linux-kernel - Re: [RFC PATCH v2 0/4] CPUs capacity information for heterogeneous systems

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Date:	Wed, 20 Jan 2016 10:25:48 +0000
From:	Juri Lelli <juri.lelli@....com>
To:	Mark Brown <broonie@...nel.org>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, vincent.guittot@...aro.org,
	robh+dt@...nel.org, mark.rutland@....com, linux@....linux.org.uk,
	sudeep.holla@....com, lorenzo.pieralisi@....com,
	catalin.marinas@....com, will.deacon@....com,
	morten.rasmussen@....com, dietmar.eggemann@....com
Subject: Re: [RFC PATCH v2 0/4] CPUs capacity information for heterogeneous
 systems

On 19/01/16 17:50, Mark Brown wrote:
> On Tue, Jan 19, 2016 at 04:05:51PM +0100, Peter Zijlstra wrote:
> > On Fri, Jan 08, 2016 at 02:09:28PM +0000, Juri Lelli wrote:
> 
> > >     cons: - not easy to come up with a clean solution, as it seems interaction
> > >             with several subsystems (e.g., cpufreq) is required
> > >           - not easy to agree upon a single benchmark (that has to be both
> > >             representative and simple enough to run at boot)
> > >           - numbers might (and do) vary from boot to boot
> 
> > This last point is a total pain for benchmarking, it means nothing is
> > every reproducible.
> 
> > Therefore, I would always augment the above (2) with the below (3), such
> > that you can overwrite the results with a known stable set of numbers:
> 
> The suggestion when the previous version was being discussed was that
> there are supposed to be some other knobs one uses for tuning and one
> was never supposed to use these numbers.

Right, DT solution might live without a sysfs interface, as you want to
use those other knobs for runtime tuning. Dynamic solution instead, and
I think this is what Peter was also pointing out, will most probably
require a sysfs interface for cases in which variation of default values
from boot to boot is not acceptable.