From: Andrew Jones <email@example.com> To: "Daniel P. Berrangé" <firstname.lastname@example.org> Cc: Barry Song <email@example.com>, Peter Maydell <firstname.lastname@example.org>, "Michael S . Tsirkin" <email@example.com>, firstname.lastname@example.org, email@example.com, firstname.lastname@example.org, "wangyanan \(Y\)" <email@example.com>, Shannon Zhao <firstname.lastname@example.org>, email@example.com, Alistair Francis <firstname.lastname@example.org>, email@example.com, Paolo Bonzini <firstname.lastname@example.org>, email@example.com, Igor Mammedov <firstname.lastname@example.org>, email@example.com, David Gibson <firstname.lastname@example.org> Subject: Re: [RFC PATCH v4 0/7] hw/arm/virt: Introduce cpu topology support Date: Tue, 22 Jun 2021 16:29:15 +0200 [thread overview] Message-ID: <20210622142915.pekttdvbi3q5vnh3@gator> (raw) In-Reply-To: <YNHvcQAMLSpVcxaE@redhat.com> 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  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  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  > > > > of this series. > > > >  https://email@example.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? Thanks, drew > > note this qemu-options.hx doc will require updating since it will apply > to more than just the PC target. > > Regards, > Daniel > -- > |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| > |: https://libvirt.org -o- https://fstop138.berrange.com :| > |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :| >
next prev parent reply other threads:[~2021-06-22 14:30 UTC|newest] Thread overview: 34+ messages / expand[flat|nested] mbox.gz Atom feed top 2021-06-22 9:34 Yanan Wang 2021-06-22 9:34 ` [RFC PATCH v4 1/7] vl: Add expose=on|off option support in -smp command line Yanan Wang 2021-06-22 9:34 ` [RFC PATCH v4 2/7] hw/arm/virt: Add separate -smp parsing function for ARM machines Yanan Wang 2021-06-22 9:34 ` [RFC PATCH v4 3/7] machine: disallow -smp expose=on for non-ARM machines Yanan Wang 2021-06-22 9:34 ` [RFC PATCH v4 4/7] device_tree: Add qemu_fdt_add_path Yanan Wang 2021-06-22 9:34 ` [RFC PATCH v4 5/7] hw/arm/virt: Add cpu-map to device tree Yanan Wang 2021-06-22 9:34 ` [RFC PATCH v4 6/7] hw/acpi/aml-build: Add Processor hierarchy node structure Yanan Wang 2021-06-22 9:34 ` [RFC PATCH v4 7/7] hw/acpi/aml-build: Generate PPTT table Yanan Wang 2021-06-22 10:18 ` [RFC PATCH v4 0/7] hw/arm/virt: Introduce cpu topology support Daniel P. Berrangé 2021-06-22 11:46 ` Andrew Jones 2021-06-22 12:31 ` wangyanan (Y) 2021-06-22 12:41 ` Daniel P. Berrangé 2021-06-22 14:04 ` wangyanan (Y) 2021-06-22 14:10 ` Daniel P. Berrangé 2021-06-22 14:15 ` Peter Maydell 2021-06-22 14:28 ` Daniel P. Berrangé 2021-06-28 11:14 ` wangyanan (Y) 2021-06-28 11:31 ` Daniel P. Berrangé 2021-06-28 11:53 ` wangyanan (Y) 2021-06-22 14:29 ` Andrew Jones [this message] 2021-06-22 15:15 ` Daniel P. Berrangé 2021-06-22 15:40 ` Igor Mammedov 2021-06-22 17:08 ` Andrew Jones 2021-06-22 17:14 ` Daniel P. Berrangé 2021-06-22 17:29 ` Andrew Jones 2021-06-22 17:39 ` Daniel P. Berrangé 2021-06-28 8:43 ` wangyanan (Y) 2021-06-28 8:58 ` Andrew Jones 2021-06-28 10:48 ` wangyanan (Y) 2021-06-30 6:36 ` wangyanan (Y) 2021-06-30 8:30 ` Andrew Jones 2021-06-30 9:37 ` wangyanan (Y) 2021-06-30 11:56 ` Andrew Jones 2021-07-01 6:15 ` wangyanan (Y)
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