Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933125AbbBJBtm (ORCPT ); Mon, 9 Feb 2015 20:49:42 -0500 Received: from ozlabs.org ([103.22.144.67]:53039 "EHLO ozlabs.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751732AbbBJBtk (ORCPT ); Mon, 9 Feb 2015 20:49:40 -0500 From: Rusty Russell To: Jason Wang , netdev@vger.kernel.org, linux-kernel@vger.kernel.org, virtualization@lists.linux-foundation.org, mst@redhat.com Cc: pagupta@redhat.com Subject: Re: [PATCH RFC v5 net-next 1/6] virtio_ring: fix virtqueue_enable_cb() when only 1 buffers were pending In-Reply-To: <1423471165-34243-2-git-send-email-jasowang@redhat.com> References: <1423471165-34243-1-git-send-email-jasowang@redhat.com> <1423471165-34243-2-git-send-email-jasowang@redhat.com> User-Agent: Notmuch/0.17 (http://notmuchmail.org) Emacs/24.3.1 (x86_64-pc-linux-gnu) Date: Tue, 10 Feb 2015 11:33:52 +1030 Message-ID: <878ug6y3uv.fsf@rustcorp.com.au> MIME-Version: 1.0 Content-Type: text/plain Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2455 Lines: 58 Jason Wang 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; 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. > Signed-off-by: Jason Wang > --- > 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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/