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

[Daniel P . Berrangé]

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 <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.
> > 
> > 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.

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.

Regards,
Daniel
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