Date: Tue, 15 Dec 2015 15:46:51 +0000
From: Juri Lelli <juri.lelli@arm.com>
To: Mark Rutland <mark.rutland@arm.com>
Cc: Mark Brown <broonie@kernel.org>, Rob Herring <robh@kernel.org>,
        linux-kernel@vger.kernel.org, linux-pm@vger.kernel.org,
        linux-arm-kernel@lists.infradead.org, devicetree@vger.kernel.org,
        peterz@infradead.org, vincent.guittot@linaro.org,
        linux@arm.linux.org.uk, sudeep.holla@arm.com,
        lorenzo.pieralisi@arm.com, catalin.marinas@arm.com,
        will.deacon@arm.com, morten.rasmussen@arm.com,
        dietmar.eggemann@arm.com, Pawel Moll <pawel.moll@arm.com>,
        Ian Campbell <ijc+devicetree@hellion.org.uk>,
        Kumar Gala <galak@codeaurora.org>,
        Maxime Ripard <maxime.ripard@free-electrons.com>,
        Olof Johansson <olof@lixom.net>,
        Gregory CLEMENT <gregory.clement@free-electrons.com>,
        Paul Walmsley <paul@pwsan.com>,
        Linus Walleij <linus.walleij@linaro.org>, Chen-Yu Tsai <wens@csie.org>,
        Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Subject: Re: [RFC PATCH 2/8] Documentation: arm: define DT cpu capacity
 bindings
Message-ID: <20151215154651.GK16007@e106622-lin>
References: <20151210153004.GA26758@sirena.org.uk>
 <20151210175820.GE14571@e106622-lin>
 <20151211174940.GQ5727@sirena.org.uk>
 <20151214123616.GC3308@e106622-lin>
 <20151214165928.GV5727@sirena.org.uk>
 <20151215122238.GG16007@e106622-lin>
 <20151215133951.GY5727@sirena.org.uk>
 <20151215140135.GI31299@leverpostej>
 <20151215150813.GZ5727@sirena.org.uk>
 <20151215153218.GA7228@leverpostej>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20151215153218.GA7228@leverpostej>
User-Agent: Mutt/1.5.21 (2010-09-15)
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 2432
Lines: 56

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
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/