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

[wangyanan (Y)]

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.
> A similar problem existed on x86 platforms. When we made that stricter
> we had cde that issued a warning for a few releases, essentially
> deprecating the config. EVentually it was turned into a fatal error.
> This gave applications time to fix their broken configs, while having
> correct configs "just work".
I understand this solution. Stop exposing topology for unqualified -smp
config and report a warning message at the transitional phase, and finally
incur an error for them.

BTW, just want to be sure, it this a common method in QEMU development
to solve this kind of compatibility issues?
> I'd suggest doing the same for arm. If the -smp args are semantically
> valid then expose the topology automatically (for new machine type).
> If the -smp args are semantically broken, then issue a warning. In
> a few releases time, turn this warning into an error.
So this topology feature will only work for the current machine type and
the following versions, right?

Thanks,
Yanan
.
> Regards,
> Daniel