Re: [PATCH v3] arm: perf: Directly handle SMP platforms with one SPI

[Will Deacon]

Hi Daniel,

Some minor comments below...

On Wed, Jan 07, 2015 at 12:28:18PM +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).
> 
> Signed-off-by: Daniel Thompson <daniel.thompson@xxxxxxxxxx>

[...]

> diff --git a/arch/arm/kernel/perf_event_cpu.c b/arch/arm/kernel/perf_event_cpu.c
> index dd9acc95ebc0..3d51c5f442eb 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
> +/*
> + * 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;
> +
> +       atomic_set(&hw_events->work_ret,
> +                  cpu_pmu->handle_irq(0, cpu_pmu));

Do you need a memory barrier here, or is that implued by enable_irq?

> +       if (atomic_dec_and_test(&cpu_pmu->remaining_work))
> +               enable_irq(cpu_pmu->muxed_spi_workaround_irq);
> +}
> +
> +/*
> + * 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)
> +{
> +       irqreturn_t ret = IRQ_NONE;
> +       cpumask_t deploy_on_mask;
> +       int cpu, work_ret;
> +       if (irq_num != cpu_pmu->muxed_spi_workaround_irq)
> +               return IRQ_NONE;

return ret ?

> +
> +       disable_irq_nosync(cpu_pmu->muxed_spi_workaround_irq);
> +
> +       cpumask_copy(&deploy_on_mask, cpu_online_mask);
> +       cpumask_clear_cpu(smp_processor_id(), &deploy_on_mask);
> +       atomic_add(cpumask_weight(&deploy_on_mask), &cpu_pmu->remaining_work);
> +       smp_mb__after_atomic();

What's this barrier needed for?

> +
> +       for_each_cpu(cpu, &deploy_on_mask) {

Why not for_each_online_cpu and then continue if cpu == smp_processor_id() ?
I assume the race against hotplug is benign, as the interrupt will no longer
be asserted to the GIC if the source CPU goes offline?

> +               struct pmu_hw_events *hw_events =
> +                   per_cpu_ptr(cpu_pmu->hw_events, cpu);
> +
> +               /*
> +                * The workaround code exits immediately without waiting to
> +                * see if the interrupt was handled by another CPU. This makes
> +                * it hard for us to decide between IRQ_HANDLED and IRQ_NONE.
> +                * However, the handler isn't shared so we don't have to worry
> +                * about being a good citizen, only about keeping the spurious
> +                * interrupt detector working. This allows us to return the
> +                * result of our *previous* attempt to deploy workaround.
> +                */
> +               work_ret = atomic_read(&hw_events->work_ret);
> +               if (work_ret != IRQ_NONE)
> +                       ret = work_ret;

Is this actually necessary, or can we always return handled?

> +
> +               if (!irq_work_queue_on(&hw_events->work, cpu))
> +                       if (atomic_dec_and_test(&cpu_pmu->remaining_work))

I'm not convinced that we can't have old work racing on the remaining work
field with a subsequent interrupt.

> +                               enable_irq(cpu_pmu->muxed_spi_workaround_irq);

"This function (enable_irq) may be called from IRQ context only when
 desc->irq_data.chip->bus_lock and desc->chip->bus_sync_unlock are NULL !"

Can we guarantee that in the general case?

> +       }
> +
> +       return ret;
> +}
> +
> +static int cpu_pmu_muxed_spi_workaround_init(struct arm_pmu *cpu_pmu)
> +{
> +       struct platform_device *pmu_device = cpu_pmu->plat_device;
> +       int cpu;
> +
> +       for_each_possible_cpu(cpu) {
> +               struct pmu_hw_events *hw_events =
> +                   per_cpu_ptr(cpu_pmu->hw_events, cpu);
> +
> +               init_irq_work(&hw_events->work, cpu_pmu_do_percpu_work);
> +               atomic_set(&hw_events->work_ret, IRQ_HANDLED);
> +       }
> +
> +       aomic_set(cpu_pmu->remaining_work, 0);

So you didn't even build this...

Will
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