Re: [PATCH v4] arm: perf: Directly handle SMP platforms with one SPI

[Will Deacon]

On Fri, Jan 09, 2015 at 04:16:10PM +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).

[...]

> diff --git a/arch/arm/kernel/perf_event.c b/arch/arm/kernel/perf_event.c
> index f7c65adaa428..e5c537b57f94 100644
> --- a/arch/arm/kernel/perf_event.c
> +++ b/arch/arm/kernel/perf_event.c
> @@ -299,8 +299,6 @@ 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;
> 
> @@ -311,14 +309,9 @@ 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 dd9acc95ebc0..76227484baa9 100644
> --- a/arch/arm/kernel/perf_event_cpu.c
> +++ b/arch/arm/kernel/perf_event_cpu.c
> @@ -59,6 +59,142 @@ int perf_num_counters(void)
>  }
>  EXPORT_SYMBOL_GPL(perf_num_counters);
> 
> +#ifdef CONFIG_SMP
> +/*
> + * 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 */
> +       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);
> +}
> +
> +/*
> + * Issue work to the other CPUs. Must be called whilst we own the
> + * hotplug locks.
> + */
> +static void cpu_pmu_queue_percpu_work(struct arm_pmu *cpu_pmu)
> +{
> +       int cpu;
> +
> +       atomic_add(num_online_cpus() - 1, &cpu_pmu->remaining_irq_work);
> +
> +       for_each_online_cpu(cpu) {
> +               struct pmu_hw_events *hw_events =
> +                   per_cpu_ptr(cpu_pmu->hw_events, cpu);
> +
> +               if (cpu == smp_processor_id())
> +                       continue;
> +
> +               /*
> +                * We assume that the IPI within irq_work_queue_on()
> +                * implies a full memory barrier making the value of
> +                * cpu_pmu->remaining_irq_work visible to the target.
> +                */
> +               if (!irq_work_queue_on(&hw_events->work, cpu))
> +                       if (atomic_dec_and_test(&cpu_pmu->remaining_irq_work))
> +                               enable_irq(cpu_pmu->muxed_spi_workaround_irq);
> +       }
> +}
> +
> +void cpu_pmu_muxed_spi_workaround_worker(struct work_struct *work)
> +{
> +       struct arm_pmu *cpu_pmu =
> +           container_of(work, struct arm_pmu, muxed_spi_workaround_work);
> +
> +       get_online_cpus();
> +       cpu_pmu_queue_percpu_work(cpu_pmu);
> +       put_online_cpus();
> +}
> +
> +/*
> + * Called when the main interrupt handler cannot determine the source
> + * of interrupt. It will deploy a workaround if we are running on an SMP
> + * platform with only a single muxed SPI.
> + *
> + * The workaround disables the interrupt and distributes irqwork to all
> + * other processors in the system. Hopefully one of them will clear the
> + * interrupt...
> + */
> +static irqreturn_t cpu_pmu_handle_irq_none(int irq_num, struct arm_pmu *cpu_pmu)
> +{
> +
> +       if (irq_num != cpu_pmu->muxed_spi_workaround_irq)
> +               return IRQ_NONE;
> +
> +       disable_irq_nosync(cpu_pmu->muxed_spi_workaround_irq);
> +
> +       if (try_get_online_cpus()) {

It's not safe to call this from interrupt context (it takes a mutex).

Can you try enabling a bunch of the debug options under "Kernel Hacking"
for things like detecting sleeping whilst atomic and then run Vince's perf
fuzzer to see what crops up please?

  https://github.com/deater/perf_event_tests/tree/master/fuzzer

Will
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