Re: [PATCH net-next v2 3/5] virtio_ring: add packed ring support

[Jason Wang <jasowang@redhat.com>]

On 2018/11/9 下午12:00, Michael S. Tsirkin wrote:
> On Fri, Nov 09, 2018 at 10:30:50AM +0800, Jason Wang wrote:
>> On 2018/11/8 下午11:56, Michael S. Tsirkin wrote:
>>> On Thu, Nov 08, 2018 at 07:51:48PM +0800, Tiwei Bie wrote:
>>>> On Thu, Nov 08, 2018 at 04:18:25PM +0800, Jason Wang wrote:
>>>>> On 2018/11/8 上午9:38, Tiwei Bie wrote:
>>>>>>>> +
>>>>>>>> +	if (vq->vq.num_free < descs_used) {
>>>>>>>> +		pr_debug("Can't add buf len %i - avail = %i\n",
>>>>>>>> +			 descs_used, vq->vq.num_free);
>>>>>>>> +		/* FIXME: for historical reasons, we force a notify here if
>>>>>>>> +		 * there are outgoing parts to the buffer.  Presumably the
>>>>>>>> +		 * host should service the ring ASAP. */
>>>>>>> I don't think we have a reason to do this for packed ring.
>>>>>>> No historical baggage there, right?
>>>>>> Based on the original commit log, it seems that the notify here
>>>>>> is just an "optimization". But I don't quite understand what does
>>>>>> the "the heuristics which KVM uses" refer to. If it's safe to drop
>>>>>> this in packed ring, I'd like to do it.
>>>>> According to the commit log, it seems like a workaround of lguest networking
>>>>> backend.
>>>> Do you know why removing this notify in Tx will break "the
>>>> heuristics which KVM uses"? Or what does "the heuristics
>>>> which KVM uses" refer to?
>>> Yes. QEMU has a mode where it disables notifications and processes TX
>>> ring periodically from a timer.  It's off by default but used to be on
>>> by default a long time ago. If ring becomes full this causes traffic
>>> stalls.
>>
>> Do you mean tx-timer? If yes, we can still enable it for packed ring
> Yes we can but I doubt anyone does.
>
>> and the
>> timer will finally fired and we can go.
> on tx ring full we probably don't want to wait for timer.
> But I think we can just prevent qemu from using tx timer
> with virtio 1.


Yes, we can.

Thanks


>
>>> As a work-around Rusty put in this hack to kick on ring full
>>> even with notifications disabled.
>>
>>  From the commit log it looks more like a performance workaround instead of a
>> bug fix.
> it's a quality of implementation issue, yes.
>
>>> It's easy enough to make sure QEMU
>>> does not combine devices with packed ring support with the timer hack.
>>> And I am guessing it's safe enough to also block that option completely
>>> e.g. when virtio 1.0 is enabled.
>>
>> I agree.
>>
>> Thanks
>>
>>
>>>>> I agree to drop it, we should not have such burden.
>>>>>
>>>>> But we should notice that, with this removed, the compare between packed vs
>>>>> split is kind of unfair. Consider the removal of lguest support recently,
>>>>> maybe we can drop this for split ring as well?
>>>>>
>>>>> Thanks
>>>>>
>>>>>
>>>>>> commit 44653eae1407f79dff6f52fcf594ae84cb165ec4
>>>>>> Author: Rusty Russell<rusty@rustcorp.com.au>
>>>>>> Date:   Fri Jul 25 12:06:04 2008 -0500
>>>>>>
>>>>>>        virtio: don't always force a notification when ring is full
>>>>>>        We force notification when the ring is full, even if the host has
>>>>>>        indicated it doesn't want to know.  This seemed like a good idea at
>>>>>>        the time: if we fill the transmit ring, we should tell the host
>>>>>>        immediately.
>>>>>>        Unfortunately this logic also applies to the receiving ring, which is
>>>>>>        refilled constantly.  We should introduce real notification thesholds
>>>>>>        to replace this logic.  Meanwhile, removing the logic altogether breaks
>>>>>>        the heuristics which KVM uses, so we use a hack: only notify if there are
>>>>>>        outgoing parts of the new buffer.
>>>>>>        Here are the number of exits with lguest's crappy network implementation:
>>>>>>        Before:
>>>>>>                network xmit 7859051 recv 236420
>>>>>>        After:
>>>>>>                network xmit 7858610 recv 118136
>>>>>>        Signed-off-by: Rusty Russell<rusty@rustcorp.com.au>
>>>>>>
>>>>>> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
>>>>>> index 72bf8bc09014..21d9a62767af 100644
>>>>>> --- a/drivers/virtio/virtio_ring.c
>>>>>> +++ b/drivers/virtio/virtio_ring.c
>>>>>> @@ -87,8 +87,11 @@ static int vring_add_buf(struct virtqueue *_vq,
>>>>>>     	if (vq->num_free < out + in) {
>>>>>>     		pr_debug("Can't add buf len %i - avail = %i\n",
>>>>>>     			 out + in, vq->num_free);
>>>>>> -		/* We notify*even if*  VRING_USED_F_NO_NOTIFY is set here. */
>>>>>> -		vq->notify(&vq->vq);
>>>>>> +		/* FIXME: for historical reasons, we force a notify here if
>>>>>> +		 * there are outgoing parts to the buffer.  Presumably the
>>>>>> +		 * host should service the ring ASAP. */
>>>>>> +		if (out)
>>>>>> +			vq->notify(&vq->vq);
>>>>>>     		END_USE(vq);
>>>>>>     		return -ENOSPC;
>>>>>>     	}
>>>>>>
>>>>>>