Re: [PATCH v2 3/3] drm/i915: optimise intel_runtime_pm_{get, put}

From: Imre Deak <imre.deak@intel.com>
To: Chris Wilson <chris@chris-wilson.co.uk>,
	David Weinehall <david.weinehall@linux.intel.com>
Cc: imre.deak@linux.intel.com, intel-gfx@lists.freedesktop.org
Subject: Re: [PATCH v2 3/3] drm/i915: optimise intel_runtime_pm_{get, put}
Date: Fri, 18 Nov 2016 17:41:50 +0200	[thread overview]
Message-ID: <1479483710.19861.23.camel@intel.com> (raw)
In-Reply-To: <20161118140040.GG28142@nuc-i3427.alporthouse.com>

On pe, 2016-11-18 at 14:00 +0000, Chris Wilson wrote:
> On Fri, Nov 18, 2016 at 03:36:47PM +0200, David Weinehall wrote:
> > Benchmarking shows that on resume we spend quite a bit of time
> > just taking and dropping these references, leaving us two options;
> > either rewriting the code not to take these references more than
> > once, which would be a rather invasive change since the involved
> > functions are used from other places, or to optimise
> > intel_runtime_pm_{get,put}().  This patch does the latter.
> > Initial benchmarking indicate improvements of a couple
> > of milliseconds on resume.
> > 
> > Original patch by Chris, with slight fixes by me.
> > 
> > v2: Fix missing return value (Patchwork)
> >     Remove extra atomic_dec() (Chris)
> > 
> > Signed-off-by: David Weinehall <david.weinehall@linux.intel.com>
> > CC: Chris Wilson <chris@chris-wilson.co.uk>
> Cc: Imre Deak <imre.deak@linux.intel.com>
> 
> I'm happy with this. Not amused that it apparently saves quite a bit
> of
> overhead with frequent pm_runtime calls.
> 
> Imre?

I think the overhead is because the RPM core takes a lock and checks if
the device needs to be woken up even if the runtime_usage count is 0.
But for us the device is awake whenever wakeref_count > 0, so yes we
can optimize things.

> -Chris
> 
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