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

From: Wanlong Gao <[email-masked]>
Date: Thu, 27 Dec 2012 11:43:30 +0800

On 12/27/2012 11:28 AM, Jason Wang wrote:
> 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.

Yeah, thank you very much for your suggestion. I'll try it.

>>> 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.

Adding Igor to CC,

As I know, hotplug support is cleaned from qemu, and Igor want to rework it but not been completed?
I'm not sure about that, Igor, could you send out your tech-preview-patches?

Thanks,
Wanlong Gao

> Thanks
>> Thanks,
>> Wanlong Gao