Re: [PATCH v4 3/4] perf/core: Remove pmu linear searching code

[Marc Zyngier <maz@kernel.org>]

On Sat, 27 May 2023 18:00:13 +0100,
Ian Rogers <irogers@google.com> wrote:
> 
> On Sat, May 27, 2023 at 6:32 AM Marc Zyngier <maz@kernel.org> wrote:
> >
> > On Sat, 27 May 2023 00:00:47 +0100,
> > Ian Rogers <irogers@google.com> wrote:
> > >
> > > On Thu, May 25, 2023 at 8:56 AM Oliver Upton <oliver.upton@linux.dev> wrote:
> > > >
> > > > On Thu, May 25, 2023 at 04:20:31PM +0200, Peter Zijlstra wrote:
> > > > > On Thu, May 25, 2023 at 07:11:41AM +0000, Oliver Upton wrote:
> > > > >
> > > > > > The PMUv3 driver does pass a name, but it relies on getting back an
> > > > > > allocated pmu id as @type is -1 in the call to perf_pmu_register().
> > > > > >
> > > > > > What actually broke is how KVM probes for a default core PMU to use for
> > > > > > a guest. kvm_pmu_probe_armpmu() creates a counter w/ PERF_TYPE_RAW and
> > > > > > reads the pmu from the returned perf_event. The linear search had the
> > > > > > effect of eventually stumbling on the correct core PMU and succeeding.
> > > > > >
> > > > > > Perf folks: is this WAI for heterogenous systems?
> > > > >
> > > > > TBH, I'm not sure. hetero and virt don't mix very well AFAIK and I'm not
> > > > > sure what ARM64 does here.
> > > > >
> > > > > IIRC the only way is to hard affine things; that is, force vCPU of
> > > > > 'type' to the pCPU mask of 'type' CPUs.
> > > >
> > > > We provide absolutely no illusion of consistency across implementations.
> > > > Userspace can select the PMU type, and then it is a userspace problem
> > > > affining vCPUs to the right pCPUs.
> > > >
> > > > And if they get that wrong, we just bail and refuse to run the vCPU.
> > > >
> > > > > If you don't do that; or let userspace 'override' that, things go
> > > > > sideways *real* fast.
> > > >
> > > > Oh yeah, and I wish PMUs were the only problem with these hetero
> > > > systems...
> > >
> > > Just to add some context from what I understand. There are inbuilt
> > > type numbers for PMUs:
> > > https://git.kernel.org/pub/scm/linux/kernel/git/acme/linux.git/tree/include/uapi/linux/perf_event.h?h=perf-tools-next#n34
> > > so the PMU generally called /sys/devices/cpu should have type 4 (ARM
> > > give it another name). For heterogeneous ARM there is a single PMU and
> > > the same events are programmed regardless of whether it is a big or a
> > > little core - the cpumask lists all CPUs.
> >
> > I think you misunderstood the way heterogeneous arm64 systems are
> > described . Each CPU type gets its own PMU type, and its own event
> > list. Case in point:
> >
> > $ grep . /sys/devices/*pmu/{type,cpus}
> > /sys/devices/apple_avalanche_pmu/type:9
> > /sys/devices/apple_blizzard_pmu/type:8
> > /sys/devices/apple_avalanche_pmu/cpus:4-9
> > /sys/devices/apple_blizzard_pmu/cpus:0-3
> >
> > Type 4 (aka PERF_EVENT_RAW) is AFAICT just a way to encode the raw
> > event number, nothing else.
> 
> Which PMU will a raw event open on?

On the PMU that matches the current CPU.

> Note, the raw events don't support
> the extended type that is present in PERF_TYPE_HARDWARE and
> PERF_TYPE_HW_CACHE:
> https://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git/tree/include/uapi/linux/perf_event.h#n41
> as the bits are already in use for being just plain config values.

I'm not sure how relevant this is to the numbering of PMUs on arm64.

> I suspect not being type 4 is a bug on apple ARM here.

If that's a bug on this machine, it's a bug on all machines, at which
point it is the de-facto API:

$ grep . /sys/devices/armv8*/{type,cpus}
/sys/devices/armv8_cortex_a53/type:8
/sys/devices/armv8_cortex_a72/type:9
/sys/devices/armv8_cortex_a53/cpus:0-3
/sys/devices/armv8_cortex_a72/cpus:4-5

See, non-Apple HW. And now for a system with homogeneous CPUs:

$ grep . /sys/devices/armv8*/{type,cpus}
/sys/devices/armv8_pmuv3_0/type:8
/sys/devices/armv8_pmuv3_0/cpus:0-159

Still no type 4. I could go on for hours, I have plenty of HW around
me!

So whatever your source of information is, it doesn't match reality.
Our PMUs are numbered arbitrarily, and have been so for... a very long
time. At least since perf_pmu_register has supported dynamic
registration (see 2e80a82a49c4c).

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.