Subject: Re: [PATCH RFC v5 net-next 1/6] virtio_ring: fix virtqueue_enable_cb() when only 1 buffers were pending

From: Jason Wang <[email-masked]>
Date: Tue, 10 Feb 2015 06:34:02 +0008

On Tue, Feb 10, 2015 at 9:03 AM, Rusty Russell <[email-masked]> 
wrote:
> Jason Wang <[email-masked]> writes:
>>  We currently does:
>> 
>>  bufs = (avail->idx - last_used_idx) * 3 / 4;
>> 
>>  This is ok now since we only try to enable the delayed callbacks 
>> when
>>  the queue is about to be full. This may not work well when there is
>>  only one pending buffer in the virtqueue (this may be the case after
>>  tx interrupt was enabled). Since virtqueue_enable_cb() will return
>>  false which may cause unnecessary triggering of napis. This patch
>>  correct this by only calculate the four thirds when bufs is not one.
> 
> I mildly prefer to avoid the branch, by changing the calculation like
> so:
> 
>         /* Set bufs >= 1, even if there's only one pending buffer */
>         bufs = (bufs + 1) * 3 / 4;

Ok.
> 
> But it's not clear to me how much this happens. 

Depends on the traffic type. In some case e.g one session TCP_RR test 
(which has at most 1 pending packet), it may happen very often.
>  I'm happy with the
> patch though, as currently virtqueue_enable_cb_delayed() is the same
> as virtqueue_enable_cb() if there's only been one buffer added.

Yes. Thanks.
> 
> Cheers,
> Rusty.
> 
>>  Signed-off-by: Jason Wang <[email-masked]>
>>  ---
>>   drivers/virtio/virtio_ring.c | 5 ++++-
>>   1 file changed, 4 insertions(+), 1 deletion(-)
>> 
>>  diff --git a/drivers/virtio/virtio_ring.c 
>> b/drivers/virtio/virtio_ring.c
>>  index 00ec6b3..545fed5 100644
>>  --- a/drivers/virtio/virtio_ring.c
>>  +++ b/drivers/virtio/virtio_ring.c
>>  @@ -636,7 +636,10 @@ bool virtqueue_enable_cb_delayed(struct 
>> virtqueue *_vq)
>>   	 * entry. Always do both to keep code simple. */
>>   	vq->vring.avail->flags &= cpu_to_virtio16(_vq->vdev, 
>> ~VRING_AVAIL_F_NO_INTERRUPT);
>>   	/* TODO: tune this threshold */
>>  -	bufs = (u16)(virtio16_to_cpu(_vq->vdev, vq->vring.avail->idx) - 
>> vq->last_used_idx) * 3 / 4;
>>  +	bufs = (u16)(virtio16_to_cpu(_vq->vdev, vq->vring.avail->idx) -
>>  +		                     vq->last_used_idx);
>>  +	if (bufs != 1)
>>  +		bufs = bufs * 3 / 4;
>>   	vring_used_event(&vq->vring) = cpu_to_virtio16(_vq->vdev, 
>> vq->last_used_idx + bufs);
>>   	virtio_mb(vq->weak_barriers);
>>   	if (unlikely((u16)(virtio16_to_cpu(_vq->vdev, 
>> vq->vring.used->idx) - vq->last_used_idx) > bufs)) {
>>  -- 
>>  1.8.3.1
>> 
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