Linux Virtualization - Jan 2017 - [PATCH V4 net-next 1/3] vhost: better detection of available buffers

On Fri, Jan 06, 2017 at 10:13:15AM +0800, Jason Wang
wrote:
> This patch tries to do several tweaks on vhost_vq_avail_empty() for a
> better performance:
> 
> - check cached avail index first which could avoid userspace memory access.
> - using unlikely() for the failure of userspace access
> - check vq->last_avail_idx instead of cached avail index as the last
>   step.
> 
> This patch is need for batching supports which needs to peek whether
> or not there's still available buffers in the ring.
> 
> Reviewed-by: Stefan Hajnoczi <stefanha at redhat.com>
> Signed-off-by: Jason Wang <jasowang at redhat.com>
> ---
>  drivers/vhost/vhost.c | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index d643260..9f11838 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -2241,11 +2241,15 @@ bool vhost_vq_avail_empty(struct vhost_dev *dev,
struct vhost_virtqueue *vq)
>  	__virtio16 avail_idx;
>  	int r;
>  
> +	if (vq->avail_idx != vq->last_avail_idx)
> +		return false;
> +
>  	r = vhost_get_user(vq, avail_idx, &vq->avail->idx);
> -	if (r)
> +	if (unlikely(r))
>  		return false;
> +	vq->avail_idx = vhost16_to_cpu(vq, avail_idx);
>  
> -	return vhost16_to_cpu(vq, avail_idx) == vq->avail_idx;
> +	return vq->avail_idx == vq->last_avail_idx;
>  }
>  EXPORT_SYMBOL_GPL(vhost_vq_avail_empty);
So again, this did not address the issue I pointed out in v1:
if we have 1 buffer in RX queue and
that is not enough to store the whole packet,
vhost_vq_avail_empty returns false, then we re-read
the descriptors again and again.

You have saved a single index access but not the more expensive
descriptor access.

I think that a way to address this could be to have this
return current index for the caller. Then as long as that
index isn't changed, you don't poke at descriptor ring.
> -- 
> 2.7.4

On 2017?01?07? 03:55, Michael S. Tsirkin wrote:
> On Fri, Jan 06, 2017 at 10:13:15AM +0800, Jason Wang wrote:
>> This patch tries to do several tweaks on vhost_vq_avail_empty() for a
>> better performance:
>>
>> - check cached avail index first which could avoid userspace memory
access.
>> - using unlikely() for the failure of userspace access
>> - check vq->last_avail_idx instead of cached avail index as the last
>>    step.
>>
>> This patch is need for batching supports which needs to peek whether
>> or not there's still available buffers in the ring.
>>
>> Reviewed-by: Stefan Hajnoczi <stefanha at redhat.com>
>> Signed-off-by: Jason Wang <jasowang at redhat.com>
>> ---
>>   drivers/vhost/vhost.c | 8 ++++++--
>>   1 file changed, 6 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
>> index d643260..9f11838 100644
>> --- a/drivers/vhost/vhost.c
>> +++ b/drivers/vhost/vhost.c
>> @@ -2241,11 +2241,15 @@ bool vhost_vq_avail_empty(struct vhost_dev
*dev, struct vhost_virtqueue *vq)
>>   	__virtio16 avail_idx;
>>   	int r;
>>   
>> +	if (vq->avail_idx != vq->last_avail_idx)
>> +		return false;
>> +
>>   	r = vhost_get_user(vq, avail_idx, &vq->avail->idx);
>> -	if (r)
>> +	if (unlikely(r))
>>   		return false;
>> +	vq->avail_idx = vhost16_to_cpu(vq, avail_idx);
>>   
>> -	return vhost16_to_cpu(vq, avail_idx) == vq->avail_idx;
>> +	return vq->avail_idx == vq->last_avail_idx;
>>   }
>>   EXPORT_SYMBOL_GPL(vhost_vq_avail_empty);
> So again, this did not address the issue I pointed out in v1:
> if we have 1 buffer in RX queue and
> that is not enough to store the whole packet,
> vhost_vq_avail_empty returns false, then we re-read
> the descriptors again and again.
>
> You have saved a single index access but not the more expensive
> descriptor access.
Looks not, if I understand the code correctly, in this case, 
get_rx_bufs() will return zero, and we will try to enable rx kick and 
exit the loop.

Thanks

On Mon, Jan 09, 2017 at 10:59:16AM +0800, Jason Wang
wrote:
> 
> 
> On 2017?01?07? 03:55, Michael S. Tsirkin wrote:
> > On Fri, Jan 06, 2017 at 10:13:15AM +0800, Jason Wang wrote:
> > > This patch tries to do several tweaks on vhost_vq_avail_empty()
for a
> > > better performance:
> > > 
> > > - check cached avail index first which could avoid userspace
memory access.
> > > - using unlikely() for the failure of userspace access
> > > - check vq->last_avail_idx instead of cached avail index as
the last
> > >    step.
> > > 
> > > This patch is need for batching supports which needs to peek
whether
> > > or not there's still available buffers in the ring.
> > > 
> > > Reviewed-by: Stefan Hajnoczi <stefanha at redhat.com>
> > > Signed-off-by: Jason Wang <jasowang at redhat.com>
> > > ---
> > >   drivers/vhost/vhost.c | 8 ++++++--
> > >   1 file changed, 6 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> > > index d643260..9f11838 100644
> > > --- a/drivers/vhost/vhost.c
> > > +++ b/drivers/vhost/vhost.c
> > > @@ -2241,11 +2241,15 @@ bool vhost_vq_avail_empty(struct
vhost_dev *dev, struct vhost_virtqueue *vq)
> > >   	__virtio16 avail_idx;
> > >   	int r;
> > > +	if (vq->avail_idx != vq->last_avail_idx)
> > > +		return false;
> > > +
> > >   	r = vhost_get_user(vq, avail_idx, &vq->avail->idx);
> > > -	if (r)
> > > +	if (unlikely(r))
> > >   		return false;
> > > +	vq->avail_idx = vhost16_to_cpu(vq, avail_idx);
> > > -	return vhost16_to_cpu(vq, avail_idx) == vq->avail_idx;
> > > +	return vq->avail_idx == vq->last_avail_idx;
> > >   }
> > >   EXPORT_SYMBOL_GPL(vhost_vq_avail_empty);
> > So again, this did not address the issue I pointed out in v1:
> > if we have 1 buffer in RX queue and
> > that is not enough to store the whole packet,
> > vhost_vq_avail_empty returns false, then we re-read
> > the descriptors again and again.
> > 
> > You have saved a single index access but not the more expensive
> > descriptor access.
> 
> Looks not, if I understand the code correctly, in this case, get_rx_bufs()
> will return zero, and we will try to enable rx kick and exit the loop.
> 
> Thanks
I mean this:

                while (vhost_can_busy_poll(vq->dev, endtime) &&
                       vhost_vq_avail_empty(vq->dev, vq))
                        cpu_relax();
                preempt_enable();
                r = vhost_get_vq_desc(vq, vq->iov, ARRAY_SIZE(vq->iov),
                                      out_num, in_num, NULL, NULL);


vhost_vq_avail_empty returns false so we break out of the loop
and call vhost_get_vq_desc.


-- 
MST