Subject: Re: [PATCH v1 1/3] virtio-net: Using single MSIX IRQ for TX/RX Q pair

From: Jason Wang <[email-masked]>
Date: Wed, 28 Oct 2015 15:54:34 +0800

On 10/28/2015 03:21 PM, Michael S. Tsirkin wrote:
> On Wed, Oct 28, 2015 at 11:13:39AM +0800, Jason Wang wrote:
>>
>> On 10/27/2015 04:38 PM, Michael S. Tsirkin wrote:
>>> On Mon, Oct 26, 2015 at 10:52:47AM -0700, Ravi Kerur wrote:
>>>> Ported earlier patch from Jason Wang (dated 12/26/2014).
>>>>
>>>> This patch tries to reduce the number of MSIX irqs required for
>>>> virtio-net by sharing a MSIX irq for each TX/RX queue pair through
>>>> channels. If transport support channel, about half of the MSIX irqs
>>>> were reduced.
>>>>
>>>> Signed-off-by: Ravi Kerur <[email-masked]>
>>> Why bother BTW? 
>> The reason is we want to save the number of interrupt vectors used.
>> Booting a guest with 256 queues with current driver will result all
>> tx/rx queues shares a single vector. This is suboptimal.
> With a single CPU? 

Even for smp guests. Or you want a per-cpu interrupt?

> But what configures so many queues? Why do it?

Something like cpu hot add.

>
>> With this
>> series, half could be saved.
> At cost of e.g. inability to balance the interrupts.

Didn't follow. Btw, most psychical cards shares irq with tx/rx queue pair.

>
>> And more complex policy could be applied on
>> top (e.g limit the number of vectors used by driver).
> If that's the motivation, I'd like to see a draft of that more complex
> policy first.

How about something like:

1) Driver provides a min and max number of vectors it needs.
2) Virtio pci can then use pci_enable_msix_range() and return the actual
number of vectors to driver.
3) Then driver can divide the virtqueues into different groups

>
>>> Looks like this is adding a bunch of overhead
>>> on data path - to what end?
>> I agree some benchmark is needed for this.
>>
>>> Maybe you have a huge number of these devices ... but in that case, how
>>> about sharing the config interrupt instead?
>>> That's only possible if host supports VIRTIO_1
>>> (so we can detect config interrupt by reading the ISR).
>>>
>>>
>>>
>>>> ---
>>>>  drivers/net/virtio_net.c | 29 ++++++++++++++++++++++++++++-
>>>>  1 file changed, 28 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
>>>> index d8838ded..d705cce 100644
>>>> --- a/drivers/net/virtio_net.c
>>>> +++ b/drivers/net/virtio_net.c
>>>> @@ -72,6 +72,9 @@ struct send_queue {
>>>>  
>>>>  	/* Name of the send queue: output.$index */
>>>>  	char name[40];
>>>> +
>>>> +	/* Name of the channel, shared with irq. */
>>>> +	char channel_name[40];
>>>>  };
>>>>  
>>>>  /* Internal representation of a receive virtqueue */
>>>> @@ -1529,6 +1532,8 @@ static int virtnet_find_vqs(struct virtnet_info *vi)
>>>>  	int ret = -ENOMEM;
>>>>  	int i, total_vqs;
>>>>  	const char **names;
>>>> +	const char **channel_names;
>>>> +	unsigned *channels;
>>>>  
>>>>  	/* We expect 1 RX virtqueue followed by 1 TX virtqueue, followed by
>>>>  	 * possible N-1 RX/TX queue pairs used in multiqueue mode, followed by
>>>> @@ -1548,6 +1553,17 @@ static int virtnet_find_vqs(struct virtnet_info *vi)
>>>>  	if (!names)
>>>>  		goto err_names;
>>>>  
>>>> +	channel_names = kmalloc_array(vi->max_queue_pairs,
>>>> +				      sizeof(*channel_names),
>>>> +				      GFP_KERNEL);
>>>> +	if (!channel_names)
>>>> +		goto err_channel_names;
>>>> +
>>>> +	channels = kmalloc_array(total_vqs, sizeof(*channels),
>>>> +				 GFP_KERNEL);
>>>> +	if (!channels)
>>>> +		goto err_channels;
>>>> +
>>>>  	/* Parameters for control virtqueue, if any */
>>>>  	if (vi->has_cvq) {
>>>>  		callbacks[total_vqs - 1] = NULL;
>>>> @@ -1562,10 +1578,15 @@ static int virtnet_find_vqs(struct virtnet_info *vi)
>>>>  		sprintf(vi->sq[i].name, "output.%d", i);
>>>>  		names[rxq2vq(i)] = vi->rq[i].name;
>>>>  		names[txq2vq(i)] = vi->sq[i].name;
>>>> +		sprintf(vi->sq[i].channel_name, "txrx.%d", i);
>>>> +		channel_names[i] = vi->sq[i].channel_name;
>>>> +		channels[rxq2vq(i)] = i;
>>>> +		channels[txq2vq(i)] = i;
>>>>  	}
>>>>  
>>>>  	ret = vi->vdev->config->find_vqs(vi->vdev, total_vqs, vqs, callbacks,
>>>> -					 names);
>>>> +					 names, channels, channel_names,
>>>> +					 vi->max_queue_pairs);
>>>>  	if (ret)
>>>>  		goto err_find;
>>>>  
>>>> @@ -1580,6 +1601,8 @@ static int virtnet_find_vqs(struct virtnet_info *vi)
>>>>  		vi->sq[i].vq = vqs[txq2vq(i)];
>>>>  	}
>>>>  
>>>> +	kfree(channels);
>>>> +	kfree(channel_names);
>>>>  	kfree(names);
>>>>  	kfree(callbacks);
>>>>  	kfree(vqs);
>>>> @@ -1587,6 +1610,10 @@ static int virtnet_find_vqs(struct virtnet_info *vi)
>>>>  	return 0;
>>>>  
>>>>  err_find:
>>>> +	kfree(channels);
>>>> +err_channels:
>>>> +	kfree(channel_names);
>>>> +err_channel_names:
>>>>  	kfree(names);
>>>>  err_names:
>>>>  	kfree(callbacks);
>>>> -- 
>>>> 1.9.1