Subject: Re: [patch 14/30] drm/i915/pmu: Replace open coded kstat_irqs() copy

From: Jani Nikula <[email-masked]>
Date: Fri, 11 Dec 2020 11:54:03 +0200

On Thu, 10 Dec 2020, Thomas Gleixner <[email-masked]> wrote:
> Driver code has no business with the internals of the irq descriptor.
>
> Aside of that the count is per interrupt line and therefore takes
> interrupts from other devices into account which share the interrupt line
> and are not handled by the graphics driver.
>
> Replace it with a pmu private count which only counts interrupts which
> originate from the graphics card.
>
> To avoid atomics or heuristics of some sort make the counter field
> 'unsigned long'. That limits the count to 4e9 on 32bit which is a lot and
> postprocessing can easily deal with the occasional wraparound.

I'll let Tvrtko and Chris review the substance here, but assuming they
don't object,

Acked-by: Jani Nikula <[email-masked]>

for merging via whichever tree makes most sense.

>
> Signed-off-by: Thomas Gleixner <[email-masked]>
> Cc: Tvrtko Ursulin <[email-masked]>
> Cc: Jani Nikula <[email-masked]>
> Cc: Joonas Lahtinen <[email-masked]>
> Cc: Rodrigo Vivi <[email-masked]>
> Cc: David Airlie <[email-masked]>
> Cc: Daniel Vetter <[email-masked]>
> Cc: [email-masked]
> Cc: [email-masked]
> ---
>  drivers/gpu/drm/i915/i915_irq.c |   34 ++++++++++++++++++++++++++++++++++
>  drivers/gpu/drm/i915/i915_pmu.c |   18 +-----------------
>  drivers/gpu/drm/i915/i915_pmu.h |    8 ++++++++
>  3 files changed, 43 insertions(+), 17 deletions(-)
>
> --- a/drivers/gpu/drm/i915/i915_irq.c
> +++ b/drivers/gpu/drm/i915/i915_irq.c
> @@ -60,6 +60,24 @@
>   * and related files, but that will be described in separate chapters.
>   */
>  
> +/*
> + * Interrupt statistic for PMU. Increments the counter only if the
> + * interrupt originated from the the GPU so interrupts from a device which
> + * shares the interrupt line are not accounted.
> + */
> +static inline void pmu_irq_stats(struct drm_i915_private *priv,
> +				 irqreturn_t res)
> +{
> +	if (unlikely(res != IRQ_HANDLED))
> +		return;
> +
> +	/*
> +	 * A clever compiler translates that into INC. A not so clever one
> +	 * should at least prevent store tearing.
> +	 */
> +	WRITE_ONCE(priv->pmu.irq_count, priv->pmu.irq_count + 1);
> +}
> +
>  typedef bool (*long_pulse_detect_func)(enum hpd_pin pin, u32 val);
>  
>  static const u32 hpd_ilk[HPD_NUM_PINS] = {
> @@ -1599,6 +1617,8 @@ static irqreturn_t valleyview_irq_handle
>  		valleyview_pipestat_irq_handler(dev_priv, pipe_stats);
>  	} while (0);
>  
> +	pmu_irq_stats(dev_priv, ret);
> +
>  	enable_rpm_wakeref_asserts(&dev_priv->runtime_pm);
>  
>  	return ret;
> @@ -1676,6 +1696,8 @@ static irqreturn_t cherryview_irq_handle
>  		valleyview_pipestat_irq_handler(dev_priv, pipe_stats);
>  	} while (0);
>  
> +	pmu_irq_stats(dev_priv, ret);
> +
>  	enable_rpm_wakeref_asserts(&dev_priv->runtime_pm);
>  
>  	return ret;
> @@ -2103,6 +2125,8 @@ static irqreturn_t ilk_irq_handler(int i
>  	if (sde_ier)
>  		raw_reg_write(regs, SDEIER, sde_ier);
>  
> +	pmu_irq_stats(i915, ret);
> +
>  	/* IRQs are synced during runtime_suspend, we don't require a wakeref */
>  	enable_rpm_wakeref_asserts(&i915->runtime_pm);
>  
> @@ -2419,6 +2443,8 @@ static irqreturn_t gen8_irq_handler(int
>  
>  	gen8_master_intr_enable(regs);
>  
> +	pmu_irq_stats(dev_priv, IRQ_HANDLED);
> +
>  	return IRQ_HANDLED;
>  }
>  
> @@ -2514,6 +2540,8 @@ static __always_inline irqreturn_t
>  
>  	gen11_gu_misc_irq_handler(gt, gu_misc_iir);
>  
> +	pmu_irq_stats(i915, IRQ_HANDLED);
> +
>  	return IRQ_HANDLED;
>  }
>  
> @@ -3688,6 +3716,8 @@ static irqreturn_t i8xx_irq_handler(int
>  		i8xx_pipestat_irq_handler(dev_priv, iir, pipe_stats);
>  	} while (0);
>  
> +	pmu_irq_stats(dev_priv, ret);
> +
>  	enable_rpm_wakeref_asserts(&dev_priv->runtime_pm);
>  
>  	return ret;
> @@ -3796,6 +3826,8 @@ static irqreturn_t i915_irq_handler(int
>  		i915_pipestat_irq_handler(dev_priv, iir, pipe_stats);
>  	} while (0);
>  
> +	pmu_irq_stats(dev_priv, ret);
> +
>  	enable_rpm_wakeref_asserts(&dev_priv->runtime_pm);
>  
>  	return ret;
> @@ -3941,6 +3973,8 @@ static irqreturn_t i965_irq_handler(int
>  		i965_pipestat_irq_handler(dev_priv, iir, pipe_stats);
>  	} while (0);
>  
> +	pmu_irq_stats(dev_priv, IRQ_HANDLED);
> +
>  	enable_rpm_wakeref_asserts(&dev_priv->runtime_pm);
>  
>  	return ret;
> --- a/drivers/gpu/drm/i915/i915_pmu.c
> +++ b/drivers/gpu/drm/i915/i915_pmu.c
> @@ -423,22 +423,6 @@ static enum hrtimer_restart i915_sample(
>  	return HRTIMER_RESTART;
>  }
>  
> -static u64 count_interrupts(struct drm_i915_private *i915)
> -{
> -	/* open-coded kstat_irqs() */
> -	struct irq_desc *desc = irq_to_desc(i915->drm.pdev->irq);
> -	u64 sum = 0;
> -	int cpu;
> -
> -	if (!desc || !desc->kstat_irqs)
> -		return 0;
> -
> -	for_each_possible_cpu(cpu)
> -		sum += *per_cpu_ptr(desc->kstat_irqs, cpu);
> -
> -	return sum;
> -}
> -
>  static void i915_pmu_event_destroy(struct perf_event *event)
>  {
>  	struct drm_i915_private *i915 =
> @@ -581,7 +565,7 @@ static u64 __i915_pmu_event_read(struct
>  				   USEC_PER_SEC /* to MHz */);
>  			break;
>  		case I915_PMU_INTERRUPTS:
> -			val = count_interrupts(i915);
> +			val = READ_ONCE(pmu->irq_count);
>  			break;
>  		case I915_PMU_RC6_RESIDENCY:
>  			val = get_rc6(&i915->gt);
> --- a/drivers/gpu/drm/i915/i915_pmu.h
> +++ b/drivers/gpu/drm/i915/i915_pmu.h
> @@ -108,6 +108,14 @@ struct i915_pmu {
>  	 */
>  	ktime_t sleep_last;
>  	/**
> +	 * @irq_count: Number of interrupts
> +	 *
> +	 * Intentionally unsigned long to avoid atomics or heuristics on 32bit.
> +	 * 4e9 interrupts are a lot and postprocessing can really deal with an
> +	 * occasional wraparound easily. It's 32bit after all.
> +	 */
> +	unsigned long irq_count;
> +	/**
>  	 * @events_attr_group: Device events attribute group.
>  	 */
>  	struct attribute_group events_attr_group;
>

-- 
Jani Nikula, Intel Open Source Graphics Center