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

From: Wanlong Gao <[email-masked]>
Date: Wed, 26 Dec 2012 18:19:14 +0800

On 12/26/2012 06:06 PM, Jason Wang wrote:
> 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 <[email-masked]>
>> Cc: "Michael S. Tsirkin" <[email-masked]>
>> Cc: Jason Wang <[email-masked]>
>> Cc: [email-masked]
>> Cc: [email-masked]
>> Signed-off-by: Wanlong Gao <[email-masked]>
>> ---
>>  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?

Callback can just give us a "hcpu", can't get the virtnet_info from callback. Am I missing something?

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

>>From http://www.linux-kvm.org/page/CPUHotPlug, I saw that qemu-kvm can support hotplug
but failed to merge to qemu.git, right?

Thanks,
Wanlong Gao