Linux Virtualization - Nov 2017 - [PATCH net-next] vhost_net: conditionally enable tx polling

On 2017?11?01? 00:36, Michael S. Tsirkin wrote:
> On Tue, Oct 31, 2017 at 06:27:20PM +0800, Jason Wang wrote:
>> We always poll tx for socket, this is sub optimal since:
>>
>> - we only want to be notified when sndbuf is available
>> - this will slightly increase the waitqueue traversing time and more
>>    important, vhost could not benefit from commit
>>    commit 9e641bdcfa4e
>>    ("net-tun: restructure tun_do_read for better sleep/wakeup
efficiency")
>>    even if we've stopped rx polling during handle_rx() since tx
poll
>>    were still left in the waitqueue.
>>
>> Pktgen from a remote host to VM over mlx4 shows 5.5% improvements on
>> rx PPS. (from 1.27Mpps to 1.34Mpps)
>>
>> Cc: Wei Xu <wexu at redhat.com>
>> Cc: Matthew Rosato <mjrosato at linux.vnet.ibm.com>
>> Signed-off-by: Jason Wang <jasowang at redhat.com>
>> ---
> Now that vhost_poll_stop happens on data path
> a lot, I'd say
>          if (poll->wqh)
> there should be unlikely().
It has been there since 8241a1e466cd ("vhost_net: stop polling socket 
during rx processing"). So it will be used for rx path too which 
unlikely() does not work as well as the case in tx.

>
>
>>   drivers/vhost/net.c | 11 ++++++++---
>>   1 file changed, 8 insertions(+), 3 deletions(-)
>>
>> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
>> index 68677d9..286c3e4 100644
>> --- a/drivers/vhost/net.c
>> +++ b/drivers/vhost/net.c
>> @@ -471,6 +471,7 @@ static void handle_tx(struct vhost_net *net)
>>   		goto out;
>>   
>>   	vhost_disable_notify(&net->dev, vq);
>> +	vhost_net_disable_vq(net, vq);
>>   
>>   	hdr_size = nvq->vhost_hlen;
>>   	zcopy = nvq->ubufs;
>> @@ -556,6 +557,8 @@ static void handle_tx(struct vhost_net *net)
>>   					% UIO_MAXIOV;
>>   			}
>>   			vhost_discard_vq_desc(vq, 1);
>> +			if (err == -EAGAIN)
>> +				vhost_net_enable_vq(net, vq);
>>   			break;
>>   		}
>>   		if (err != len)
> I would probably just enable it unconditionally here. Why not?
>
I thought we only care about the case of tun_sock_write_space() and for 
the errors other than -EAGAIN, they have nothing to do with polling.
>> @@ -1145,9 +1148,11 @@ static long vhost_net_set_backend(struct
vhost_net *n, unsigned index, int fd)
>>   		r = vhost_vq_init_access(vq);
>>   		if (r)
>>   			goto err_used;
>> -		r = vhost_net_enable_vq(n, vq);
>> -		if (r)
>> -			goto err_used;
>> +		if (index == VHOST_NET_VQ_RX) {
>> +			r = vhost_net_enable_vq(n, vq);
>> +			if (r)
>> +				goto err_used;
>> +		}
>>   
>>   		oldubufs = nvq->ubufs;
>>   		nvq->ubufs = ubufs;
> This last chunk seems questionable. If queue has stuff in it
> when we connect the backend, we'll miss a wakeup.
> I suspect this can happen during migration.
Unless qemu pass a tap which s already had pending tx packets.

I can remove this chuck, but if guest does not transmit any packet, rx 
can't benefit from this.

Thanks
>
>
>> -- 
>> 2.7.4

On Wed, Nov 01, 2017 at 08:51:36PM +0800, Jason Wang
wrote:
> 
> 
> On 2017?11?01? 00:36, Michael S. Tsirkin wrote:
> > On Tue, Oct 31, 2017 at 06:27:20PM +0800, Jason Wang wrote:
> > > We always poll tx for socket, this is sub optimal since:
> > > 
> > > - we only want to be notified when sndbuf is available
> > > - this will slightly increase the waitqueue traversing time and
more
> > >    important, vhost could not benefit from commit
> > >    commit 9e641bdcfa4e
> > >    ("net-tun: restructure tun_do_read for better
sleep/wakeup efficiency")
> > >    even if we've stopped rx polling during handle_rx() since
tx poll
> > >    were still left in the waitqueue.
> > > 
> > > Pktgen from a remote host to VM over mlx4 shows 5.5% improvements
on
> > > rx PPS. (from 1.27Mpps to 1.34Mpps)
> > > 
> > > Cc: Wei Xu <wexu at redhat.com>
> > > Cc: Matthew Rosato <mjrosato at linux.vnet.ibm.com>
> > > Signed-off-by: Jason Wang <jasowang at redhat.com>
> > > ---
> > Now that vhost_poll_stop happens on data path
> > a lot, I'd say
> >          if (poll->wqh)
> > there should be unlikely().
> 
> It has been there since 8241a1e466cd ("vhost_net: stop polling socket
during
> rx processing"). So it will be used for rx path too which unlikely()
does
> not work as well as the case in tx.
Worth testing, does not have to block this patch.
> 
> > 
> > 
> > >   drivers/vhost/net.c | 11 ++++++++---
> > >   1 file changed, 8 insertions(+), 3 deletions(-)
> > > 
> > > diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> > > index 68677d9..286c3e4 100644
> > > --- a/drivers/vhost/net.c
> > > +++ b/drivers/vhost/net.c
> > > @@ -471,6 +471,7 @@ static void handle_tx(struct vhost_net *net)
> > >   		goto out;
> > >   	vhost_disable_notify(&net->dev, vq);
> > > +	vhost_net_disable_vq(net, vq);
> > >   	hdr_size = nvq->vhost_hlen;
> > >   	zcopy = nvq->ubufs;
> > > @@ -556,6 +557,8 @@ static void handle_tx(struct vhost_net *net)
> > >   					% UIO_MAXIOV;
> > >   			}
> > >   			vhost_discard_vq_desc(vq, 1);
> > > +			if (err == -EAGAIN)
> > > +				vhost_net_enable_vq(net, vq);
> > >   			break;
> > >   		}
> > >   		if (err != len)
> > I would probably just enable it unconditionally here. Why not?
> > 
> 
> I thought we only care about the case of tun_sock_write_space() and for the
> errors other than -EAGAIN, they have nothing to do with polling.
We could thinkably get ENOMEM I guess. If we miss a code things
get stuck - It's just easier not to add extra code IMHO.
> > > @@ -1145,9 +1148,11 @@ static long vhost_net_set_backend(struct
vhost_net *n, unsigned index, int fd)
> > >   		r = vhost_vq_init_access(vq);
> > >   		if (r)
> > >   			goto err_used;
> > > -		r = vhost_net_enable_vq(n, vq);
> > > -		if (r)
> > > -			goto err_used;
> > > +		if (index == VHOST_NET_VQ_RX) {
> > > +			r = vhost_net_enable_vq(n, vq);
> > > +			if (r)
> > > +				goto err_used;
> > > +		}
> > >   		oldubufs = nvq->ubufs;
> > >   		nvq->ubufs = ubufs;
> > This last chunk seems questionable. If queue has stuff in it
> > when we connect the backend, we'll miss a wakeup.
> > I suspect this can happen during migration.
> 
> Unless qemu pass a tap which s already had pending tx packets.
> 
> I can remove this chuck, but if guest does not transmit any packet, rx
can't
> benefit from this.
> 
> Thanks
Not sure I understand the last sentence. vhost will stay
polling the socket - why is that a problem?

> > 
> > 
> > > -- 
> > > 2.7.4
> 
> _______________________________________________
> Virtualization mailing list
> Virtualization at lists.linux-foundation.org
> https://lists.linuxfoundation.org/mailman/listinfo/virtualization

On 2017?11?01? 23:03, Michael S. Tsirkin wrote:
> On Wed, Nov 01, 2017 at 08:51:36PM +0800, Jason Wang wrote:
>>
>> On 2017?11?01? 00:36, Michael S. Tsirkin wrote:
>>> On Tue, Oct 31, 2017 at 06:27:20PM +0800, Jason Wang wrote:
>>>> We always poll tx for socket, this is sub optimal since:
>>>>
>>>> - we only want to be notified when sndbuf is available
>>>> - this will slightly increase the waitqueue traversing time and
more
>>>>     important, vhost could not benefit from commit
>>>>     commit 9e641bdcfa4e
>>>>     ("net-tun: restructure tun_do_read for better
sleep/wakeup efficiency")
>>>>     even if we've stopped rx polling during handle_rx()
since tx poll
>>>>     were still left in the waitqueue.
>>>>
>>>> Pktgen from a remote host to VM over mlx4 shows 5.5%
improvements on
>>>> rx PPS. (from 1.27Mpps to 1.34Mpps)
>>>>
>>>> Cc: Wei Xu <wexu at redhat.com>
>>>> Cc: Matthew Rosato <mjrosato at linux.vnet.ibm.com>
>>>> Signed-off-by: Jason Wang <jasowang at redhat.com>
>>>> ---
>>> Now that vhost_poll_stop happens on data path
>>> a lot, I'd say
>>>           if (poll->wqh)
>>> there should be unlikely().
>> It has been there since 8241a1e466cd ("vhost_net: stop polling
socket during
>> rx processing"). So it will be used for rx path too which
unlikely() does
>> not work as well as the case in tx.
> Worth testing, does not have to block this patch.
Ok.
>>>
>>>>    drivers/vhost/net.c | 11 ++++++++---
>>>>    1 file changed, 8 insertions(+), 3 deletions(-)
>>>>
>>>> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
>>>> index 68677d9..286c3e4 100644
>>>> --- a/drivers/vhost/net.c
>>>> +++ b/drivers/vhost/net.c
>>>> @@ -471,6 +471,7 @@ static void handle_tx(struct vhost_net
*net)
>>>>    		goto out;
>>>>    	vhost_disable_notify(&net->dev, vq);
>>>> +	vhost_net_disable_vq(net, vq);
>>>>    	hdr_size = nvq->vhost_hlen;
>>>>    	zcopy = nvq->ubufs;
>>>> @@ -556,6 +557,8 @@ static void handle_tx(struct vhost_net
*net)
>>>>    					% UIO_MAXIOV;
>>>>    			}
>>>>    			vhost_discard_vq_desc(vq, 1);
>>>> +			if (err == -EAGAIN)
>>>> +				vhost_net_enable_vq(net, vq);
>>>>    			break;
>>>>    		}
>>>>    		if (err != len)
>>> I would probably just enable it unconditionally here. Why not?
>>>
>> I thought we only care about the case of tun_sock_write_space() and for
the
>> errors other than -EAGAIN, they have nothing to do with polling.
> We could thinkably get ENOMEM I guess. If we miss a code things
> get stuck - It's just easier not to add extra code IMHO.
ENOBUFS actually. I agree to remove the specific checking of -EAGAIN here.
>
>>>> @@ -1145,9 +1148,11 @@ static long vhost_net_set_backend(struct
vhost_net *n, unsigned index, int fd)
>>>>    		r = vhost_vq_init_access(vq);
>>>>    		if (r)
>>>>    			goto err_used;
>>>> -		r = vhost_net_enable_vq(n, vq);
>>>> -		if (r)
>>>> -			goto err_used;
>>>> +		if (index == VHOST_NET_VQ_RX) {
>>>> +			r = vhost_net_enable_vq(n, vq);
>>>> +			if (r)
>>>> +				goto err_used;
>>>> +		}
>>>>    		oldubufs = nvq->ubufs;
>>>>    		nvq->ubufs = ubufs;
>>> This last chunk seems questionable. If queue has stuff in it
>>> when we connect the backend, we'll miss a wakeup.
>>> I suspect this can happen during migration.
>> Unless qemu pass a tap which s already had pending tx packets.
>>
>> I can remove this chuck, but if guest does not transmit any packet, rx
can't
>> benefit from this.
>>
>> Thanks
> Not sure I understand the last sentence. vhost will stay
> polling the socket - why is that a problem?
No function problem at all. If guest does not transmit any packet, the 
patch does not have any effect.

Will remove this chunk in next version.

Thanks
>
>>>
>>>> -- 
>>>> 2.7.4
>> _______________________________________________
>> Virtualization mailing list
>> Virtualization at lists.linux-foundation.org
>> https://lists.linuxfoundation.org/mailman/listinfo/virtualization