Date:   Fri, 11 Dec 2020 10:13:21 +0000
From:   Tvrtko Ursulin <tvrtko.ursulin@...ux.intel.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Marc Zyngier <maz@...nel.org>,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
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        "James E.J. Bottomley" <James.Bottomley@...senPartnership.com>,
        Helge Deller <deller@....de>,
        afzal mohammed <afzal.mohd.ma@...il.com>,
        linux-parisc@...r.kernel.org, Russell King <linux@...linux.org.uk>,
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Subject: Re: [patch 14/30] drm/i915/pmu: Replace open coded kstat_irqs() copy


On 10/12/2020 19:25, Thomas Gleixner 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.

After my failed hasty sketch from last night I had a different one which 
was kind of heuristics based (re-reading the upper dword and retrying if 
it changed on 32-bit). But you are right - it is okay to at least start 
like this today and if later there is a need we can either do that or 
deal with wrap at PMU read time.

So thanks for dealing with it, some small comments below but overall it 
is fine.

> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> Cc: Tvrtko Ursulin <tvrtko.ursulin@...ux.intel.com>
> Cc: Jani Nikula <jani.nikula@...ux.intel.com>
> Cc: Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>
> Cc: Rodrigo Vivi <rodrigo.vivi@...el.com>
> Cc: David Airlie <airlied@...ux.ie>
> Cc: Daniel Vetter <daniel@...ll.ch>
> Cc: intel-gfx@...ts.freedesktop.org
> Cc: dri-devel@...ts.freedesktop.org
> ---
>   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,

We never use priv as a local name, it should be either i915 or dev_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);

Curious, probably more educational for me - given x86_32 and x86_64, and 
the context of it getting called, what is the difference from just doing 
irq_count++?

> +}
> +
>   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;
>   }

In this file you can also drop the #include <linux/irq.h> line.

>   
> -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);

I guess same curiosity about READ_ONCE like in the increment site.

>   			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;
> 

Regards,

Tvrtko

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