Re: [PATCH v6] arm: perf: Directly handle SMP platforms with one SPI

[Daniel Thompson <daniel.thompson@linaro.org>]

On 31/03/15 17:20, Will Deacon wrote:
> Hi Daniel,
>
> On Wed, Mar 04, 2015 at 01:25:45PM +0000, Daniel Thompson wrote:
>> Some ARM platforms mux the PMU interrupt of every core into a single
>> SPI. On such platforms if the PMU of any core except 0 raises an interrupt
>> then it cannot be serviced and eventually, if you are lucky, the spurious
>> irq detection might forcefully disable the interrupt.
>>
>> On these SoCs it is not possible to determine which core raised the
>> interrupt so workaround this issue by queuing irqwork on the other
>> cores whenever the primary interrupt handler is unable to service the
>> interrupt.
>>
>> The u8500 platform has an alternative workaround that dynamically alters
>> the affinity of the PMU interrupt. This workaround logic is no longer
>> required so the original code is removed as is the hook it relied upon.
>>
>> Tested on imx6q (which has fours cores/PMUs all muxed to a single SPI)
>> using a simple soak, combined perf and CPU hotplug soak and using
>> perf fuzzer's fast_repro.sh.
>
> [...]
>
>> diff --git a/arch/arm/include/asm/pmu.h b/arch/arm/include/asm/pmu.h
>> index b1596bd59129..dfef7904b790 100644
>> --- a/arch/arm/include/asm/pmu.h
>> +++ b/arch/arm/include/asm/pmu.h
>> @@ -87,6 +87,14 @@ struct pmu_hw_events {
>>           * already have to allocate this struct per cpu.
>>           */
>>          struct arm_pmu          *percpu_pmu;
>> +
>> +#ifdef CONFIG_SMP
>> +       /*
>> +        * This is used to schedule workaround logic on platforms where all
>> +        * the PMUs are attached to a single SPI.
>> +        */
>> +       struct irq_work work;
>> +#endif
>>   };
>>
>>   struct arm_pmu {
>> @@ -117,6 +125,10 @@ struct arm_pmu {
>>          struct platform_device  *plat_device;
>>          struct pmu_hw_events    __percpu *hw_events;
>>          struct notifier_block   hotplug_nb;
>> +#ifdef CONFIG_SMP
>> +       int                     muxed_spi_workaround_irq;
>> +       atomic_t                remaining_irq_work;
>> +#endif
>>   };
>>
>>   #define to_arm_pmu(p) (container_of(p, struct arm_pmu, pmu))
>> diff --git a/arch/arm/kernel/perf_event.c b/arch/arm/kernel/perf_event.c
>> index 557e128e4df0..e3fc1a0ce969 100644
>> --- a/arch/arm/kernel/perf_event.c
>> +++ b/arch/arm/kernel/perf_event.c
>> @@ -305,8 +305,6 @@ validate_group(struct perf_event *event)
>>   static irqreturn_t armpmu_dispatch_irq(int irq, void *dev)
>>   {
>>          struct arm_pmu *armpmu;
>> -       struct platform_device *plat_device;
>> -       struct arm_pmu_platdata *plat;
>>          int ret;
>>          u64 start_clock, finish_clock;
>>
>> @@ -317,14 +315,9 @@ static irqreturn_t armpmu_dispatch_irq(int irq, void *dev)
>>           * dereference.
>>           */
>>          armpmu = *(void **)dev;
>> -       plat_device = armpmu->plat_device;
>> -       plat = dev_get_platdata(&plat_device->dev);
>>
>>          start_clock = sched_clock();
>> -       if (plat && plat->handle_irq)
>> -               ret = plat->handle_irq(irq, armpmu, armpmu->handle_irq);
>> -       else
>> -               ret = armpmu->handle_irq(irq, armpmu);
>> +       ret = armpmu->handle_irq(irq, armpmu);
>>          finish_clock = sched_clock();
>>
>>          perf_sample_event_took(finish_clock - start_clock);
>> diff --git a/arch/arm/kernel/perf_event_cpu.c b/arch/arm/kernel/perf_event_cpu.c
>> index 61b53c46edfa..d5bbd79abd4c 100644
>> --- a/arch/arm/kernel/perf_event_cpu.c
>> +++ b/arch/arm/kernel/perf_event_cpu.c
>> @@ -59,6 +59,116 @@ int perf_num_counters(void)
>>   }
>>   EXPORT_SYMBOL_GPL(perf_num_counters);
>>
>> +#ifdef CONFIG_SMP
>> +
>> +static cpumask_t down_prepare_cpu_mask;
>> +static DEFINE_SPINLOCK(down_prepare_cpu_lock);
>> +
>> +/*
>> + * Workaround logic that is distributed to all cores if the PMU has only
>> + * a single IRQ and the CPU receiving that IRQ cannot handle it. Its
>> + * job is to try to service the interrupt on the current CPU. It will
>> + * also enable the IRQ again if all the other CPUs have already tried to
>> + * service it.
>> + */
>> +static void cpu_pmu_do_percpu_work(struct irq_work *w)
>> +{
>> +       struct pmu_hw_events *hw_events =
>> +           container_of(w, struct pmu_hw_events, work);
>> +       struct arm_pmu *cpu_pmu = hw_events->percpu_pmu;
>> +
>> +       /* Ignore the return code, we can do nothing useful with it */
>> +       (void) cpu_pmu->handle_irq(0, cpu_pmu);
>> +
>> +       if (atomic_dec_and_test(&cpu_pmu->remaining_irq_work))
>> +               enable_irq(cpu_pmu->muxed_spi_workaround_irq);
>> +}
>> +
>> +/*
>> + * Workaround for systems where all PMU interrupts are targeting a
>> + * single SPI.
>> + *
>> + * The workaround will disable the interrupt and distribute irqwork to all
>> + * the other processors in the system. Hopefully one of them will clear the
>> + * interrupt...
>> + *
>> + * The workaround is only deployed when all PMU interrupts are aimed
>> + * at a single core. As a result the workaround is never re-entered
>> + * making it safe for us to use static data to maintain state.
>> + */
>> +static void cpu_pmu_deploy_muxed_spi_workaround(struct arm_pmu *cpu_pmu)
>> +{
>> +       static cpumask_t irqwork_mask;
>> +       int cpu;
>> +
>> +       disable_irq_nosync(cpu_pmu->muxed_spi_workaround_irq);
>> +       spin_lock(&down_prepare_cpu_lock);
>> +
>> +       /*
>> +        * Combining cpu_online_mask and down_prepare_cpu_mask gives
>> +        * us the CPUs that are currently online and cannot die until
>> +        * we release down_prepare_cpu_lock.
>> +        */
>> +       cpumask_andnot(&irqwork_mask, cpu_online_mask, &down_prepare_cpu_mask);
>> +       cpumask_clear_cpu(smp_processor_id(), &irqwork_mask);
>> +       atomic_add(cpumask_weight(&irqwork_mask), &cpu_pmu->remaining_irq_work);
>
> AFAICT, this is a hack to avoid get_online_cpus (which can sleep) from irq
> context? Is there no way we can do try_get_online_cpus, and just return
> IRQ_NONE if that fails? At some point, the hotplug operation will complete
> and we'll be able to service the pending interrupt. I think that would allow
> us to kill the down_prepare_cpu_lock.

It certainly doesn't look easy but I will take a closer look...

 From the perf side its reasonably easy. I don't think we can return 
IRQ_NONE (we might own the h.p. lock on the CPU handling the interrupt) 
so we have to disable the interrupt. However we could probably arrange 
to re-enable it from the hotplug notifications when this is needed.

Unfortunately I'm not sure the cpu_hotplug side is so easy.