linux-kernel - Re: [PATCH v2 0/5] Cavium ThunderX uncore PMU support

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Date:	Mon, 4 Jul 2016 11:11:32 +0100
From:	Will Deacon <will.deacon@....com>
To:	Jan Glauber <jan.glauber@...iumnetworks.com>
Cc:	Mark Rutland <mark.rutland@....com>, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v2 0/5] Cavium ThunderX uncore PMU support

On Tue, Jun 28, 2016 at 04:04:59PM +0200, Jan Glauber wrote:
> On Tue, Jun 28, 2016 at 11:24:20AM +0100, Will Deacon wrote:
> > On Wed, Mar 09, 2016 at 05:21:02PM +0100, Jan Glauber wrote:
> > > This patch series provides access to various counters on the ThunderX SOC.
> > > 
> > > For details of the uncore implementation see patch #1.
> > > 
> > > Patches #2-5 add the various ThunderX specific PMUs.
> > > 
> > > As suggested I've put the files under drivers/perf/uncore. I would
> > > prefer this location over drivers/bus because not all of the uncore
> > > drivers are bus related.
> > 
> > What's the status of these patches? Were you planning to send a new
> > version?
>
> I was half-way through with addressing Mark's review comments when
> got side-tracked.
> 
> The principle question these patches raised remains open though in my
> opinion, how to determine the socket a device belongs to.
> 
> There is no first-class interface to ask a device or the firmware
> which socket the device lives on.
> 
> The options I see are:
> A) Using NUMA node information, depends on CONFIG_NUMA
> B) Decoding the socket bits of the PCI BAR address
> C) Using PCI topology information
> 
> A is what I tried, but I agree that depending on CONFIG_NUMA is not a good
> solution. B would be easy but looks not very future-proof. So option C
> is what is left...

Sorry to go full circle on this, but "depends on NUMA" sounds better
than deriving NUMA topology from PCI to me. The only worry I have is if
the NUMA information ends up being insufficient in the long-term, and we
end up with a mixture of the three options above in order to figure out
the PMU topology.

As long as you're happy that the PMU:NUMA topology remains 1:1, then I
have no objections. The moment you need extra hacks on the side, we should
probably drop the NUMA dependency altogether and figure it out some other
way.

Will