Re: [RFC PATCH v4 0/7] hw/arm/virt: Introduce cpu topology support

[Andrew Jones]

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?

Thanks,
drew

> 
> note this qemu-options.hx doc will require updating since it will apply
> to more than just the PC target.
> 
> Regards,
> Daniel
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