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

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Date:	Tue, 15 Dec 2015 15:46:51 +0000
From:	Juri Lelli <juri.lelli@....com>
To:	Mark Rutland <mark.rutland@....com>
Cc:	Mark Brown <broonie@...nel.org>, 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 15/12/15 15:32, Mark Rutland wrote:
> 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.
> 

Yeah, and I guess the path towards out of three patches changing this
values for a specifica platform (without exposing the same changes
upstream) is not too far away.

> > 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 take is that ballpark is fine, but it's a per platform/configuration
ballpark that we need. Not a per core-type one.

> > 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.
> 

I didn't really followed that, so I might be wrong here, but isn't
already happened a discussion about how we want/like to stop exposing
bogomips info or rely on it for anything but in kernel delay loops?

Thanks,

- Juri
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