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[209.132.180.67]) by mx.google.com with ESMTP id bb7-v6si3505228plb.322.2018.11.08.20.01.00; Thu, 08 Nov 2018 20:01:16 -0800 (PST) Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67; Authentication-Results: mx.google.com; spf=pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) smtp.mailfrom=linux-kernel-owner@vger.kernel.org; dmarc=fail (p=NONE sp=NONE dis=NONE) header.from=redhat.com Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727655AbeKINjQ (ORCPT + 99 others); Fri, 9 Nov 2018 08:39:16 -0500 Received: from mx1.redhat.com ([209.132.183.28]:55714 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727311AbeKINjP (ORCPT ); Fri, 9 Nov 2018 08:39:15 -0500 Received: from smtp.corp.redhat.com (int-mx03.intmail.prod.int.phx2.redhat.com [10.5.11.13]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 5814089AE6; Fri, 9 Nov 2018 04:00:33 +0000 (UTC) Received: from redhat.com (ovpn-120-200.rdu2.redhat.com [10.10.120.200]) by smtp.corp.redhat.com (Postfix) with ESMTP id 6A4F567647; Fri, 9 Nov 2018 04:00:29 +0000 (UTC) Date: Thu, 8 Nov 2018 23:00:28 -0500 From: "Michael S. Tsirkin" To: Jason Wang Cc: Tiwei Bie , virtualization@lists.linux-foundation.org, linux-kernel@vger.kernel.org, netdev@vger.kernel.org, virtio-dev@lists.oasis-open.org, wexu@redhat.com, jfreimann@redhat.com Subject: Re: [PATCH net-next v2 3/5] virtio_ring: add packed ring support Message-ID: <20181108225858-mutt-send-email-mst@kernel.org> References: <20180711022711.7090-1-tiwei.bie@intel.com> <20180711022711.7090-4-tiwei.bie@intel.com> <20181107123933-mutt-send-email-mst@kernel.org> <20181108013759.GA20591@debian> <2d46a41e-bc00-276a-e19a-105c9dffc75a@redhat.com> <20181108115148.GA15701@debian> <20181108103155-mutt-send-email-mst@kernel.org> <67bd6a88-00f2-ed13-ad13-bdfe92ceeffc@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <67bd6a88-00f2-ed13-ad13-bdfe92ceeffc@redhat.com> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.26]); Fri, 09 Nov 2018 04:00:33 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Nov 09, 2018 at 10:30:50AM +0800, Jason Wang wrote: > > On 2018/11/8 下午11:56, Michael S. Tsirkin wrote: > > On Thu, Nov 08, 2018 at 07:51:48PM +0800, Tiwei Bie wrote: > > > On Thu, Nov 08, 2018 at 04:18:25PM +0800, Jason Wang wrote: > > > > On 2018/11/8 上午9:38, Tiwei Bie wrote: > > > > > > > + > > > > > > > + if (vq->vq.num_free < descs_used) { > > > > > > > + pr_debug("Can't add buf len %i - avail = %i\n", > > > > > > > + descs_used, vq->vq.num_free); > > > > > > > + /* FIXME: for historical reasons, we force a notify here if > > > > > > > + * there are outgoing parts to the buffer. Presumably the > > > > > > > + * host should service the ring ASAP. */ > > > > > > I don't think we have a reason to do this for packed ring. > > > > > > No historical baggage there, right? > > > > > Based on the original commit log, it seems that the notify here > > > > > is just an "optimization". But I don't quite understand what does > > > > > the "the heuristics which KVM uses" refer to. If it's safe to drop > > > > > this in packed ring, I'd like to do it. > > > > > > > > According to the commit log, it seems like a workaround of lguest networking > > > > backend. > > > Do you know why removing this notify in Tx will break "the > > > heuristics which KVM uses"? Or what does "the heuristics > > > which KVM uses" refer to? > > Yes. QEMU has a mode where it disables notifications and processes TX > > ring periodically from a timer. It's off by default but used to be on > > by default a long time ago. If ring becomes full this causes traffic > > stalls. > > > Do you mean tx-timer? If yes, we can still enable it for packed ring Yes we can but I doubt anyone does. > and the > timer will finally fired and we can go. on tx ring full we probably don't want to wait for timer. But I think we can just prevent qemu from using tx timer with virtio 1. > > > As a work-around Rusty put in this hack to kick on ring full > > even with notifications disabled. > > > From the commit log it looks more like a performance workaround instead of a > bug fix. it's a quality of implementation issue, yes. > > > It's easy enough to make sure QEMU > > does not combine devices with packed ring support with the timer hack. > > And I am guessing it's safe enough to also block that option completely > > e.g. when virtio 1.0 is enabled. > > > I agree. > > Thanks > > > > > > I agree to drop it, we should not have such burden. > > > > > > > > But we should notice that, with this removed, the compare between packed vs > > > > split is kind of unfair. Consider the removal of lguest support recently, > > > > maybe we can drop this for split ring as well? > > > > > > > > Thanks > > > > > > > > > > > > > commit 44653eae1407f79dff6f52fcf594ae84cb165ec4 > > > > > Author: Rusty Russell > > > > > Date: Fri Jul 25 12:06:04 2008 -0500 > > > > > > > > > > virtio: don't always force a notification when ring is full > > > > > We force notification when the ring is full, even if the host has > > > > > indicated it doesn't want to know. This seemed like a good idea at > > > > > the time: if we fill the transmit ring, we should tell the host > > > > > immediately. > > > > > Unfortunately this logic also applies to the receiving ring, which is > > > > > refilled constantly. We should introduce real notification thesholds > > > > > to replace this logic. Meanwhile, removing the logic altogether breaks > > > > > the heuristics which KVM uses, so we use a hack: only notify if there are > > > > > outgoing parts of the new buffer. > > > > > Here are the number of exits with lguest's crappy network implementation: > > > > > Before: > > > > > network xmit 7859051 recv 236420 > > > > > After: > > > > > network xmit 7858610 recv 118136 > > > > > Signed-off-by: Rusty Russell > > > > > > > > > > diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c > > > > > index 72bf8bc09014..21d9a62767af 100644 > > > > > --- a/drivers/virtio/virtio_ring.c > > > > > +++ b/drivers/virtio/virtio_ring.c > > > > > @@ -87,8 +87,11 @@ static int vring_add_buf(struct virtqueue *_vq, > > > > > if (vq->num_free < out + in) { > > > > > pr_debug("Can't add buf len %i - avail = %i\n", > > > > > out + in, vq->num_free); > > > > > - /* We notify*even if* VRING_USED_F_NO_NOTIFY is set here. */ > > > > > - vq->notify(&vq->vq); > > > > > + /* FIXME: for historical reasons, we force a notify here if > > > > > + * there are outgoing parts to the buffer. Presumably the > > > > > + * host should service the ring ASAP. */ > > > > > + if (out) > > > > > + vq->notify(&vq->vq); > > > > > END_USE(vq); > > > > > return -ENOSPC; > > > > > } > > > > > > > > > >