Linux Virtualization - Dec 2012 - [RFC PATCH] virtio-net: reset virtqueue affinity when doing cpu hotplug

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

On 12/26/2012 03:06 PM, 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.
Hi Wanlong:

Thanks for looking at this.
> Cc: Rusty Russell <rusty at rustcorp.com.au>
> Cc: "Michael S. Tsirkin" <mst at redhat.com>
> Cc: Jason Wang <jasowang at redhat.com>
> Cc: virtualization at lists.linux-foundation.org
> Cc: netdev at vger.kernel.org
> Signed-off-by: Wanlong Gao <gaowanlong at cn.fujitsu.com>
> ---
>  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;
> +	}
> +
Why don't you just do this in callback?

btw. Does qemu/kvm support cpu-hotplug now?
> +	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:

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


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

On Wed, 2012-12-26 at 15:06 +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 at rustcorp.com.au>
> Cc: "Michael S. Tsirkin" <mst at redhat.com>
> Cc: Jason Wang <jasowang at redhat.com>
> Cc: virtualization at lists.linux-foundation.org
> Cc: netdev at vger.kernel.org
> Signed-off-by: Wanlong Gao <gaowanlong at cn.fujitsu.com>
> ---
>  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:
It looks like this patch assumes virtio_net supports a single instance.

Try your patch with two instances, I am pretty sure it wont do very
well.

It seems to me you need something else than a single boolean.

A sequence number for example should be better...

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 at rustcorp.com.au>
>> Cc: "Michael S. Tsirkin" <mst at redhat.com>
>> Cc: Jason Wang <jasowang at redhat.com>
>> Cc: virtualization at lists.linux-foundation.org
>> Cc: netdev at vger.kernel.org
>> Signed-off-by: Wanlong Gao <gaowanlong at 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?
>
>
> 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

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