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

From: Jason Wang <[email-masked]>
Date: Thu, 27 Dec 2012 11:28:59 +0800

On 12/26/2012 06:19 PM, Wanlong Gao wrote:
> 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?

Well, I think you can just embed the notifier block into virtnet_info,
then use something like container_of in the callback to make the
notifier per device. This also solve the concern of Eric.
>> 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?

Not sure, I just try latest qemu, it even does not have a cpu_set command.

Thanks
> Thanks,
> Wanlong Gao