From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-1.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,URIBL_BLOCKED autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 9E5CAC43387 for ; Mon, 7 Jan 2019 06:32:04 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 6015A20859 for ; Mon, 7 Jan 2019 06:32:04 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726458AbfAGGcD (ORCPT ); Mon, 7 Jan 2019 01:32:03 -0500 Received: from mx1.redhat.com ([209.132.183.28]:50646 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725306AbfAGGcD (ORCPT ); Mon, 7 Jan 2019 01:32:03 -0500 Received: from smtp.corp.redhat.com (int-mx04.intmail.prod.int.phx2.redhat.com [10.5.11.14]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 30903C0587F7; Mon, 7 Jan 2019 06:32:02 +0000 (UTC) Received: from [10.72.12.209] (ovpn-12-209.pek2.redhat.com [10.72.12.209]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 22CE15D9CD; Mon, 7 Jan 2019 06:31:49 +0000 (UTC) Subject: Re: [PATCH RFC 1/2] virtio-net: bql support To: "Michael S. Tsirkin" Cc: linux-kernel@vger.kernel.org, maxime.coquelin@redhat.com, tiwei.bie@intel.com, wexu@redhat.com, jfreimann@redhat.com, "David S. Miller" , virtualization@lists.linux-foundation.org, netdev@vger.kernel.org References: <20181205225323.12555-2-mst@redhat.com> <21384cb5-99a6-7431-1039-b356521e1bc3@redhat.com> <20181226101528-mutt-send-email-mst@kernel.org> <0fa99d9b-e510-d7eb-db1b-831bd7610ce9@redhat.com> <20181230134106-mutt-send-email-mst@kernel.org> <20190102085457-mutt-send-email-mst@kernel.org> <17d2ab21-1c9a-2bb9-166f-2863d019cb0b@redhat.com> <20190106221506-mutt-send-email-mst@kernel.org> <20190106225951-mutt-send-email-mst@kernel.org> From: Jason Wang Message-ID: <88db987e-b519-5c1f-f64f-6f65f8415799@redhat.com> Date: Mon, 7 Jan 2019 14:31:47 +0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.2.1 MIME-Version: 1.0 In-Reply-To: <20190106225951-mutt-send-email-mst@kernel.org> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Content-Language: en-US X-Scanned-By: MIMEDefang 2.79 on 10.5.11.14 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.32]); Mon, 07 Jan 2019 06:32:02 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2019/1/7 下午12:01, Michael S. Tsirkin wrote: > On Mon, Jan 07, 2019 at 11:51:55AM +0800, Jason Wang wrote: >> On 2019/1/7 上午11:17, Michael S. Tsirkin wrote: >>> On Mon, Jan 07, 2019 at 10:14:37AM +0800, Jason Wang wrote: >>>> On 2019/1/2 下午9:59, Michael S. Tsirkin wrote: >>>>> On Wed, Jan 02, 2019 at 11:28:43AM +0800, Jason Wang wrote: >>>>>> On 2018/12/31 上午2:45, Michael S. Tsirkin wrote: >>>>>>> On Thu, Dec 27, 2018 at 06:00:36PM +0800, Jason Wang wrote: >>>>>>>> On 2018/12/26 下午11:19, Michael S. Tsirkin wrote: >>>>>>>>> On Thu, Dec 06, 2018 at 04:17:36PM +0800, Jason Wang wrote: >>>>>>>>>> On 2018/12/6 上午6:54, Michael S. Tsirkin wrote: >>>>>>>>>>> When use_napi is set, let's enable BQLs. Note: some of the issues are >>>>>>>>>>> similar to wifi. It's worth considering whether something similar to >>>>>>>>>>> commit 36148c2bbfbe ("mac80211: Adjust TSQ pacing shift") might be >>>>>>>>>>> benefitial. >>>>>>>>>> I've played a similar patch several days before. The tricky part is the mode >>>>>>>>>> switching between napi and no napi. We should make sure when the packet is >>>>>>>>>> sent and trakced by BQL,  it should be consumed by BQL as well. I did it by >>>>>>>>>> tracking it through skb->cb.  And deal with the freeze by reset the BQL >>>>>>>>>> status. Patch attached. >>>>>>>>>> >>>>>>>>>> But when testing with vhost-net, I don't very a stable performance, >>>>>>>>> So how about increasing TSQ pacing shift then? >>>>>>>> I can test this. But changing default TCP value is much more than a >>>>>>>> virtio-net specific thing. >>>>>>> Well same logic as wifi applies. Unpredictable latencies related >>>>>>> to radio in one case, to host scheduler in the other. >>>>>>> >>>>>>>>>> it was >>>>>>>>>> probably because we batch the used ring updating so tx interrupt may come >>>>>>>>>> randomly. We probably need to implement time bounded coalescing mechanism >>>>>>>>>> which could be configured from userspace. >>>>>>>>> I don't think it's reasonable to expect userspace to be that smart ... >>>>>>>>> Why do we need time bounded? used ring is always updated when ring >>>>>>>>> becomes empty. >>>>>>>> We don't add used when means BQL may not see the consumed packet in time. >>>>>>>> And the delay varies based on the workload since we count packets not bytes >>>>>>>> or time before doing the batched updating. >>>>>>>> >>>>>>>> Thanks >>>>>>> Sorry I still don't get it. >>>>>>> When nothing is outstanding then we do update the used. >>>>>>> So if BQL stops userspace from sending packets then >>>>>>> we get an interrupt and packets start flowing again. >>>>>> Yes, but how about the cases of multiple flows. That's where I see unstable >>>>>> results. >>>>>> >>>>>> >>>>>>> It might be suboptimal, we might need to tune it but I doubt running >>>>>>> timers is a solution, timer interrupts cause VM exits. >>>>>> Probably not a timer but a time counter (or event byte counter) in vhost to >>>>>> add used and signal guest if it exceeds a value instead of waiting the >>>>>> number of packets. >>>>>> >>>>>> >>>>>> Thanks >>>>> Well we already have VHOST_NET_WEIGHT - is it too big then? >>>> I'm not sure, it might be too big. >>>> >>>> >>>>> And maybe we should expose the "MORE" flag in the descriptor - >>>>> do you think that will help? >>>>> >>>> I don't know. But how a "more" flag can help here? >>>> >>>> Thanks >>> It sounds like we should be a bit more aggressive in updating used ring. >>> But if we just do it naively we will harm performance for sure as that >>> is how we are doing batching right now. >> >> I agree but the problem is to balance the PPS and throughput. More batching >> helps for PPS but may damage TCP throughput. > That is what more flag is supposed to be I think - it is only set if > there's a socket that actually needs the skb freed in order to go on. I'm not quite sure I get, but is this something similar to what you want? https://lists.linuxfoundation.org/pipermail/virtualization/2014-October/027667.html Which enables tx interrupt for TCP packets, and you want to add used more aggressively for those sockets? Thanks >>> Instead we could make guest >>> control batching using the more flag - if that's not set we write out >>> the used ring. >> >> It's under the control of guest, so I'm afraid we still need some more guard >> (e.g time/bytes counters) on host. >> >> Thanks > Point is if guest does not care about the skb being freed, then there is no > rush host side to mark buffer used. > >