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

[Andrew Jones]

On Tue, Jun 22, 2021 at 05:40:13PM +0200, Igor Mammedov wrote:
> On Tue, 22 Jun 2021 16:29:15 +0200
> Andrew Jones <drjones@redhat.com> 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. 

I like this proposal, but Peter generally does not want currently working
command lines to stop working. The 'virt' machine type always points to
the latest machine type, so '-M virt -smp sockets=2,cores=4' will
currently work (even though no topology is generated), but, if we were to
merge patches that outlawed that using the compat machinery for 6.2, then
it'll stop working, possibly breaking scripts. The 'strict' parameter
allows one to opt into the strict parsing and actually get the topology
described. Everyone else will still have working command lines without
topology no matter what they use. If Peter agrees to making the smp parser
strict from 6.2 on (possibly by issuing a warning for a release or two
instead of an error, like Daniel suggested), then that's indeed better.
If not, then I don't know how to get a useful -smp command line parser
without the additional parameter.

Thanks,
drew

> 
> 
> > 
> > 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-    
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> > >   
> > 
>