Re: [PATCH 01/32] perf: Allow a PMU to have a parent

[Mark Rutland]

On Tue, Jun 06, 2023 at 03:18:59PM +0200, Peter Zijlstra wrote:
> On Tue, Jun 06, 2023 at 02:06:24PM +0100, Mark Rutland wrote:
> > On Thu, Apr 06, 2023 at 09:49:38PM +0200, Peter Zijlstra wrote:
> > > On Thu, Apr 06, 2023 at 05:44:45PM +0100, Jonathan Cameron wrote:
> > > > On Thu, 6 Apr 2023 14:40:40 +0200
> > > > Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> > > > 
> > > > > On Thu, Apr 06, 2023 at 11:16:07AM +0100, Jonathan Cameron wrote:
> > > > > 
> > > > > > In the long run I agree it would be good.  Short term there are more instances of
> > > > > > struct pmu that don't have parents than those that do (even after this series).
> > > > > > We need to figure out what to do about those before adding checks on it being
> > > > > > set.  
> > > > > 
> > > > > Right, I don't think you've touched *any* of the x86 PMUs for example,
> > > > > and getting everybody that boots an x86 kernel a warning isn't going to
> > > > > go over well :-)
> > > > > 
> > > > 
> > > > It was tempting :) "Warning: Parentless PMU: try a different architecture."
> > > 
> > > Haha!
> > > 
> > > > I'd love some inputs on what the x86 PMU devices parents should be?
> > > > CPU counters in general tend to just spin out of deep in the architecture code.
> > > 
> > > For the 'simple' ones I suppose we can use the CPU device.
> > 
> > Uh, *which* CPU device? Do we have a container device for all CPUs?
> 
> drivers/base/cpu.c:per_cpu(cpu_sys_devices, cpu) for whatever the core
> pmu is for that cpu ?

... but the struct pmu covers several CPUs, so I don't have a single 'cpu', no?

If I have a system where cpu{0,1,2} are Cortex-A53 and cpu{3,4} are Cortex-A57,
I have two struct pmu instances, each associated with several CPUs. When I
probe each of those I determine a cpumask for each.

> > > > My overall favorite is an l2 cache related PMU that is spun up in
> > > > arch/arm/kernel/irq.c init_IRQ()
> > 
> > That's an artifact of the L2 cache controller driver getting initialized there;
> > ideally we'd have a device for the L2 cache itself (which presumably should
> > hang off an aggregate CPU device).
> 
> /sys/devices/system/cpu/cpuN/cache/indexM
> 
> has a struct device somewhere in
> drivers/base/cacheinfo.c:ci_index_dev or somesuch.

I guess, but I don't think the L2 cache controller (the PL310) is actually tied
to that today.

> > > Yeah, we're going to have a ton of them as well. Some of them are PCI
> > > devices and have a clear parent, others, not so much :/
> > 
> > In a number of places the only thing we have is the PMU driver, and we don't
> > have a driver (or device) for the HW block it's a part of. Largely that's
> > interconnect PMUs; we could create container devices there.
> 
> Dont they have a PCI device? But yeah, some are going to be a wee bit
> challenging.

The system might not even have PCI, so it's arguable that they should just hang
off an MMIO bus (which is effectively what the platofrm bus is).

Thanks,
Mark.