From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mga01.intel.com (mga01.intel.com [192.55.52.88]) by dpdk.org (Postfix) with ESMTP id 0E8144CA9; Mon, 22 Oct 2018 09:16:34 +0200 (CEST) X-Amp-Result: UNKNOWN X-Amp-Original-Verdict: FILE UNKNOWN X-Amp-File-Uploaded: False Received: from fmsmga001.fm.intel.com ([10.253.24.23]) by fmsmga101.fm.intel.com with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384; 22 Oct 2018 00:16:34 -0700 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.54,411,1534834800"; d="scan'208";a="101300942" Received: from btwcube1.sh.intel.com (HELO debian) ([10.67.104.158]) by fmsmga001.fm.intel.com with ESMTP; 22 Oct 2018 00:16:32 -0700 Date: Mon, 22 Oct 2018 15:15:10 +0800 From: Tiwei Bie To: Maxime Coquelin Cc: dev@dpdk.org, zhihong.wang@intel.com, jfreimann@redhat.com, stable@dpdk.org Message-ID: <20181022071510.GA30145@debian> References: <20181019140058.4981-1-maxime.coquelin@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20181019140058.4981-1-maxime.coquelin@redhat.com> User-Agent: Mutt/1.10.1 (2018-07-13) Subject: Re: [dpdk-dev] [PATCH] vhost: avoid memory barriers when no descriptors dequeued X-BeenThere: dev@dpdk.org X-Mailman-Version: 2.1.15 Precedence: list List-Id: DPDK patches and discussions List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Mon, 22 Oct 2018 07:16:35 -0000 On Fri, Oct 19, 2018 at 04:00:58PM +0200, Maxime Coquelin wrote: > In both split and packed dequeue paths, flush_shadow_used_ring > and vhost_ring_call variants gets called even if not packets > have been dequeued, and so no descriptors updates happened. > > It has an impact on CPU pipeline, as memory barriers are used > in these functions. > > This patch don't call these functions if no descriptors have > been dequeued. The performance gain with split ring when > dequeue zero-copy is disabled should be null, but should be > noticeable with packed ring or dequeue zero-copy enabled. > > Fixes: ae999ce49dcb ("vhost: add Tx support for packed ring") > Fixes: 915cf9404225 ("vhost: use shadow used ring in dequeue path") > Cc: stable@dpdk.org > > Signed-off-by: Maxime Coquelin > --- > lib/librte_vhost/virtio_net.c | 16 ++++++++++------ > 1 file changed, 10 insertions(+), 6 deletions(-) [...] > > - if (likely(dev->dequeue_zero_copy == 0)) { > + if (likely(dev->dequeue_zero_copy == 0 && i != 0)) { > do_data_copy_dequeue(vq); > if (unlikely(i < count)) > vq->shadow_used_idx = i; When i is 0, we may need to update vq->shadow_used_idx to 0, e.g. when error happens after update_shadow_used_ring_split() in the first iteration of the loop. > @@ -1475,8 +1477,10 @@ virtio_dev_tx_packed(struct virtio_net *dev, struct vhost_virtqueue *vq, > } > } > > - flush_shadow_used_ring_packed(dev, vq); > - vhost_vring_call_packed(dev, vq); > + if (likely(vq->shadow_used_idx)) { > + flush_shadow_used_ring_packed(dev, vq); > + vhost_vring_call_packed(dev, vq); > + } > } > > VHOST_LOG_DEBUG(VHOST_DATA, "(%d) %s\n", dev->vid, __func__); > @@ -1550,7 +1554,7 @@ virtio_dev_tx_packed(struct virtio_net *dev, struct vhost_virtqueue *vq, > } > } > > - if (likely(dev->dequeue_zero_copy == 0)) { > + if (likely(dev->dequeue_zero_copy == 0 && i != 0)) { Ditto > do_data_copy_dequeue(vq); > if (unlikely(i < count)) > vq->shadow_used_idx = i; > -- > 2.17.1 >