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Jason Wang

2018-Nov-08 08:18 UTC

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

On 2018/11/8 ??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. 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 at 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 at 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;
>   	}
>
>

Tiwei Bie

2018-Nov-08 11:51 UTC

head link

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

On Thu, Nov 08, 2018 at 04:18:25PM +0800, Jason Wang
wrote:> 
> On 2018/11/8 ??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?

> 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 at 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 at 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;
> >   	}
> > 
> >

Michael S. Tsirkin

2018-Nov-08 14:14 UTC

head link

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

On Thu, Nov 08, 2018 at 04:18:25PM +0800, Jason Wang
wrote:> 
> On 2018/11/8 ??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. 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.
I don't think this ever triggers to be frank. When would it?
> Consider the removal of lguest support recently,
> maybe we can drop this for split ring as well?
> 
> Thanks
If it's helpful, then for sure we can drop it for virtio 1.
Can you see any perf differences at all? With which device?
> 
> > 
> > commit 44653eae1407f79dff6f52fcf594ae84cb165ec4
> > Author: Rusty Russell<rusty at 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 at 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;
> >   	}
> > 
> >

Michael S. Tsirkin

2018-Nov-08 15:56 UTC

head link

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

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 ??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.  As a work-around Rusty put in this hack to kick on ring full
even with notifications disabled.  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 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 at 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 at 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;
> > >   	}
> > > 
> > >

Jason Wang

2018-Nov-09 02:25 UTC

head link

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

On 2018/11/8 ??10:14, Michael S. Tsirkin wrote:> On Thu, Nov 08, 2018 at 04:18:25PM +0800, Jason Wang wrote:
>> On 2018/11/8 ??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. 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.
> I don't think this ever triggers to be frank. When would it?

I think it can happen e.g in the path of XDP transmission in 
__virtnet_xdp_xmit_one():


 ??????? err = virtqueue_add_outbuf(sq->vq, sq->sg, 1, xdpf, GFP_ATOMIC);
 ??????? if (unlikely(err))
 ??????????????? return -ENOSPC; /* Caller handle free/refcnt */

>
>> Consider the removal of lguest support recently,
>> maybe we can drop this for split ring as well?
>>
>> Thanks
> If it's helpful, then for sure we can drop it for virtio 1.
> Can you see any perf differences at all? With which device?

I don't test but consider the case of XDP_TX in guest plus vhost_net in 
host. Since vhost_net is half duplex, it's pretty easier to trigger this 
condition.

Thanks

>
>>> commit 44653eae1407f79dff6f52fcf594ae84cb165ec4
>>> Author: Rusty Russell<rusty at 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 at 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;
>>>    	}
>>>
>>>

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