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

[Juri Lelli <juri.lelli@arm.com>]

Hi,

On 15/12/15 17:45, Mark Brown wrote:
> On Tue, Dec 15, 2015 at 05:28:37PM +0000, Mark Rutland wrote:
> > On Tue, Dec 15, 2015 at 05:17:13PM +0000, Mark Brown wrote:
> 
> > > Obviously people are going to get upset if we introduce performance
> > > regressions - but that's true always, we can also introduce problems
> > > with numbers people have put in DT.  It seems like it'd be harder to
> > > manage regressions due to externally provided magic numbers since
> > > there's inherently less information there.
> 
> > It's certainly still possible to have regressions in that case. Those
> > regressions would be due to code changes in the kernel, given the DT
> > didn't change.
> 
> > I'm not sure I follow w.r.t. "inherently less information", unless you
> > mean trying to debug without access to that DTB?
> 
> If what the kernel knows about the system is that it's got a bunch of
> cores with numbers assigned to them then all it's really got is those
> numbers.  If something changes that causes problems for some systems
> (eg, because the numbers have been picked poorly but in a way that
> happened to work well with the old code) that's not a lot to go on, the
> more we know about the system the more likely it is that we'll be able
> to adjust the assumptions in whatever new thing we do that causes
> problems for any particular systems where we run into trouble.
> 
> > > My point there is that if we're not that concerned about the specific
> > > number something in kernel is safer.
> 
> > I don't entirely disagree there. I think an in-kernel benchmark is
> > likely safer.
> 
> Yes, I think that something where we just observe the system performance
> at runtime is likely one of the best solutions if we can get something
> that gives reasonable results.
> 
> > > That does have the issue that we need to scale with regard to the
> > > frequency the benchmark gets run at.  That's not an insurmountable
> > > obstacle but it's not completely trivial either.
> 
> > If we change clock frequency, then regardless of where the information
> > comes from we need to perform scaling, no?
> 
> Yes, it's just a question of making the benchmarking bit talk to the
> scaling bit so we know where we're at when we do the benchmark.  Like I
> say it should be doable.
> 
> > One nice thing about doing a benchmark to derive the numbers is that
> > when the kernel is that when the frequency is fixed but the kernel
> > cannot query it, the numbers will be representative.
> 
> Definitely.

OK, let's see how a dynamic approach could look like. As said, since it
was actually our first thought too, I already have a possible
implementation of such a thing. I'll be OOO until early Jan, but I'll
try to rebase what I have and post it here as soon as I'm back; and then
we see which solution looks better.

Thanks a lot for the feedback!

Best,

- Juri