On Tue, Feb 10, 2015 at 9:03 AM, Rusty Russell <rusty@xxxxxxxxxxxxxxx>
wrote:
Jason Wang <jasowang@xxxxxxxxxx> 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@xxxxxxxxxx>
---
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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