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 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 C7BA4C43387 for ; Mon, 7 Jan 2019 04:01:32 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 9ED1C20859 for ; Mon, 7 Jan 2019 04:01:32 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726509AbfAGEBb (ORCPT ); Sun, 6 Jan 2019 23:01:31 -0500 Received: from mx1.redhat.com ([209.132.183.28]:45300 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726246AbfAGEBb (ORCPT ); Sun, 6 Jan 2019 23:01:31 -0500 Received: from smtp.corp.redhat.com (int-mx02.intmail.prod.int.phx2.redhat.com [10.5.11.12]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id D775A821CC; Mon, 7 Jan 2019 04:01:30 +0000 (UTC) Received: from redhat.com (ovpn-120-33.rdu2.redhat.com [10.10.120.33]) by smtp.corp.redhat.com (Postfix) with ESMTP id 4409760C44; Mon, 7 Jan 2019 04:01:24 +0000 (UTC) Date: Sun, 6 Jan 2019 23:01:23 -0500 From: "Michael S. Tsirkin" To: Jason Wang 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 Subject: Re: [PATCH RFC 1/2] virtio-net: bql support Message-ID: <20190106225951-mutt-send-email-mst@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> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: X-Scanned-By: MIMEDefang 2.79 on 10.5.11.12 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.28]); Mon, 07 Jan 2019 04:01:30 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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. > > > 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. > > >