Re: [Intel-gfx] [PATCH v3 2/4] drm/i915/pmu: Use kstat_irqs to get interrupt count

From: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
To: Jerry Snitselaar <jsnitsel@redhat.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	linux-kernel@vger.kernel.org,
	Tvrtko Ursulin <tvrtko.ursulin@intel.com>,
	Chris Wilson <chris@chris-wilson.co.uk>
Cc: Peter Zijlstra <peterz@infradead.org>,
	David Airlie <airlied@linux.ie>,
	Matthew Garrett <mjg59@google.com>,
	intel-gfx@lists.freedesktop.org,
	James Bottomley <James.Bottomley@HansenPartnership.com>,
	Jason Gunthorpe <jgg@ziepe.ca>,
	Jarkko Sakkinen <jarkko@kernel.org>,
	dri-devel@lists.freedesktop.org, linux-integrity@vger.kernel.org,
	Peter Huewe <peterhuewe@gmx.de>
Subject: Re: [Intel-gfx] [PATCH v3 2/4] drm/i915/pmu: Use kstat_irqs to get interrupt count
Date: Thu, 10 Dec 2020 09:53:00 +0200	[thread overview]
Message-ID: <160758677957.5062.15497765500689083558@jlahtine-mobl.ger.corp.intel.com> (raw)
In-Reply-To: <875z5e99ez.fsf@nanos.tec.linutronix.de>

+ Tvrtko and Chris for comments

Code seems to be added in:

commit 0cd4684d6ea9a4ffec33fc19de4dd667bb90d0a5
Author: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Date:   Tue Nov 21 18:18:50 2017 +0000

    drm/i915/pmu: Add interrupt count metric

I think later in the thread there was a suggestion to replace this with
simple counter increment in IRQ handler.

Regards, Joonas

Quoting Thomas Gleixner (2020-12-06 18:38:44)
> On Fri, Dec 04 2020 at 18:43, Jerry Snitselaar wrote:
> 
> > Now that kstat_irqs is exported, get rid of count_interrupts in
> > i915_pmu.c
> > --- 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(struct hrtimer *hrtimer)
> >       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;
> > -}
> 
> May I ask why this has been merged in the first place?
> 
> Nothing in a driver has ever to fiddle with the internals of an irq
> descriptor. We have functions for properly accessing them. Just because
> C allows to fiddle with everything is not a justification. If the
> required function is not exported then adding the export with a proper
> explanation is not asked too much.
> 
> Also this lacks protection or at least a comment why this can be called
> safely and is not subject to a concurrent removal of the irq descriptor.
> The same problem exists when calling kstat_irqs(). It's even documented
> at the top of the function.
> 
> Thanks,
> 
>         tglx
> 
> 
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