Subject: [RFC PATCH 5/5] arm: perf: Use FIQ to handle PMU events.

From: [email-masked] (Daniel Thompson)
Date: Tue, 20 Jan 2015 10:18:10 +0000

On 19/01/15 16:35, Joshua Clayton wrote:
> On Tuesday, January 13, 2015 04:35:31 PM Daniel Thompson wrote:
>> Using FIQ (if it is available) gives perf a better insight into the
>> system by allowing code run with interrupts disabled to be profiled.
>>
>> Signed-off-by: Daniel Thompson <[email-masked]>
>> ---
>>  arch/arm/include/asm/pmu.h       |  4 ++++
>>  arch/arm/kernel/perf_event.c     |  2 +-
>>  arch/arm/kernel/perf_event_cpu.c | 35 ++++++++++++++++++++++++++++++++---
>>  arch/arm/kernel/traps.c          |  3 ++-
>>  4 files changed, 39 insertions(+), 5 deletions(-)
>>
>> diff --git a/arch/arm/include/asm/pmu.h b/arch/arm/include/asm/pmu.h
>> index b1596bd59129..2a7ea97a4a14 100644
>> --- a/arch/arm/include/asm/pmu.h
>> +++ b/arch/arm/include/asm/pmu.h
>> @@ -123,6 +123,8 @@ struct arm_pmu {
>>
>>  extern const struct dev_pm_ops armpmu_dev_pm_ops;
>>
>> +irqreturn_t armpmu_dispatch_irq(int irq, void *dev);
>> +
>>  int armpmu_register(struct arm_pmu *armpmu, int type);
>>
>>  u64 armpmu_event_update(struct perf_event *event);
>> @@ -136,6 +138,8 @@ int armpmu_map_event(struct perf_event *event,
>>  						[PERF_COUNT_HW_CACHE_RESULT_MAX],
>>  		     u32 raw_event_mask);
>>
>> +void cpu_pmu_handle_fiq(int irq);
>> +
>>  struct pmu_probe_info {
>>  	unsigned int cpuid;
>>  	unsigned int mask;
>> diff --git a/arch/arm/kernel/perf_event.c b/arch/arm/kernel/perf_event.c
>> index f7c65adaa428..5ae9adf7f18e 100644
>> --- a/arch/arm/kernel/perf_event.c
>> +++ b/arch/arm/kernel/perf_event.c
>> @@ -296,7 +296,7 @@ validate_group(struct perf_event *event)
>>  	return 0;
>>  }
>>
>> -static irqreturn_t armpmu_dispatch_irq(int irq, void *dev)
>> +irqreturn_t armpmu_dispatch_irq(int irq, void *dev)
>>  {
>>  	struct arm_pmu *armpmu;
>>  	struct platform_device *plat_device;
>> diff --git a/arch/arm/kernel/perf_event_cpu.c
>> b/arch/arm/kernel/perf_event_cpu.c index a80309087a7b..5c4e9ce23389 100644
>> --- a/arch/arm/kernel/perf_event_cpu.c
>> +++ b/arch/arm/kernel/perf_event_cpu.c
>> @@ -36,6 +36,9 @@
>>  /* Set at runtime when we know what CPU type we are. */
>>  static struct arm_pmu *cpu_pmu;
>>
>> +/* Allows us to find out if an IRQ is for us (mostly used from NMI context)
>> */ +static DEFINE_PER_CPU(int, cpu_pmu_irqs);
>> +
>>  /*
>>   * Despite the names, these two functions are CPU-specific and are used
>>   * by the OProfile/perf code.
>> @@ -127,6 +130,24 @@ static void cpu_pmu_free_irq(struct arm_pmu *cpu_pmu)
>>  	}
>>  }
>>
>> +/*
>> + * This handler is called *unconditionally* from the default NMI/FIQ
>> + * handler. The irq may not be anything to do with us so the main
>> + * job of this function is to figure out if the irq passed in is ours
>> + * or not.
>> + */
> 
> This comment is an indicator that all the code in cpu_pmu_handle_fiq is
> in the wrong place.
> It (or something like it) belongs at the level of the default 
> FIQ handler, and not in perf_event_cpu.c

I'm not sure about that.

If we moved this code into the default FIQ handler that means the PMU
driver would have to explicitly share its irq value with the default FIQ
handler (which doesn't really care about that).

I'm inclined to view this code as the effect of avoiding indirection in
the default FIQ handler.

Regular irq code has nothing like this because &armpmu_dispatch_irq, irq
and get_cpu_ptr(&cpu_pmu->hw_events->percpu_pmu) would all be looked up
from the irqaction and any unwanted events are naturally filtered by the
irq dispatch.

After your review I'm very tempted to put together a patch that
dispatches NMIs indirectly from the default FIQ handler. However I still
don't have much of an answer to Russell's concerns about code review.

>> +void cpu_pmu_handle_fiq(int irq)
>> +{
>> +	int cpu = smp_processor_id();
>> +
>> +	if (irq != get_cpu_var(cpu_pmu_irqs))
>> +		return;
>> +
>> +	(void)armpmu_dispatch_irq(irq,
>> +				  get_cpu_ptr(&cpu_pmu->hw_events->percpu_pmu));
>> +}
>> +
>> +
>>  static int cpu_pmu_request_irq(struct arm_pmu *cpu_pmu, irq_handler_t
>> handler) {
>>  	int i, err, irq, irqs;
>> @@ -170,9 +191,16 @@ static int cpu_pmu_request_irq(struct arm_pmu *cpu_pmu,
>> irq_handler_t handler) continue;
>>  			}
>>
>> -			err = request_irq(irq, handler,
>> -					  IRQF_NOBALANCING | IRQF_NO_THREAD, "arm-pmu",
>> -					  per_cpu_ptr(&hw_events->percpu_pmu, i));
>> +			err = request_nmi_irq(
>> +			    irq, IRQF_NOBALANCING, "arm-pmu",
>> +			    per_cpu_ptr(&hw_events->percpu_pmu, i));
>> +			if (err) {
>> +				err = request_irq(
>> +				    irq, handler,
>> +				    IRQF_NOBALANCING | IRQF_NO_THREAD,
>> +				    "arm-pmu",
>> +				    per_cpu_ptr(&hw_events->percpu_pmu, i));
>> +			}
>>  			if (err) {
>>  				pr_err("unable to request IRQ%d for ARM PMU counters\n",
>>  					irq);
>> @@ -180,6 +208,7 @@ static int cpu_pmu_request_irq(struct arm_pmu *cpu_pmu,
>> irq_handler_t handler) }
>>
>>  			cpumask_set_cpu(i, &cpu_pmu->active_irqs);
>> +			per_cpu(cpu_pmu_irqs, i) = irq;
>>  		}
>>  	}
>>
>> diff --git a/arch/arm/kernel/traps.c b/arch/arm/kernel/traps.c
>> index 74c752b9db68..c581e07517ff 100644
>> --- a/arch/arm/kernel/traps.c
>> +++ b/arch/arm/kernel/traps.c
>> @@ -38,6 +38,7 @@
>>  #include <asm/tls.h>
>>  #include <asm/system_misc.h>
>>  #include <asm/opcodes.h>
>> +#include <asm/pmu.h>
>>
>>
>>  static const char *handler[]= {
>> @@ -485,7 +486,7 @@ asmlinkage void __exception_irq_entry
>> handle_fiq_as_nmi(struct pt_regs *regs) irq = gic_ack_fiq();
>>
>>  	if (irq) {
>> -		/* empty - no SPI handlers (yet) */
>> +		cpu_pmu_handle_fiq(irq);
>>  	} else {
>>  #ifdef CONFIG_SMP
>>  		ipi_cpu_backtrace(regs);
>> --
>> 1.9.3
>>
>>
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>