From: "Michael S. Tsirkin" <mst@redhat.com>
To: Rusty Russell <rusty@rustcorp.com.au>
Cc: pagupta@redhat.com, netdev@vger.kernel.org,
linux-kernel@vger.kernel.org,
virtualization@lists.linux-foundation.org
Subject: Re: [PATCH RFC v5 net-next 1/6] virtio_ring: fix virtqueue_enable_cb() when only 1 buffers were pending
Date: Tue, 10 Feb 2015 11:18:39 +0100 [thread overview]
Message-ID: <20150210101839.GA9505@redhat.com> (raw)
In-Reply-To: <878ug6y3uv.fsf@rustcorp.com.au>
On Tue, Feb 10, 2015 at 11:33:52AM +1030, Rusty Russell 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;
Or bus * 3/4 + 1
> But it's not clear to me how much this happens. 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.
>
> Cheers,
> Rusty.
But isn't this by design?
The documentation says:
* This re-enables callbacks but hints to the other side to delay
* interrupts until most of the available buffers have been processed;
Clearly, this implies that when there's one buffer it must behave
exactly the same.
So I'm not very happy - this changes the meaning of the API without
updating the documentation.
> > 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
> >
> > _______________________________________________
> > Virtualization mailing list
> > Virtualization@lists.linux-foundation.org
> > https://lists.linuxfoundation.org/mailman/listinfo/virtualization
next prev parent reply other threads:[~2015-02-10 10:18 UTC|newest]
Thread overview: 24+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-02-09 8:39 [PATCH RFC v5 net-next 0/6] enable tx interrupts for virtio-net Jason Wang
2015-02-09 8:39 ` [PATCH RFC v5 net-next 1/6] virtio_ring: fix virtqueue_enable_cb() when only 1 buffers were pending Jason Wang
2015-02-10 1:03 ` Rusty Russell
2015-02-10 6:26 ` Jason Wang
2015-02-10 10:18 ` Michael S. Tsirkin [this message]
2015-02-10 23:58 ` Rusty Russell
2015-02-11 5:41 ` Jason Wang
2015-02-09 8:39 ` [PATCH RFC v5 net-next 2/6] virtio_ring: try to disable event index callbacks in virtqueue_disable_cb() Jason Wang
2015-02-10 1:07 ` Rusty Russell
2015-02-10 10:24 ` Michael S. Tsirkin
2015-02-11 5:55 ` Jason Wang
2015-02-09 8:39 ` [PATCH RFC v5 net-next 3/6] virtio_net: enable tx interrupt Jason Wang
2015-02-09 8:39 ` [PATCH RFC v5 net-next 4/6] virtio-net: add basic interrupt coalescing support Jason Wang
2015-02-10 1:32 ` Rusty Russell
2015-02-10 6:51 ` Jason Wang
2015-02-10 10:25 ` Michael S. Tsirkin
2015-02-10 10:40 ` Michael S. Tsirkin
2015-02-13 2:52 ` Rusty Russell
2015-02-13 12:41 ` Pawel Moll
2015-02-16 3:07 ` Rusty Russell
2015-02-13 18:19 ` Cornelia Huck
2015-02-16 3:19 ` Rusty Russell
2015-02-09 8:39 ` [PATCH RFC v5 net-next 5/6] vhost: let vhost_signal() returns whether signalled Jason Wang
2015-02-09 8:39 ` [PATCH RFC v5 net-next 6/6] vhost_net: interrupt coalescing support Jason Wang
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