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

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Message-ID: <20160426135354.GR27312@arm.com>
Date:	Tue, 26 Apr 2016 14:53:54 +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,
	David Daney <ddaney@...iumnetworks.com>
Subject: Re: [PATCH v2 0/5] Cavium ThunderX uncore PMU support

On Tue, Apr 26, 2016 at 02:08:09PM +0200, Jan Glauber wrote:
> On Mon, Apr 25, 2016 at 02:19:07PM +0100, Will Deacon wrote:
> > On Mon, Apr 25, 2016 at 02:02:22PM +0200, Jan Glauber wrote:
> > > On Mon, Apr 25, 2016 at 12:22:07PM +0100, Will Deacon wrote:
> > > > On Mon, Apr 04, 2016 at 02:19:54PM +0200, Jan Glauber wrote:
> > > > > can you have a look at these patches?
> > > > 
> > > > Looks like Mark reviewed this last week -- are you planning to respin?
> > > 
> > > Yes, of course. I just had no time yet and I'm a bit lost on how to
> > > proceed without using the NUMA node information which Mark did not like
> > > to be used.
> > > 
> > > The only way to know which device is on which node would be to look
> > > at the PCI topology (which is also the source of the NUMA node_id).
> > > We could do this manually in order to not depend on CONFIG_NUMA,
> > > but I would like to know if that is acceptable before respinning the
> > > patches.
> > 
> > That doesn't feel like it really addresses Mark's concerns -- it's just
> > another way to get the information that isn't a first-class PMU topology
> > description from firmware.
> > 
> > Now, I don't actually mind using the NUMA topology so much in the cases
> > where it genuinely correlates with the PMU topology. My objection is more
> > that we end up sticking everything on node 0 if !CONFIG_NUMA, which could
> > result in working with an incorrect PMU topology and passing all of that
> > through to userspace.
> > 
> > So I'd prefer either making the driver depend on NUMA, or at the very least
> > failing to probe  the PMU if we discover a socketed system and NUMA is not
> > selected. Do either of those work as a compromise?
> > 
> > Will
> 
> That sounds like a good compromise.
> 
> So I could do the following:
> 
> 1) In the uncore setup check for CONFIG_NUMA, if set use the NUMA
>    information to determine the device node
> 
> 2) If CONFIG_NUMA is not set we check if we run on a socketed system
> 
>    a) In that case we return an error and give a message that CONFIG_NUMA needs
>       to be enabled
>    b) Otherwise we have a single node system and use node_id = 0

That sounds sensible to me. How do you "check if we run on a socketed
system"? My assumption would be that you could figure this out from the
firmware tables?

> David noted that it would also be possible to extract the node id from
> the physical address of the device, but I'm not sure that classifies as
> 'first-class' topology description...

I'd rather avoid this sort of probing, as it inevitably breaks when it
sees new hardware that doesn't follow the unwritten assumptions of the
old hardware.

Will