Re: [PATCH V5 4/7] driver/perf/arm_pmu_platform: Add support for BRBE attributes detection

[Mark Rutland <mark.rutland@arm.com>]

On Mon, Nov 21, 2022 at 12:06:31PM +0530, Anshuman Khandual wrote:
> 
> 
> On 11/18/22 23:31, Mark Rutland wrote:
> > On Mon, Nov 07, 2022 at 11:55:11AM +0530, Anshuman Khandual wrote:
> >> This adds arm pmu infrastrure to probe BRBE implementation's attributes via
> >> driver exported callbacks later. The actual BRBE feature detection will be
> >> added by the driver itself.
> >>
> >> CPU specific BRBE entries, cycle count, format support gets detected during
> >> PMU init. This information gets saved in per-cpu struct pmu_hw_events which
> >> later helps in operating BRBE during a perf event context.
> > 
> > Do we expect this to vary between CPUs handled by the same struct arm_pmu ?
> 
> BRBE registers are per CPU, and the spec does not assert about BRBE properties
> being the same across the system, served via same the struct arm_pmu.

The same is true of the PMU, and struct arm_pmu does not cover the whole
system, it covers each *micro-architecture* within the system.

I think BRBE should be treated the same, i.e. uniform *within* a struct
arm_pmu.

> Hence it would be inaccurate to make that assumption, which might have just
> avoided all these IPI based probes during boot.

FWIW, I would be happy to IPI all CPUs during boot to verify uniformity of CPUs
within an arm_pmu; I just don't think that BRBE should be treated differently
from the rest of the PMU features.

[...]

> >> +	hw_events = per_cpu_ptr(armpmu->hw_events, smp_processor_id());
> >> +	armpmu->brbe_probe(hw_events);
> >> +}
> >> +
> >> +static int armpmu_request_brbe(struct arm_pmu *armpmu)
> >> +{
> >> +	int cpu, err = 0;
> >> +
> >> +	for_each_cpu(cpu, &armpmu->supported_cpus) {
> >> +		err = smp_call_function_single(cpu, arm_brbe_probe_cpu, armpmu, 1);
> > 
> > Why does this need to be called on each CPU in the supported_cpus mask?
> 
> Is not supported_cpus derived after partitioning the IRQ in pmu_parse_percpu_irq().
> The idea is to fill up BRBE buffer attributes, on all such supported cpus which could
> trigger PMU interrupt. Is the concern, that not all cpus in supported_cpus mask might
> not be online during boot, hence IPIs could not be served, hence BRBE attributed for
> them could not be fetched ?

As above, I think this is solvable if we mandate that BRBE must be uniform
*within* an arm_pmu's supported CPUs; then we only need one CPU in the
supported_cpus mask to be present at boot time, as with the rest of the PMU
code.

We could *verify* that when onlining a CPU.

> > I don't see anything here to handle late hotplug, so this looks suspicious.
> 
> Right, I should add cpu hotplug handling, otherwise risk loosing BRBE support on cpus
> which might have been offline during boot i.e when above IPI based probe happened ?
> 
> > Either we're missing something, or it's redundant at boot time.
> 
> Should we add cpu hotplug online-offline handlers like some other PMU drivers ? Let
> me know if there are some other concerns.
> 
> cpuhp_setup_state_multi(CPUHP_AP_ONLINE_DYN, DRVNAME,
> 			arm_brbe_cpu_startup,
> 		        arm_brbe_cpu_teardown)

We *could* add that, but that's going to require ordering against the existing
hooks for probing arm_pmu.

Why can't this hang off the exising hooks for arm_pmu? We're treating this as
part of the PMU anyway, so I don't understand why we should probe it
separately.

Thanks,
Mark.

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