From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Michael S. Tsirkin" Subject: Re: [PATCH V4 net-next 1/3] vhost: better detection of available buffers Date: Tue, 10 Jan 2017 04:57:39 +0200 Message-ID: <20170110045213-mutt-send-email-mst@kernel.org> References: <1483668797-24112-1-git-send-email-jasowang@redhat.com> <1483668797-24112-2-git-send-email-jasowang@redhat.com> <20170106214903-mutt-send-email-mst@kernel.org> <20170110010957-mutt-send-email-mst@kernel.org> <5eb3b349-bbae-6438-d8ea-7df82a7babb4@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit Cc: virtualization@lists.linux-foundation.org, netdev@vger.kernel.org, kvm@vger.kernel.org, stephen@networkplumber.org, wexu@redhat.com, stefanha@redhat.com To: Jason Wang Return-path: Received: from mx1.redhat.com ([209.132.183.28]:49164 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S967986AbdAJC5k (ORCPT ); Mon, 9 Jan 2017 21:57:40 -0500 Content-Disposition: inline In-Reply-To: <5eb3b349-bbae-6438-d8ea-7df82a7babb4@redhat.com> Sender: netdev-owner@vger.kernel.org List-ID: On Tue, Jan 10, 2017 at 10:22:42AM +0800, Jason Wang wrote: > > > On 2017年01月10日 07:10, Michael S. Tsirkin wrote: > > On Mon, Jan 09, 2017 at 10:59:16AM +0800, Jason Wang wrote: > > > > > > On 2017年01月07日 03:55, Michael S. Tsirkin wrote: > > > > On Fri, Jan 06, 2017 at 10:13:15AM +0800, Jason Wang wrote: > > > > > This patch tries to do several tweaks on vhost_vq_avail_empty() for a > > > > > better performance: > > > > > > > > > > - check cached avail index first which could avoid userspace memory access. > > > > > - using unlikely() for the failure of userspace access > > > > > - check vq->last_avail_idx instead of cached avail index as the last > > > > > step. > > > > > > > > > > This patch is need for batching supports which needs to peek whether > > > > > or not there's still available buffers in the ring. > > > > > > > > > > Reviewed-by: Stefan Hajnoczi > > > > > Signed-off-by: Jason Wang > > > > > --- > > > > > drivers/vhost/vhost.c | 8 ++++++-- > > > > > 1 file changed, 6 insertions(+), 2 deletions(-) > > > > > > > > > > diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c > > > > > index d643260..9f11838 100644 > > > > > --- a/drivers/vhost/vhost.c > > > > > +++ b/drivers/vhost/vhost.c > > > > > @@ -2241,11 +2241,15 @@ bool vhost_vq_avail_empty(struct vhost_dev *dev, struct vhost_virtqueue *vq) > > > > > __virtio16 avail_idx; > > > > > int r; > > > > > + if (vq->avail_idx != vq->last_avail_idx) > > > > > + return false; > > > > > + > > > > > r = vhost_get_user(vq, avail_idx, &vq->avail->idx); > > > > > - if (r) > > > > > + if (unlikely(r)) > > > > > return false; > > > > > + vq->avail_idx = vhost16_to_cpu(vq, avail_idx); > > > > > - return vhost16_to_cpu(vq, avail_idx) == vq->avail_idx; > > > > > + return vq->avail_idx == vq->last_avail_idx; > > > > > } > > > > > EXPORT_SYMBOL_GPL(vhost_vq_avail_empty); > > > > So again, this did not address the issue I pointed out in v1: > > > > if we have 1 buffer in RX queue and > > > > that is not enough to store the whole packet, > > > > vhost_vq_avail_empty returns false, then we re-read > > > > the descriptors again and again. > > > > > > > > You have saved a single index access but not the more expensive > > > > descriptor access. > > > Looks not, if I understand the code correctly, in this case, get_rx_bufs() > > > will return zero, and we will try to enable rx kick and exit the loop. > > > > > > Thanks > > I mean this: > > > > while (vhost_can_busy_poll(vq->dev, endtime) && > > vhost_vq_avail_empty(vq->dev, vq)) > > cpu_relax(); > > preempt_enable(); > > r = vhost_get_vq_desc(vq, vq->iov, ARRAY_SIZE(vq->iov), > > out_num, in_num, NULL, NULL); > > > > > > vhost_vq_avail_empty returns false so we break out of the loop > > and call vhost_get_vq_desc. > > > > > > But this is the code for polling tx vq not rx I think? > > Thanks Oh, right. I'll re-read this. -- MST