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

[Mark Rutland]

On Wed, Apr 27, 2016 at 12:51:56PM +0200, Jan Glauber wrote:
> On Tue, Apr 26, 2016 at 02:53:54PM +0100, Will Deacon wrote:
> 
> [...]
> 
> > > 
> > > 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?
> 
> There are probably multiple ways to detect a socketed system, with some quite
> hardware specific. I would like to avoid parsing DT (and ACPI) though,
> if possible.
> 
> A generic approach would be to do a query of the multiprocessor affinity
> register (MPIDR_EL1) on all CPUs. The AFF2 part (bits 23:16) contains the 
> socket number on ThunderX. If this is non-zero on any CPU I would assume a
> socketed system.
> 
> Would that be feasible?

As with checking the physical address of a peripheral, this is an
unwritten assumption, and I suspect that similarly, it will inevitably
break (e.g. if Aff3 becomes used).

If you expect kernels relevant to your platform to have NUMA support,
you can simply depend on NUMA to determine whether or not you have NUMA
nodes.

Regarding relying on NUMA nodes, I have two concerns:

In general a NUMA node is not necessarily a socket, as you can have NUMA
properties even within a socket. If you can guarantee that for your
platform NUMA nodes will always be sockets, then I guess using NUMA
nodes is ok, though I imagine that as with the physical address map and
organisation of CPU IDs, that's difficult to have set in stone.

Linux NUMA node IDs are arbitrary tokens, and may not necessarily idmap
to documented socket IDs for your platform (even if they happen to
today). If you're happy to have users figure out how those IDs map to
clusters, that's fine, but otherwise you need to expose additional
information such that users get what they expect (at which point, if you
have said information we probably don't need NUMA information).

Thanks,
Mark.