From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Michael S. Tsirkin" Subject: Re: [PATCH net] Revert "vhost: cache used event for better performance" Date: Wed, 26 Jul 2017 19:08:26 +0300 Message-ID: <20170726190745-mutt-send-email-mst@kernel.org> References: <1501056197-3368-1-git-send-email-jasowang@redhat.com> <20170726155435-mutt-send-email-mst@kernel.org> <117542fc-eef9-c043-7c9e-daafceb7db4e@redhat.com> <6b3a9a98-c095-1729-3528-dd521f136797@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: base64 Return-path: Content-Disposition: inline In-Reply-To: <6b3a9a98-c095-1729-3528-dd521f136797@redhat.com> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: virtualization-bounces@lists.linux-foundation.org Errors-To: virtualization-bounces@lists.linux-foundation.org To: Jason Wang Cc: netdev@vger.kernel.org, linux-kernel@vger.kernel.org, kvm@vger.kernel.org, virtualization@lists.linux-foundation.org List-Id: virtualization@lists.linuxfoundation.org T24gV2VkLCBKdWwgMjYsIDIwMTcgYXQgMDk6Mzc6MTVQTSArMDgwMCwgSmFzb24gV2FuZyB3cm90 ZToKPiAKPiAKPiBPbiAyMDE35bm0MDfmnIgyNuaXpSAyMToxOCwgSmFzb24gV2FuZyB3cm90ZToK PiA+IAo+ID4gCj4gPiBPbiAyMDE35bm0MDfmnIgyNuaXpSAyMDo1NywgTWljaGFlbCBTLiBUc2ly a2luIHdyb3RlOgo+ID4gPiBPbiBXZWQsIEp1bCAyNiwgMjAxNyBhdCAwNDowMzoxN1BNICswODAw LCBKYXNvbiBXYW5nIHdyb3RlOgo+ID4gPiA+IFRoaXMgcmV2ZXJ0cyBjb21taXQgODA5ZWNiOWJj YTZhOTQyNGNjZDM5MmQ2N2UzNjgxNjBmOGI3NmM5Mi4gU2luY2UgaXQKPiA+ID4gPiB3YXMgcmVw b3J0ZWQgdG8gYnJlYWsgdmhvc3RfbmV0LiBXZSB3YW50IHRvIGNhY2hlIHVzZWQgZXZlbnQgYW5k IHVzZQo+ID4gPiA+IGl0IHRvIGNoZWNrIGZvciBub3RpZmljYXRpb24uIFdlIHRyeSB0byB2YWxp ZCBjYWNoZWQgdXNlZCBldmVudCBieQo+ID4gPiA+IGNoZWNraW5nIHdoZXRoZXIgb3Igbm90IGl0 IHdhcyBhaGVhZCBvZiBuZXcsIGJ1dCB0aGlzIGlzIG5vdCBjb3JyZWN0Cj4gPiA+ID4gYWxsIHRo ZSB0aW1lLCBpdCBjb3VsZCBiZSBzdGFsZSBhbmQgdGhlcmUncyBubyB3YXkgdG8ga25vdyBhYm91 dCB0aGlzLgo+ID4gPiA+IAo+ID4gPiA+IFNpZ25lZC1vZmYtYnk6IEphc29uIFdhbmc8amFzb3dh bmdAcmVkaGF0LmNvbT4KPiA+ID4gQ291bGQgeW91IHN1cHBseSBhIGJpdCBtb3JlIGRhdGEgaGVy ZSBwbGVhc2U/ICBIb3cgZG9lcyBpdCBnZXQgc3RhbGU/Cj4gPiA+IFdoYXQgZG9lcyBndWVzdCBu ZWVkIHRvIGRvIHRvIG1ha2UgaXQgc3RhbGU/ICBUaGlzIHdpbGwgYmUgaGVscGZ1bCBpZgo+ID4g PiBhbnlvbmUgd2FudHMgdG8gYnJpbmcgaXQgYmFjaywgb3IgaWYgd2Ugd2FudCB0byBleHRlbmQg dGhlIHByb3RvY29sLgo+ID4gPiAKPiA+IAo+ID4gVGhlIHByb2JsZW0gd2UgZG9uJ3Qga25vdyB3 aGV0aGVyIG9yIG5vdCBndWVzdCBoYXMgcHVibGlzaGVkIGEgbmV3IHVzZWQKPiA+IGV2ZW50LiBU aGUgY2hlY2sgdnJpbmdfbmVlZF9ldmVudCh2cS0+bGFzdF91c2VkX2V2ZW50LCBuZXcgKyB2cS0+ bnVtLAo+ID4gbmV3KSBpcyBub3Qgc3VmZmljaWVudCB0byBjaGVjayBmb3IgdGhpcy4KPiA+IAo+ ID4gVGhhbmtzCj4gCj4gTW9yZSBub3RlcywgdGhlIHByZXZpb3VzIGFzc3VtcHRpb24gaXMgdGhh dCB3ZSBkb24ndCBtb3ZlIHVzZWQgZXZlbnQgYmFjaywKPiBidXQgdGhpcyBjb3VsZCBoYXBwZW4g aW4gZmFjdCBpZiBpZHggaXMgd3JhcHBlciBhcm91bmQuCgpZb3UgbWVhbiBpZiB0aGUgMTYgYml0 IGluZGV4IHdyYXBzIGFyb3VuZCBhZnRlciA2NEsgZW50cmllcy4KTWFrZXMgc2Vuc2UuCgo+IFdp bGwgcmVwb3N0IGFuZCBhZGQKPiB0aGlzIGludG8gY29tbWl0IGxvZy4KPiAKPiBUaGFua3MKX19f X19fX19fX19fX19fX19fX19fX19fX19fX19fX19fX19fX19fX19fX19fX18KVmlydHVhbGl6YXRp b24gbWFpbGluZyBsaXN0ClZpcnR1YWxpemF0aW9uQGxpc3RzLmxpbnV4LWZvdW5kYXRpb24ub3Jn Cmh0dHBzOi8vbGlzdHMubGludXhmb3VuZGF0aW9uLm9yZy9tYWlsbWFuL2xpc3RpbmZvL3ZpcnR1 YWxpemF0aW9u From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751822AbdGZQI3 (ORCPT ); Wed, 26 Jul 2017 12:08:29 -0400 Received: from mx1.redhat.com ([209.132.183.28]:58884 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751756AbdGZQI1 (ORCPT ); Wed, 26 Jul 2017 12:08:27 -0400 DMARC-Filter: OpenDMARC Filter v1.3.2 mx1.redhat.com 2C05B88E60 Authentication-Results: ext-mx01.extmail.prod.ext.phx2.redhat.com; dmarc=none (p=none dis=none) header.from=redhat.com Authentication-Results: ext-mx01.extmail.prod.ext.phx2.redhat.com; spf=fail smtp.mailfrom=mst@redhat.com Date: Wed, 26 Jul 2017 19:08:26 +0300 From: "Michael S. Tsirkin" To: Jason Wang Cc: virtualization@lists.linux-foundation.org, kvm@vger.kernel.org, netdev@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH net] Revert "vhost: cache used event for better performance" Message-ID: <20170726190745-mutt-send-email-mst@kernel.org> References: <1501056197-3368-1-git-send-email-jasowang@redhat.com> <20170726155435-mutt-send-email-mst@kernel.org> <117542fc-eef9-c043-7c9e-daafceb7db4e@redhat.com> <6b3a9a98-c095-1729-3528-dd521f136797@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <6b3a9a98-c095-1729-3528-dd521f136797@redhat.com> X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.25]); Wed, 26 Jul 2017 16:08:27 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Jul 26, 2017 at 09:37:15PM +0800, Jason Wang wrote: > > > On 2017年07月26日 21:18, Jason Wang wrote: > > > > > > On 2017年07月26日 20:57, Michael S. Tsirkin wrote: > > > On Wed, Jul 26, 2017 at 04:03:17PM +0800, Jason Wang wrote: > > > > This reverts commit 809ecb9bca6a9424ccd392d67e368160f8b76c92. Since it > > > > was reported to break vhost_net. We want to cache used event and use > > > > it to check for notification. We try to valid cached used event by > > > > checking whether or not it was ahead of new, but this is not correct > > > > all the time, it could be stale and there's no way to know about this. > > > > > > > > Signed-off-by: Jason Wang > > > Could you supply a bit more data here please? How does it get stale? > > > What does guest need to do to make it stale? This will be helpful if > > > anyone wants to bring it back, or if we want to extend the protocol. > > > > > > > The problem we don't know whether or not guest has published a new used > > event. The check vring_need_event(vq->last_used_event, new + vq->num, > > new) is not sufficient to check for this. > > > > Thanks > > More notes, the previous assumption is that we don't move used event back, > but this could happen in fact if idx is wrapper around. You mean if the 16 bit index wraps around after 64K entries. Makes sense. > Will repost and add > this into commit log. > > Thanks