, gso = true;
> >>  module_param(csum, bool, 0444);
> >>  module_param(gso, bool, 0444);
> >>  
> >> +static bool cpu_hotplug = false;
> >> +
> >>  /* FIXME: MTU in config. */
> >>  #define MAX_PACKET_LEN (ETH_HLEN + VLAN_HLEN + ETH_DATA_LEN)
> >>  #define GOOD_COPY_LEN	128
> >> @@ -1041,6 +1044,26 @@ static void virtnet_set_affinity(struct virtnet_info *vi, bool set)
> >>  		vi->affinity_hint_set = false;
> >>  }
> >>  
> >> +static int virtnet_cpu_callback(struct notifier_block *nfb,
> >> +			       unsigned long action, void *hcpu)
> >> +{
> >> +	switch(action) {
> >> +	case CPU_ONLINE:
> >> +	case CPU_ONLINE_FROZEN:
> >> +	case CPU_DEAD:
> >> +	case CPU_DEAD_FROZEN:
> >> +		cpu_hotplug = true;
> >> +		break;
> >> +	default:
> >> +		break;
> >> +	}
> >> +	return NOTIFY_OK;
> >> +}
> >> +
> >> +static struct notifier_block virtnet_cpu_notifier = {
> >> +	.notifier_call = virtnet_cpu_callback,
> >> +};
> >> +
> >>  static void virtnet_get_ringparam(struct net_device *dev,
> >>  				struct ethtool_ringparam *ring)
> >>  {
> >> @@ -1131,7 +1154,14 @@ static int virtnet_change_mtu(struct net_device *dev, int new_mtu)
> >>   */
> >>  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) :
> >> +	int txq;
> >> +
> >> +	if (unlikely(cpu_hotplug == true)) {
> >> +		virtnet_set_affinity(netdev_priv(dev), true);
> >> +		cpu_hotplug = false;
> >> +	}
> >> +
> >> +	txq = skb_rx_queue_recorded(skb) ? skb_get_rx_queue(skb) :
> >>  		  smp_processor_id();
> >>  
> >>  	while (unlikely(txq >= dev->real_num_tx_queues))
> >> @@ -1248,6 +1278,8 @@ static void virtnet_del_vqs(struct virtnet_info *vi)
> >>  {
> >>  	struct virtio_device *vdev = vi->vdev;
> >>  
> >> +	unregister_hotcpu_notifier(&virtnet_cpu_notifier);
> >> +
> >>  	virtnet_set_affinity(vi, false);
> >>  
> >>  	vdev->config->del_vqs(vdev);
> >> @@ -1372,6 +1404,11 @@ static int init_vqs(struct virtnet_info *vi)
> >>  		goto err_free;
> >>  
> >>  	virtnet_set_affinity(vi, true);
> >> +
> >> +	ret = register_hotcpu_notifier(&virtnet_cpu_notifier);
> >> +	if (ret)
> >> +		goto err_free;
> >> +
> >>  	return 0;
> >>  
> >>  err_free:
> >> -- 
> >> 1.8.0