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

[Andrew Jones <drjones@redhat.com>]

On Wed, Jun 30, 2021 at 05:37:42PM +0800, wangyanan (Y) wrote:
> On 2021/6/30 16:30, Andrew Jones wrote:
> > On Wed, Jun 30, 2021 at 02:36:31PM +0800, wangyanan (Y) wrote:
> > > Hi Drew, Igor,
> > > 
> > > I have a question below, hope for some explanation... :)
> > > 
> > > I'm trying to rearrange the smp_parse() helper to make it more scalable.
> > > But I wonder why we are currently using maxcpus to calculate the missing
> > > sockets while using *cpus* to calculate the missing cores and threads?
> > > 
> > > This makes the following cmdlines work fine,
> > > -smp cpus=8,maxcpus=12  <==>  -smp
> > > cpus=8,maxcpus=12,sockets=12,cores=1,threads=1
> > > -smp cpus=8,maxcpus=12,cores=6  <==>  -smp
> > > cpus=8,maxcpus=12,sockets=2,cores=6,threads=1
> > > -smp cpus=8,maxcpus=12,threads=2  <==>  -smp
> > > cpus=8,maxcpus=12,sockets=6,cores=1,threads=2
> > > 
> > > but the following ones break the invalid CPU topology check:
> > > -smp cpus=8,maxcpus=12,sockets=2  <==>  -smp
> > > cpus=8,maxcpus=12,sockets=2,cores=4,threads=1
> > > -smp cpus=8,maxcpus=12,sockets=4,threads=1  <==>  -smp
> > > cpus=8,maxcpus=12,sockets=4,cores=2,threads=1
> > > -smp maxcpus=12  <==>  -smp cpus=1,maxcpus=12,sockets=1,cores=1,threads=1
> > > -smp maxcpus=12,sockets=2  <==>  -smp
> > > cpus=2,maxcpus=12,sockets=2,cores=1,threads=1
> > > 
> > > IMO we should uniformly use maxcpus to calculate the missing sockets
> > > also cores and threads, which will allow all the above cmdlines work.
> > > Or maybe I missed something? I read the related discussion in [1] but
> > > didn't get an unambiguous conclusion.
> > > 
> > > [1] https://patchwork.kernel.org/project/qemu-devel/patch/1535553121-80352-1-git-send-email-imammedo@redhat.com/
> > I agree that maxcpus should be used for all calculations.
> Thanks. From my view uniformly using maxcpus to calculate the missing
> values won't break any existing working cmdlines, but will allow some now
> being invalid and incomplete cmdlines to be valid. I will use maxcpus and
> test the parser for all possible parameter collections.
> > I think we need
> > to write -smp parsing from scratch using a set of clean requirements and
> > then use the machine compat stuff to switch to it. And also properly
> > document it with something like "Since 6.2..."
> I agree to rewrite the -smp parsing. But what's the meaning of clean
> requirements?
> Sorry I didn't get it.

I think -smp evolved without all the details considered up front. Now that
we've considered the details/requirements more completely, then let's
apply our knowledge of them to an implementation that gets them all
covered. Here's a quick list from the top of my head, there might be
some missing 

 - maxcpus should be used for computation of missing values
 - we should assume cores over sockets over threads
 - we should allow extending the topology with arch-specific
   members, such as dies, which will always default to 1 when
   not provided, rather than be computed
 - we should store the results in the smp machine properties

Thanks,
drew

> 
> Thanks,
> Yanan
> .
> > 
> > > Regards,
> > > Yanan
> > > .
> > > 
> > > On 2021/6/28 16:58, Andrew Jones wrote:
> > > > On Mon, Jun 28, 2021 at 04:43:05PM +0800, wangyanan (Y) wrote:
> > > > > Hi,
> > > > > On 2021/6/23 1:39, Daniel P. Berrangé wrote:
> > > > > > 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.
> > > > > If based on this point of view that we haven't made any promises for the
> > > > > precise behavior of the computed topology, things may get much easier.
> > > > > I have the following understanding (also a proposal):
> > > > > 
> > > > > We will introduce the support for exposing cpu topology since machine
> > > > > type 6.2 and we will also describe the computed topology for the guest.
> > > > > We will not make any stricter parsing logic, however the -smp content in
> > > > > qemu-options.hx should be rearranged to clearly explain how the missing
> > > > > values will exactly be computed. And this is what QEMU is responsible for.
> > > > > 
> > > > > We know that a well designed cpu topology configuration can gain much
> > > > > benefit for the guest, while a badly designed one will also probably cause
> > > > > negative impact. But the users should be responsible for the design of the
> > > > > -smp cmdlines. If they are using an incomplete cmdline for a 6.2 machine,
> > > > > then they should have known what the computed values will be and that
> > > > > the computed topology will be exposed to the guest.
> > > > > > 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.
> > > > > If we really decide to prefer cores over sockets over threads for new
> > > > > machine
> > > > > types, then I think we should also record this change in qemu-option.hx.
> > > > > 
> > > > I agree. The proposal sounds good to me. I'd like to hear Eduardo's
> > > > opinion too (CC'ed).
> > > > 
> > > > Thanks,
> > > > drew
> > > > 
> > > > 
> > > > .
> > 
> > .
>