Re: [PATCH V4 net-next 1/3] vhost: better detection of available buffers

[Jason Wang <jasowang@redhat.com>]

On 2017年01月10日 07:10, Michael S. Tsirkin wrote:
> On Mon, Jan 09, 2017 at 10:59:16AM +0800, Jason Wang wrote:
>>
>> On 2017年01月07日 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@redhat.com>
>>>> Signed-off-by: Jason Wang <jasowang@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.
>
>

But this is the code for polling tx vq not rx I think?

Thanks

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