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=-5.3 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE, SPF_PASS,USER_AGENT_SANE_1 autolearn=no 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 2961FC2B9F4 for ; Mon, 28 Jun 2021 10:49:04 +0000 (UTC) Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 91A4561C49 for ; Mon, 28 Jun 2021 10:49:03 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 91A4561C49 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=huawei.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Received: from localhost ([::1]:50840 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1lxop4-0005jI-MI for qemu-devel@archiver.kernel.org; Mon, 28 Jun 2021 06:49:02 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:35002) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1lxooR-0004r5-GI; Mon, 28 Jun 2021 06:48:23 -0400 Received: from szxga01-in.huawei.com ([45.249.212.187]:2428) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1lxooN-0001ZZ-RQ; Mon, 28 Jun 2021 06:48:23 -0400 Received: from dggemv704-chm.china.huawei.com (unknown [172.30.72.53]) by szxga01-in.huawei.com (SkyGuard) with ESMTP id 4GD44h0D2YzZkgy; Mon, 28 Jun 2021 18:45:08 +0800 (CST) Received: from dggpemm500023.china.huawei.com (7.185.36.83) by dggemv704-chm.china.huawei.com (10.3.19.47) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2176.2; Mon, 28 Jun 2021 18:48:12 +0800 Received: from [10.174.187.128] (10.174.187.128) by dggpemm500023.china.huawei.com (7.185.36.83) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA256) id 15.1.2176.2; Mon, 28 Jun 2021 18:48:11 +0800 Subject: Re: [RFC PATCH v4 0/7] hw/arm/virt: Introduce cpu topology support To: Andrew Jones , Eduardo Habkost References: <7fcc5f2d-cc84-3464-15cc-3bebb07f8190@huawei.com> <20210622142915.pekttdvbi3q5vnh3@gator> <20210622174013.52422c73@redhat.com> <20210622172934.537l7e27sxd6car6@gator> <20210628085805.5y7bxvqprx75hwi4@gator> From: "wangyanan (Y)" Message-ID: <5c8c0f85-4d4f-61db-b1e8-adaae2b78106@huawei.com> Date: Mon, 28 Jun 2021 18:48:11 +0800 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:78.0) Gecko/20100101 Thunderbird/78.4.0 MIME-Version: 1.0 In-Reply-To: <20210628085805.5y7bxvqprx75hwi4@gator> Content-Type: text/plain; charset="utf-8"; format=flowed Content-Transfer-Encoding: 8bit Content-Language: en-US X-Originating-IP: [10.174.187.128] X-ClientProxiedBy: dggeme705-chm.china.huawei.com (10.1.199.101) To dggpemm500023.china.huawei.com (7.185.36.83) X-CFilter-Loop: Reflected Received-SPF: pass client-ip=45.249.212.187; envelope-from=wangyanan55@huawei.com; helo=szxga01-in.huawei.com X-Spam_score_int: -41 X-Spam_score: -4.2 X-Spam_bar: ---- X-Spam_report: (-4.2 / 5.0 requ) BAYES_00=-1.9, NICE_REPLY_A=-0.001, RCVD_IN_DNSWL_MED=-2.3, RCVD_IN_MSPIKE_H4=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Barry Song , Peter Maydell , =?UTF-8?Q?Daniel_P=2e_Berrang=c3=a9?= , Paolo Bonzini , "Michael S . Tsirkin" , qemu-devel@nongnu.org, Shannon Zhao , Igor Mammedov , qemu-arm@nongnu.org, Alistair Francis , prime.zeng@hisilicon.com, yangyicong@huawei.com, yuzenghui@huawei.com, wanghaibin.wang@huawei.com, zhukeqian1@huawei.com, David Gibson Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 2021/6/28 16:58, Andrew Jones wrote: > On Mon, Jun 28, 2021 at 04:43:05PM +0800, wangyanan (Y) wrote: >> Hi, >> On 2021/6/23 1:39, Daniel P. Berrangé wrote: >>> On Tue, Jun 22, 2021 at 07:29:34PM +0200, Andrew Jones wrote: >>>> On Tue, Jun 22, 2021 at 06:14:25PM +0100, Daniel P. Berrangé wrote: >>>>> On Tue, Jun 22, 2021 at 05:40:13PM +0200, Igor Mammedov wrote: >>>>>> On Tue, 22 Jun 2021 16:29:15 +0200 >>>>>> Andrew Jones wrote: >>>>>> >>>>>>> On Tue, Jun 22, 2021 at 03:10:57PM +0100, Daniel P. Berrangé wrote: >>>>>>>> On Tue, Jun 22, 2021 at 10:04:52PM +0800, wangyanan (Y) wrote: >>>>>>>>> Hi Daniel, >>>>>>>>> >>>>>>>>> On 2021/6/22 20:41, Daniel P. Berrangé wrote: >>>>>>>>>> On Tue, Jun 22, 2021 at 08:31:22PM +0800, wangyanan (Y) wrote: >>>>>>>>>>> On 2021/6/22 19:46, Andrew Jones wrote: >>>>>>>>>>>> On Tue, Jun 22, 2021 at 11:18:09AM +0100, Daniel P. Berrangé wrote: >>>>>>>>>>>>> On Tue, Jun 22, 2021 at 05:34:06PM +0800, Yanan Wang wrote: >>>>>>>>>>>>>> Hi, >>>>>>>>>>>>>> >>>>>>>>>>>>>> This is v4 of the series [1] that I posted to introduce support for >>>>>>>>>>>>>> generating cpu topology descriptions to guest. Comments are welcome! >>>>>>>>>>>>>> >>>>>>>>>>>>>> Description: >>>>>>>>>>>>>> Once the view of an accurate virtual cpu topology is provided to guest, >>>>>>>>>>>>>> with a well-designed vCPU pinning to the pCPU we may get a huge benefit, >>>>>>>>>>>>>> e.g., the scheduling performance improvement. See Dario Faggioli's >>>>>>>>>>>>>> research and the related performance tests in [2] for reference. So here >>>>>>>>>>>>>> we go, this patch series introduces cpu topology support for ARM platform. >>>>>>>>>>>>>> >>>>>>>>>>>>>> In this series, instead of quietly enforcing the support for the latest >>>>>>>>>>>>>> machine type, a new parameter "expose=on|off" in -smp command line is >>>>>>>>>>>>>> introduced to leave QEMU users a choice to decide whether to enable the >>>>>>>>>>>>>> feature or not. This will allow the feature to work on different machine >>>>>>>>>>>>>> types and also ideally compat with already in-use -smp command lines. >>>>>>>>>>>>>> Also we make much stricter requirement for the topology configuration >>>>>>>>>>>>>> with "expose=on". >>>>>>>>>>>>> Seeing this 'expose=on' parameter feels to me like we're adding a >>>>>>>>>>>>> "make-it-work=yes" parameter. IMHO this is just something that should >>>>>>>>>>>>> be done by default for the current machine type version and beyond. >>>>>>>>>>>>> I don't see the need for a parameter to turnthis on, especially since >>>>>>>>>>>>> it is being made architecture specific. >>>>>>>>>>>> I agree. >>>>>>>>>>>> >>>>>>>>>>>> Yanan, we never discussed an "expose" parameter in the previous versions >>>>>>>>>>>> of this series. We discussed a "strict" parameter though, which would >>>>>>>>>>>> allow existing command lines to "work" using assumptions of what the user >>>>>>>>>>>> meant and strict=on users to get what they mean or an error saying that >>>>>>>>>>>> they asked for something that won't work or would require unreasonable >>>>>>>>>>>> assumptions. Why was this changed to an "expose" parameter? >>>>>>>>>>> Yes, we indeed discuss a new "strict" parameter but not a "expose" in v2 [1] >>>>>>>>>>> of this series. >>>>>>>>>>> [1] https://patchwork.kernel.org/project/qemu-devel/patch/20210413080745.33004-6-wangyanan55@huawei.com/ >>>>>>>>>>> >>>>>>>>>>> And in the discussion, we hoped things would work like below with "strict" >>>>>>>>>>> parameter: >>>>>>>>>>> Users who want to describe cpu topology should provide cmdline like >>>>>>>>>>> >>>>>>>>>>> -smp strict=on,cpus=4,sockets=2,cores=2,threads=1 >>>>>>>>>>> >>>>>>>>>>> and in this case we require an more accurate -smp configuration and >>>>>>>>>>> then generate the cpu topology description through ACPI/DT. >>>>>>>>>>> >>>>>>>>>>> While without a strict description, no cpu topology description would >>>>>>>>>>> be generated, so they get nothing through ACPI/DT. >>>>>>>>>>> >>>>>>>>>>> It seems to me that the "strict" parameter actually serves as a knob to >>>>>>>>>>> turn on/off the exposure of topology, and this is the reason I changed >>>>>>>>>>> the name. >>>>>>>>>> Yes, the use of 'strict=on' is no better than expose=on IMHO. >>>>>>>>>> >>>>>>>>>> If I give QEMU a cli >>>>>>>>>> >>>>>>>>>> -smp cpus=4,sockets=2,cores=2,threads=1 >>>>>>>>>> >>>>>>>>>> then I expect that topology to be exposed to the guest. I shouldn't >>>>>>>>>> have to add extra flags to make that happen. >>>>>>>>>> >>>>>>>>>> Looking at the thread, it seems the concern was around the fact that >>>>>>>>>> the settings were not honoured historically and thus the CLI values >>>>>>>>>> could be garbage. ie -smp cpus=4,sockets=8,cores=3,thread=9 >>>>>>>>> This "-smp cpus=4,sockets=8,cores=3,threads=9" behaviors as a wrong >>>>>>>>> configuration, and the parsing function already report error for this case. >>>>>>>>> >>>>>>>>> We hope more complete config like "-smp 4,sockets=2,cores=2,threads=1" >>>>>>>>> for exposure of topology, and the incomplete ones like "-smp 4,sockets=1" >>>>>>>>> or "-smp 4, cores=1" are not acceptable any more because we are starting >>>>>>>>> to expose the topology. >>>>>>>> Incomplete specified topologies *are* acceptable. >>>>>>>> >>>>>>>> The smp_parse method will automatically fill in any missing values. >>>>>>>> >>>>>>>> ie, >>>>>>>> >>>>>>>> -smp 4,cores=1 >>>>>>>> -smp cores=1 >>>>>>>> -smp threads=1 >>>>>>>> -smp sockets=4 >>>>>>>> >>>>>>>> are all functionally identical to >>>>>>>> >>>>>>>> -smp 4,sockets=4,cores=1,dies=1,threads=1 >>>>>>>> >>>>>>>> >>>>>>>> The QEMU man page says this explicitly >>>>>>>> >>>>>>>> For the PC target, the number of cores per die, the >>>>>>>> number of threads per cores, the number of dies per packages and the >>>>>>>> total number of sockets can be specified. Missing values will be >>>>>>>> computed. If any on the three values is given, the total number of >>>>>>>> CPUs n can be omitted. >>>>>>> It doesn't say how it will compute them though, which for the default >>>>>>> smp_parse and for x86 is to prefer sockets over cores over threads. >>>>>>> That's not necessarily what the user expects. IMO, we need a 'strict=on' >>>>>>> parameter that doesn't allow any collection of smp parameters which >>>>>>> require unreasonable assumptions. Reasonable assumptions are threads=1, >>>>>>> when threads is not specified and the rest of the math adds up. Also, >>>>>>> maxcpus == cpus when maxcpus isn't specified is reasonable. But, it's not >>>>>>> as reasonable to decide how to divide cores among sockets or to assume >>>>>>> threads=1 when only sockets and cores are given. How do we know the user >>>>>>> didn't forget to specify threads if we can't check the math? >>>>>> or just outlaw all invalid topologies incl. incomplete by default >>>>>> (without requiring extra option), and permit them only for old machine >>>>>> types ()using compat machinery) without topo info provided to guest. >>>>>> And maybe later deprecate invalid topologies altogether. >>>>> This feels like it is creating pain for users to fix a problem that >>>>> isn't shown to actually be causing any common issues. >>>>> >>>>> We've supposed that users are having problems when forgetting to >>>>> specify "threads" and not having the compute value be desirable, >>>>> but where are the bug reports to back this up ? >>>>> >>>>> The partial topologies are valid and have well defined semantics. >>>>> Those semantics may not match everyone's preference, but that >>>>> doesn't make them invalid. >>>>> >>>> If we adopt the [undocumented] semantics of x86 for arm, then we may >>>> surprise some users that expect e.g. '-smp 16' to give them a single >>>> socket with 16 cores, because they'll start getting 16 sockets with 1 >>>> core each. That's because if we don't describe a topology to an arm linux >>>> guest then it assumes cores. Maybe we shouldn't worry about this, but I'd >>>> prefer we require explicit inputs from users and, if necessary, for them >>>> to explicitly opt-in to requiring those explicit inputs. >>> Even for x86, defaulting to maximising sockets over cores is sub-optimal. >>> In real world x86 hardware it is very rare to have sockets > 2 or 4. For >>> large CPU counts, you generally have large cores-per-socket counts on x86. >>> >>> The QEMU preference for sockets over cores on x86 (and PPC too IIUC) >>> is a fairly arbitrary historical decision. >>> >>> It can cause problems with guest OS licensing because both Windows >>> and RHEL have been known to charge differently for sockets vs cores, >>> with high core counts being cheaper. >>> >>> We are not tied into the precise behaviour of the computed topology >>> values, as we have no made any promises. All that's required is that >>> we keep ABI compat for existing machine types. >> If based on this point of view that we haven't made any promises for the >> precise behavior of the computed topology, things may get much easier. >> I have the following understanding (also a proposal): >> >> We will introduce the support for exposing cpu topology since machine >> type 6.2 and we will also describe the computed topology for the guest. >> We will not make any stricter parsing logic, however the -smp content in >> qemu-options.hx should be rearranged to clearly explain how the missing >> values will exactly be computed. And this is what QEMU is responsible for. >> >> We know that a well designed cpu topology configuration can gain much >> benefit for the guest, while a badly designed one will also probably cause >> negative impact. But the users should be responsible for the design of the >> -smp cmdlines. If they are using an incomplete cmdline for a 6.2 machine, >> then they should have known what the computed values will be and that >> the computed topology will be exposed to the guest. >>> So we could decide to change the computed topology so that it prefers >>> high core counts, over sockets, whem using new machine types only. >>> That would seem to benefit all arches, by making QEMU more reflective >>> of real world CPUs topology. >> If we really decide to prefer cores over sockets over threads for new >> machine >> types, then I think we should also record this change in qemu-option.hx. >> > I agree. The proposal sounds good to me. I'd like to hear Eduardo's > opinion too (CC'ed). Thanks, Drew. I also hope that more people will reach an agreement on this issue, so that I can prepare v5 with confidence, although it will probably be late for 6.1. Thanks, Yanan . > Thanks, > drew > > > .