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([2804:431:c7c6:6b49:9e31:c2b4:54cb:eeb6]) by smtp.gmail.com with ESMTPSA id y14sm11261238qtw.70.2021.03.29.11.32.38 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 29 Mar 2021 11:32:40 -0700 (PDT) Subject: Re: [PATCH 1/2] spapr: number of SMP sockets must be equal to NUMA nodes To: =?UTF-8?Q?C=c3=a9dric_Le_Goater?= , David Gibson References: <20210319183453.4466-1-danielhb413@gmail.com> <20210319183453.4466-2-danielhb413@gmail.com> <2025f26f-5883-4e86-02af-5b83a8d52465@gmail.com> <9870aaba-9921-5c5d-113c-5be6cd098cf2@kaod.org> From: Daniel Henrique Barboza Message-ID: <91e406bf-c9c6-0734-1f69-081d3633332b@gmail.com> Date: Mon, 29 Mar 2021 15:32:37 -0300 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.8.1 MIME-Version: 1.0 In-Reply-To: <9870aaba-9921-5c5d-113c-5be6cd098cf2@kaod.org> Content-Type: text/plain; charset=windows-1252; format=flowed Content-Language: en-US Content-Transfer-Encoding: 8bit Received-SPF: pass client-ip=2607:f8b0:4864:20::f2a; envelope-from=danielhb413@gmail.com; helo=mail-qv1-xf2a.google.com X-Spam_score_int: -18 X-Spam_score: -1.9 X-Spam_bar: - X-Spam_report: (-1.9 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, FREEMAIL_ENVFROM_END_DIGIT=0.25, FREEMAIL_FROM=0.001, NICE_REPLY_A=-0.001, RCVD_IN_DNSWL_NONE=-0.0001, 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: Laurent Vivier , Thomas Huth , Srikar Dronamraju , Michael Ellerman , qemu-devel@nongnu.org, groug@kaod.org, qemu-ppc@nongnu.org, Igor Mammedov Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 3/29/21 12:32 PM, Cédric Le Goater wrote: > On 3/29/21 6:20 AM, David Gibson wrote: >> On Thu, Mar 25, 2021 at 09:56:04AM +0100, Cédric Le Goater wrote: >>> On 3/25/21 3:10 AM, David Gibson wrote: >>>> On Tue, Mar 23, 2021 at 02:21:33PM -0300, Daniel Henrique Barboza wrote: >>>>> >>>>> >>>>> On 3/22/21 10:03 PM, David Gibson wrote: >>>>>> On Fri, Mar 19, 2021 at 03:34:52PM -0300, Daniel Henrique Barboza wrote: >>>>>>> Kernel commit 4bce545903fa ("powerpc/topology: Update >>>>>>> topology_core_cpumask") cause a regression in the pseries machine when >>>>>>> defining certain SMP topologies [1]. The reasoning behind the change is >>>>>>> explained in kernel commit 4ca234a9cbd7 ("powerpc/smp: Stop updating >>>>>>> cpu_core_mask"). In short, cpu_core_mask logic was causing troubles with >>>>>>> large VMs with lots of CPUs and was changed by cpu_cpu_mask because, as >>>>>>> far as the kernel understanding of SMP topologies goes, both masks are >>>>>>> equivalent. >>>>>>> >>>>>>> Further discussions in the kernel mailing list [2] shown that the >>>>>>> powerpc kernel always considered that the number of sockets were equal >>>>>>> to the number of NUMA nodes. The claim is that it doesn't make sense, >>>>>>> for Power hardware at least, 2+ sockets being in the same NUMA node. The >>>>>>> immediate conclusion is that all SMP topologies the pseries machine were >>>>>>> supplying to the kernel, with more than one socket in the same NUMA node >>>>>>> as in [1], happened to be correctly represented in the kernel by >>>>>>> accident during all these years. >>>>>>> >>>>>>> There's a case to be made for virtual topologies being detached from >>>>>>> hardware constraints, allowing maximum flexibility to users. At the same >>>>>>> time, this freedom can't result in unrealistic hardware representations >>>>>>> being emulated. If the real hardware and the pseries kernel don't >>>>>>> support multiple chips/sockets in the same NUMA node, neither should we. >>>>>>> >>>>>>> Starting in 6.0.0, all sockets must match an unique NUMA node in the >>>>>>> pseries machine. qtest changes were made to adapt to this new >>>>>>> condition. >>>>>> >>>>>> Oof. I really don't like this idea. It means a bunch of fiddly work >>>>>> for users to match these up, for no real gain. I'm also concerned >>>>>> that this will require follow on changes in libvirt to not make this a >>>>>> really cryptic and irritating point of failure. >>>>> >>>>> Haven't though about required Libvirt changes, although I can say that there >>>>> will be some amount to be mande and it will probably annoy existing users >>>>> (everyone that has a multiple socket per NUMA node topology). >>>>> >>>>> There is not much we can do from the QEMU layer aside from what I've proposed >>>>> here. The other alternative is to keep interacting with the kernel folks to >>>>> see if there is a way to keep our use case untouched. >>>> >>>> Right. Well.. not necessarily untouched, but I'm hoping for more >>>> replies from Cédric to my objections and mpe's. Even with sockets >>>> being a kinda meaningless concept in PAPR, I don't think tying it to >>>> NUMA nodes makes sense. >>> >>> I did a couple of replies in different email threads but maybe not >>> to all. I felt it was going nowhere :/ Couple of thoughts, >> >> I think I saw some of those, but maybe not all. >> >>> Shouldn't we get rid of the socket concept, die also, under pseries >>> since they don't exist under PAPR ? We only have numa nodes, cores, >>> threads AFAICT. >> >> Theoretically, yes. I'm not sure it's really practical, though, since >> AFAICT, both qemu and the kernel have the notion of sockets (though >> not dies) built into generic code. > > Yes. But, AFAICT, these topology notions have not reached "arch/powerpc" > and PPC Linux only has a NUMA node id, on pseries and powernv. > >> It does mean that one possible approach here - maybe the best one - is >> to simply declare that sockets are meaningless under, so we simply >> don't expect what the guest kernel reports to match what's given to >> qemu. >> >> It'd be nice to avoid that if we can: in a sense it's just cosmetic, >> but it is likely to surprise and confuse people. >> >>> Should we diverged from PAPR and add extra DT properties "qemu,..." ? >>> There are a couple of places where Linux checks for the underlying >>> hypervisor already. >>> >>>>> This also means that >>>>> 'ibm,chip-id' will probably remain in use since it's the only place where >>>>> we inform cores per socket information to the kernel. >>>> >>>> Well.. unless we can find some other sensible way to convey that >>>> information. I haven't given up hope for that yet. >>> >>> Well, we could start by fixing the value in QEMU. It is broken >>> today. >> >> Fixing what value, exactly? > > The value of the "ibm,chip-id" since we are keeping the property under > QEMU. David, I believe this has to do with the discussing we had last Friday. I mentioned that the ibm,chip-id property is being calculated in a way that promotes the same ibm,chip-id in CPUs that belongs to different NUMA nodes, e.g.: -smp 4,cores=4,maxcpus=8,threads=1 \ -numa node,nodeid=0,cpus=0-1,cpus=4-5,memdev=ram-node0 \ -numa node,nodeid=1,cpus=2-3,cpus=6-7,memdev=ram-node1 $ dtc -I dtb -O dts fdt.dtb | grep -B2 ibm,chip-id ibm,associativity = <0x05 0x00 0x00 0x00 0x00 0x00>; ibm,pft-size = <0x00 0x19>; ibm,chip-id = <0x00>; -- ibm,associativity = <0x05 0x00 0x00 0x00 0x00 0x01>; ibm,pft-size = <0x00 0x19>; ibm,chip-id = <0x00>; -- ibm,associativity = <0x05 0x01 0x01 0x01 0x01 0x02>; ibm,pft-size = <0x00 0x19>; ibm,chip-id = <0x00>; -- ibm,associativity = <0x05 0x01 0x01 0x01 0x01 0x03>; ibm,pft-size = <0x00 0x19>; ibm,chip-id = <0x00>; We assign ibm,chip-id=0x0 to CPUs 0-3, but CPUs 2-3 are located in a different NUMA node than 0-1. This would mean that the same socket would belong to different NUMA nodes at the same time. I believe this is what Cedric wants to be addressed. Given that the property is called after the OPAL property ibm,chip-id, the kernel expects that the property will have the same semantics as in OPAL. Thanks, DHB > >>> This is all coming from some work we did last year to evaluate our HW >>> (mostly for XIVE) on 2s, 4s, 16s systems on baremetal, KVM and PowerVM. >>> We saw some real problems because Linux did not have a clear view of the >>> topology. See the figures here : >>> >>> http://patchwork.ozlabs.org/project/linuxppc-dev/patch/20210303174857.1760393-9-clg@kaod.org/ >>> >>> The node id is a key parameter for system resource management, memory >>> allocation, interrupt affinity, etc. Linux scales much better if used >>> correctly. >> >> Well, sure. And we have all the ibm,associativity stuff to convey the >> node ids to the guest (which has its own problems, but not that are >> relevant here). What's throwing me is why getting node IDs correct >> has anything to do with socket numbers. > >