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

[Mark Rutland]

Hi Daniel,

On Wed, Nov 26, 2014 at 04:59:07PM +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.

I agree that the workaround for this design "feature" should live in the
architectural perf code rather than in board files.

I had intended to implement that atop of my heterogeneous CPUs series
[1] (so as to only target the subset of cores for a given PMU), but I
only had an affinity bouncing implementation, and the general approach
looks like it has the potential to be far better in terms of latency.

Hwoever, I have some concerns with the implementation below.

> 
> Tested on imx6q (which has fours cores/PMUs all muxed to a single SPI).
> 
> Signed-off-by: Daniel Thompson <daniel.thompson@xxxxxxxxxx>
> ---
> 
> Notes:
>     v2:
>      * Fixed build problems on systems without SMP.
> 
>     v1:
>      * Thanks to Lucas Stach, Russell King and Thomas Gleixner for
>        critiquing an older, completely different way to tackle the
>        same problem.
> 
> 
>  arch/arm/include/asm/pmu.h       |  12 ++++
>  arch/arm/kernel/perf_event.c     |  13 ++--
>  arch/arm/kernel/perf_event_cpu.c | 126 +++++++++++++++++++++++++++++++++++++++
>  arch/arm/mach-ux500/cpu-db8500.c |  29 ---------
>  4 files changed, 143 insertions(+), 37 deletions(-)
> 
> diff --git a/arch/arm/include/asm/pmu.h b/arch/arm/include/asm/pmu.h
> index 0b648c541293..771201ff0988 100644
> --- a/arch/arm/include/asm/pmu.h
> +++ b/arch/arm/include/asm/pmu.h
> @@ -81,6 +81,12 @@ struct pmu_hw_events {
>         raw_spinlock_t          pmu_lock;
>  };
> 
> +struct arm_pmu_work {
> +       struct irq_work work;
> +       struct arm_pmu *arm_pmu;
> +       atomic_t ret;
> +};
> +
>  struct arm_pmu {
>         struct pmu      pmu;
>         cpumask_t       active_irqs;
> @@ -108,6 +114,12 @@ struct arm_pmu {
>         u64             max_period;
>         struct platform_device  *plat_device;
>         struct pmu_hw_events    *(*get_hw_events)(void);
> +#ifdef CONFIG_SMP
> +       irqreturn_t     (*handle_irq_none)(struct arm_pmu *);

I don't think this needs to live on the struct arm_pmu; we're unlikely
to need a different callback given this is already generic to CPUs, so
we can just call it directly.

Once Will's arm-perf-3.19 tag hits mainline (with my perf cleanups
series) the CCI will be decoupled [2] and the arm_pmu framework will
only be used by CPU PMUs.

> +       int             single_irq;

With my heterogeneous PMUs series you shouldn't need this, there will be
an irq_map with a single entry instead.

> +       struct arm_pmu_work __percpu *work;

In my cleanups series I folded all the percpu data into the
pmu_hw_events. It would be nice to fold this too if possible, so as to
make allocation and freeing simpler. We already have a percpu_pmu
pointer with my series applied, so we don't need the arm_pmu pointer.

If it feels like we're straying too far from hw events data we can
rename the structure to pmu_cpu_data or something like that.

> +       atomic_t        remaining_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 b50a770f8c99..ba67d6309e1e 100644
> --- a/arch/arm/kernel/perf_event.c
> +++ b/arch/arm/kernel/perf_event.c
> @@ -306,22 +306,19 @@ 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;
> 
>         if (irq_is_percpu(irq))
>                 dev = *(void **)dev;
>         armpmu = 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, dev, armpmu->handle_irq);
> -       else
> -               ret = armpmu->handle_irq(irq, dev);
> +       ret = armpmu->handle_irq(irq, dev);
> +#ifdef CONFIG_SMP
> +       if (ret == IRQ_NONE && armpmu->handle_irq_none)
> +               ret = armpmu->handle_irq_none(dev);
> +#endif
>         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 eb2c4d55666b..5605d4a4c01f 100644
> --- a/arch/arm/kernel/perf_event_cpu.c
> +++ b/arch/arm/kernel/perf_event_cpu.c
> @@ -88,6 +88,120 @@ static void cpu_pmu_disable_percpu_irq(void *data)
>         disable_percpu_irq(irq);
>  }
> 
> +#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 arm_pmu_work *work = container_of(w, struct arm_pmu_work, work);
> +       struct arm_pmu *cpu_pmu = work->arm_pmu;
> +
> +       atomic_set(&work->ret,
> +                  cpu_pmu->handle_irq(cpu_pmu->single_irq, cpu_pmu));
> +
> +       if (atomic_dec_and_test(&cpu_pmu->remaining_work))
> +               enable_irq(cpu_pmu->single_irq);
> +}
> +
> +/*
> + * This callback, which is enabled only on SMP platforms that are
> + * running with a single IRQ, is called when the PMU handler running in
> + * the current CPU cannot service the interrupt.
> + *
> + * It will disable the interrupt and distribute irqwork to all other
> + * processors in the system. Hopefully one of them will clear the
> + * interrupt...
> + */
> +static irqreturn_t cpu_pmu_handle_irq_none(struct arm_pmu *cpu_pmu)
> +{
> +       int num_online = num_online_cpus();
> +       irqreturn_t ret = IRQ_NONE;
> +       int cpu, cret;

s/cret/cpu_ret/ -- it's slightly more typing but easier to read IMO.

> +
> +       if (num_online <= 1)
> +               return IRQ_NONE;

Surely num_online can never be below one here?

We didn't get the online CPUs, so can't other CPUs may shut down between
the call to num_online_cpus() and portions of the below code?

Could that prevent us from re-enabling the interrupt? Or are we
protected from that someehow?

> +
> +       disable_irq_nosync(cpu_pmu->single_irq);
> +       atomic_add(num_online, &cpu_pmu->remaining_work);
> +       smp_mb__after_atomic();
> +
> +       for_each_online_cpu(cpu) {
> +               struct arm_pmu_work *work = per_cpu_ptr(cpu_pmu->work, cpu);
> +
> +               if (cpu == smp_processor_id())
> +                       continue;
> +
> +               /*
> +                * We can be extremely relaxed about memory ordering
> +                * here. All we are doing is gathering information
> +                * about the past to help us give a return value that
> +                * will keep the spurious interrupt detector both happy
> +                * *and* functional. We are not shared so we can
> +                * tolerate the occasional spurious IRQ_HANDLED.
> +                */
> +               cret = atomic_read(&work->ret);
> +               if (cret != IRQ_NONE)
> +                       ret = cret;

I'm a little confused as to what's going on here. Why are we checking
the return code for the work triggered by the _previous_ interrupt
before queueing up the next work item?

As far as I can tell, work_ret was never initialised, so what does this
do for the first round?

> +
> +               if (!irq_work_queue_on(&work->work, cpu))
> +                       atomic_dec(&cpu_pmu->remaining_work);

What context does queued work run in? Will we sample the expected set of
registers (e.g. can we sample userspace)?

> +       }
> +
> +       if (atomic_dec_and_test(&cpu_pmu->remaining_work))
> +               enable_irq(cpu_pmu->single_irq);

If we initialise remaining work to the number of CPUs minus one, we can
drop this last bit, and another CPU will always re-enable the interrupt.

> +
> +       return ret;
> +}
> +
> +static int cpu_pmu_single_irq_workaround_init(struct arm_pmu *cpu_pmu)
> +{
> +       struct platform_device *pmu_device = cpu_pmu->plat_device;
> +       int cpu;
> +
> +       cpu_pmu->handle_irq_none = cpu_pmu_handle_irq_none;
> +       cpu_pmu->single_irq = platform_get_irq(pmu_device, 0);
> +
> +       cpu_pmu->work = alloc_percpu(struct arm_pmu_work);
> +       if (!cpu_pmu->work) {
> +               pr_err("no memory for shared IRQ workaround\n");
> +               return -ENOMEM;
> +       }
> +
> +       for_each_possible_cpu(cpu) {
> +               struct arm_pmu_work *w = per_cpu_ptr(cpu_pmu->work, cpu);
> +
> +               init_irq_work(&w->work, cpu_pmu_do_percpu_work);
> +               w->arm_pmu = cpu_pmu;
> +       }
> +
> +       return 0;
> +}
> +
> +static void cpu_pmu_single_irq_workaround_term(struct arm_pmu *cpu_pmu)
> +{
> +       cpu_pmu->handle_irq_none = cpu_pmu_handle_irq_none;

Huh? We set that up identically in cpu_pmu_single_irq_workaround_init.

I think the handle_irq_none member should go.

> +       free_percpu(cpu_pmu->work);
> +}
> +
> +#else /* CONFIG_SMP */
> +
> +static int cpu_pmu_single_irq_workaround_init(struct arm_pmu *cpu_pmu)
> +{
> +       return 0;
> +}
> +
> +static void cpu_pmu_single_irq_workaround_term(struct arm_pmu *cpu_pmu)
> +{
> +}
> +
> +#endif /* CONFIG_SMP */
> +
>  static void cpu_pmu_free_irq(struct arm_pmu *cpu_pmu)
>  {
>         int i, irq, irqs;
> @@ -107,6 +221,8 @@ static void cpu_pmu_free_irq(struct arm_pmu *cpu_pmu)
>                         if (irq >= 0)
>                                 free_irq(irq, cpu_pmu);
>                 }
> +
> +               cpu_pmu_single_irq_workaround_term(cpu_pmu);
>         }
>  }
> 
> @@ -162,6 +278,16 @@ static int cpu_pmu_request_irq(struct arm_pmu *cpu_pmu, irq_handler_t handler)
> 
>                         cpumask_set_cpu(i, &cpu_pmu->active_irqs);
>                 }
> +
> +               /*
> +                * If we are running SMP and have only one interrupt source
> +                * then get ready to share that single irq among the cores.
> +                */
> +               if (nr_cpu_ids > 1 && irqs == 1) {
> +                       err = cpu_pmu_single_irq_workaround_init(cpu_pmu);
> +                       if (err)
> +                               return err;
> +               }

What does this do for systems using PPIs, where there's a single percpu
interrupt (e.g. Krait)?

Thanks,
Mark.

[1] http://lists.infradead.org/pipermail/linux-arm-kernel/2014-November/300748.html
[2] https://git.kernel.org/cgit/linux/kernel/git/will/linux.git/tag/?h=perf/updates&id=arm-perf-3.19
[3] https://git.kernel.org/cgit/linux/kernel/git/will/linux.git/commit/?h=perf/updates&id=c6f85cb4305bd80658d19f7b097a7c36ef9912e2
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