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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>,
intel-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
"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>,
linux-arm-kernel@...ts.infradead.org,
Mark Rutland <mark.rutland@....com>,
Catalin Marinas <catalin.marinas@....com>,
Will Deacon <will@...nel.org>,
Christian Borntraeger <borntraeger@...ibm.com>,
Heiko Carstens <hca@...ux.ibm.com>, linux-s390@...r.kernel.org,
Pankaj Bharadiya <pankaj.laxminarayan.bharadiya@...el.com>,
Chris Wilson <chris@...is-wilson.co.uk>,
Wambui Karuga <wambui.karugax@...il.com>,
Linus Walleij <linus.walleij@...aro.org>,
linux-gpio@...r.kernel.org, Lee Jones <lee.jones@...aro.org>,
Jon Mason <jdmason@...zu.us>,
Dave Jiang <dave.jiang@...el.com>,
Allen Hubbe <allenbh@...il.com>, linux-ntb@...glegroups.com,
Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
Rob Herring <robh@...nel.org>,
Bjorn Helgaas <bhelgaas@...gle.com>,
Michal Simek <michal.simek@...inx.com>,
linux-pci@...r.kernel.org,
Karthikeyan Mitran <m.karthikeyan@...iveil.co.in>,
Hou Zhiqiang <Zhiqiang.Hou@....com>,
Tariq Toukan <tariqt@...dia.com>,
"David S. Miller" <davem@...emloft.net>,
Jakub Kicinski <kuba@...nel.org>, netdev@...r.kernel.org,
linux-rdma@...r.kernel.org, Saeed Mahameed <saeedm@...dia.com>,
Leon Romanovsky <leon@...nel.org>,
Boris Ostrovsky <boris.ostrovsky@...cle.com>,
Juergen Gross <jgross@...e.com>,
Stefano Stabellini <sstabellini@...nel.org>,
xen-devel@...ts.xenproject.org
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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