Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751382AbbBJG0S (ORCPT ); Tue, 10 Feb 2015 01:26:18 -0500 Received: from mx1.redhat.com ([209.132.183.28]:55089 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750824AbbBJG0Q (ORCPT ); Tue, 10 Feb 2015 01:26:16 -0500 Date: Tue, 10 Feb 2015 06:34:02 +0008 From: Jason Wang Subject: Re: [PATCH RFC v5 net-next 1/6] virtio_ring: fix virtqueue_enable_cb() when only 1 buffers were pending To: Rusty Russell Cc: netdev@vger.kernel.org, linux-kernel@vger.kernel.org, virtualization@lists.linux-foundation.org, mst@redhat.com, pagupta@redhat.com Message-Id: <1423549562.897.0@smtp.corp.redhat.com> In-Reply-To: <878ug6y3uv.fsf@rustcorp.com.au> References: <1423471165-34243-1-git-send-email-jasowang@redhat.com> <1423471165-34243-2-git-send-email-jasowang@redhat.com> <878ug6y3uv.fsf@rustcorp.com.au> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8; format=flowed Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2829 Lines: 78 On Tue, Feb 10, 2015 at 9:03 AM, Rusty Russell wrote: > 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; 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 >> --- >> 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/