linux-kernel - Re: [RFC PATCH 2/8] Documentation: arm: define DT cpu capacity bindings

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Date:	Tue, 15 Dec 2015 15:32:19 +0000
From:	Mark Rutland <mark.rutland@....com>
To:	Mark Brown <broonie@...nel.org>
Cc:	Juri Lelli <juri.lelli@....com>, Rob Herring <robh@...nel.org>,
	linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, devicetree@...r.kernel.org,
	peterz@...radead.org, vincent.guittot@...aro.org,
	linux@....linux.org.uk, sudeep.holla@....com,
	lorenzo.pieralisi@....com, catalin.marinas@....com,
	will.deacon@....com, morten.rasmussen@....com,
	dietmar.eggemann@....com, Pawel Moll <pawel.moll@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Kumar Gala <galak@...eaurora.org>,
	Maxime Ripard <maxime.ripard@...e-electrons.com>,
	Olof Johansson <olof@...om.net>,
	Gregory CLEMENT <gregory.clement@...e-electrons.com>,
	Paul Walmsley <paul@...an.com>,
	Linus Walleij <linus.walleij@...aro.org>,
	Chen-Yu Tsai <wens@...e.org>,
	Thomas Petazzoni <thomas.petazzoni@...e-electrons.com>
Subject: Re: [RFC PATCH 2/8] Documentation: arm: define DT cpu capacity
 bindings

On Tue, Dec 15, 2015 at 03:08:13PM +0000, Mark Brown wrote:
> On Tue, Dec 15, 2015 at 02:01:36PM +0000, Mark Rutland wrote:
> 
> > I really don't want to see a table of magic numbers in the kernel.
> 
> Right, there's pitfalls there too although not being part of an ABI
> does make them more manageable.  

I think that people are very likely to treat them exactly like an ABI,
w.r.t. any regressions in performance that result from their addition,
modification, or removal. That becomes really horrible when new CPUs
appear.

> One thing it's probably helpful to establish here is how much the
> specific numbers are going to matter in the grand scheme of things.  If
> the specific numbers *are* super important then nobody is going to want
> to touch them as they'll be prone to getting tweaked.  If instead the
> numbers just need to be ballpark accurate so the scheduler starts off in
> roughly the right place and the specific numbers don't matter it's a lot
> easier and having a table in the kernel until we think of something
> better (if that ever happens) gets a lot easier.

I agree that we first need to figure out the importance of these
numbers. I disagree that our first step should be to add a table.

> My expectation is that we just need good enough, not perfect, and that
> seems to match what Juri is saying about the expectation that most of
> the fine tuning is done via other knobs.

My expectation is that if a ballpark figure is good enough, it should be
possible to implement something trivial like bogomips / loop_per_jiffy
calculation.

Thanks,
Mark.
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