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

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Date:	Tue, 19 Jan 2016 16:05:51 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Juri Lelli <juri.lelli@....com>
Cc:	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,
	broonie@...nel.org
Subject: Re: [RFC PATCH v2 0/4] CPUs capacity information for heterogeneous
 systems

On Fri, Jan 08, 2016 at 02:09:28PM +0000, Juri Lelli wrote:
> Second version of this RFC proposes an alternative solution (w.r.t. v1) to the
> problem of how do we init CPUs original capacity: we run a bogus benchmark (for
> this RFC I simple stole int_sqrt from lib/ and I run that in a loop to perform
> some integer computation, I'm sure there are better benchmarks around) on the
> first cpu of each frequency domain (assuming no u-arch differences inside
> domains), measure time to complete a fixed number of iterations and then
> normalize results to SCHED_CAPACITY_SCALE (1024). I didn't spend much time in
> polishing this up or thinking about a better benchmark, as this is an RFC and
> I'd like discussion happening before we make this solution better
> working/looking. However, surprisingly, results are not that bad already:

>  2. Dynamic profiling at boot (v2)
> 
>     pros: - does not require a standardized definition of capacity
>           - cannot be incorrectly tuned (once benchmark is fixed)
>           - does not require user/integrator work
> 
>     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:

>  3. sysfs (v1)
> 
>     pros: - clean and super easy to implement
>           - values don't require to be physical properties, defining them is
>             probably easier
> 
>     cons: - CPUs capacity have to be provided after boot (by some init script?)
>           - API is modified, still some discussion/review is needed
>           - values can still be incorrectly used for runtime tuning purposes