Re: [RFC PATCH 3/3] KVM: selftests: Add vgic initialization for dirty log perf test for ARM

[Jing Zhang <jingzhangos@google.com>]

On Wed, Jan 12, 2022 at 3:37 AM Marc Zyngier <maz@kernel.org> wrote:
>
> On Tue, 11 Jan 2022 22:16:01 +0000,
> Jing Zhang <jingzhangos@google.com> wrote:
> >
> > On Tue, Jan 11, 2022 at 2:30 AM Marc Zyngier <maz@kernel.org> wrote:
> > >
> > > On Mon, 10 Jan 2022 21:04:41 +0000,
> > > Jing Zhang <jingzhangos@google.com> wrote:
> > > >
> > > > For ARM64, if no vgic is setup before the dirty log perf test, the
> > > > userspace irqchip would be used, which would affect the dirty log perf
> > > > test result.
> > >
> > > Doesn't it affect *all* performance tests? How much does this change
> > > contributes to the performance numbers you give in the cover letter?
> > >
> > This bottleneck showed up after adding the fast path patch. I didn't
> > try other performance tests with this, but I think it is a good idea
> > to add a vgic setup for all performance tests. I can post another
> > patch later to make it available for all performance tests after
> > finishing this one and verifying all other performance tests.
> > Below is the test result without adding the vgic setup. It shows
> > 20~30% improvement for the different number of vCPUs.
> > +-------+------------------------+
> >     | #vCPU | dirty memory time (ms) |
> >     +-------+------------------------+
> >     | 1     | 965                    |
> >     +-------+------------------------+
> >     | 2     | 1006                    |
> >     +-------+------------------------+
> >     | 4     | 1128                    |
> >     +-------+------------------------+
> >     | 8     | 2005                   |
> >     +-------+------------------------+
> >     | 16    | 3903                   |
> >     +-------+------------------------+
> >     | 32    | 7595                   |
> >     +-------+------------------------+
> >     | 64    | 15783                  |
> >     +-------+------------------------+
>
> So please use these numbers in your cover letter when you repost your
> series, as the improvement you'd observe on actual workloads is likely
> to be less than what you claim due to this change in the test itself
> (in other words, if you are going to benchamark something, don't
> change the benchmark halfway).
Sure. Will clarify this in the cover letter in future posts.
Thanks,
Jing
>
>         M.
>
> --
> Without deviation from the norm, progress is not possible.