Messages in this thread | | | Date | Mon, 21 Nov 2022 12:06:31 +0530 | Subject | Re: [PATCH V5 4/7] driver/perf/arm_pmu_platform: Add support for BRBE attributes detection | From | Anshuman Khandual <> |
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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. Hence it would be inaccurate to make that assumption, which might have just avoided all these IPI based probes during boot.
> >> Cc: Will Deacon <will@kernel.org> >> Cc: Mark Rutland <mark.rutland@arm.com> >> Cc: linux-arm-kernel@lists.infradead.org >> Cc: linux-kernel@vger.kernel.org >> Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com> >> --- >> drivers/perf/arm_pmu_platform.c | 34 +++++++++++++++++++++++++++++++++ >> 1 file changed, 34 insertions(+) >> >> diff --git a/drivers/perf/arm_pmu_platform.c b/drivers/perf/arm_pmu_platform.c >> index 933b96e243b8..acdc445081aa 100644 >> --- a/drivers/perf/arm_pmu_platform.c >> +++ b/drivers/perf/arm_pmu_platform.c >> @@ -172,6 +172,36 @@ static int armpmu_request_irqs(struct arm_pmu *armpmu) >> return err; >> } >> >> +static void arm_brbe_probe_cpu(void *info) >> +{ >> + struct pmu_hw_events *hw_events; >> + struct arm_pmu *armpmu = info; >> + >> + /* >> + * Return from here, if BRBE driver has not been >> + * implemented for this PMU. This helps prevent >> + * kernel crash later when brbe_probe() will be >> + * called on the PMU. >> + */ >> + if (!armpmu->brbe_probe) >> + return; > > Since this is a field on struct arm_pmu, why doesn't armpmu_request_brbe() > check this before calling smp_call_function_single(), to avoid the redundant > IPI?
Makes sense, I will move the check inside armpmu_request_brbe() with return code -ENODEV when not available.
> >> + >> + 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 ?
> > 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)
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