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

Jason Wang
2015-02-10T06:26:18Z

On Tue, Feb 10, 2015 at 9:03 AM, Rusty Russell <rusty@rustcorp.com.au>
wrote:
> Jason Wang <jasowang@redhat.com> 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 <jasowang@redhat.com>
>> ---
>> 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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