Subject: Re: [RFC PATCH 8/8] powerpc/papr_scm: Use FORM2 associativity details

From: David Gibson <[email-masked]>
Date: Tue, 15 Jun 2021 16:34:56 +1000

On Tue, Jun 15, 2021 at 11:27:50AM +0530, Aneesh Kumar K.V wrote:
> David Gibson <[email-masked]> writes:
> 
> > On Mon, Jun 14, 2021 at 10:10:03PM +0530, Aneesh Kumar K.V wrote:
> >> FORM2 introduce a concept of secondary domain which is identical to the
> >> conceept of FORM1 primary domain. Use secondary domain as the numa node
> >> when using persistent memory device. For DAX kmem use the logical domain
> >> id introduced in FORM2. This new numa node
> >> 
> >> Signed-off-by: Aneesh Kumar K.V <[email-masked]>
> >> ---
> >>  arch/powerpc/mm/numa.c                    | 28 +++++++++++++++++++++++
> >>  arch/powerpc/platforms/pseries/papr_scm.c | 26 +++++++++++++--------
> >>  arch/powerpc/platforms/pseries/pseries.h  |  1 +
> >>  3 files changed, 45 insertions(+), 10 deletions(-)
> >> 
> >> diff --git a/arch/powerpc/mm/numa.c b/arch/powerpc/mm/numa.c
> >> index 86cd2af014f7..b9ac6d02e944 100644
> >> --- a/arch/powerpc/mm/numa.c
> >> +++ b/arch/powerpc/mm/numa.c
> >> @@ -265,6 +265,34 @@ static int associativity_to_nid(const __be32 *associativity)
> >>  	return nid;
> >>  }
> >>  
> >> +int get_primary_and_secondary_domain(struct device_node *node, int *primary, int *secondary)
> >> +{
> >> +	int secondary_index;
> >> +	const __be32 *associativity;
> >> +
> >> +	if (!numa_enabled) {
> >> +		*primary = NUMA_NO_NODE;
> >> +		*secondary = NUMA_NO_NODE;
> >> +		return 0;
> >> +	}
> >> +
> >> +	associativity = of_get_associativity(node);
> >> +	if (!associativity)
> >> +		return -ENODEV;
> >> +
> >> +	if (of_read_number(associativity, 1) >= primary_domain_index) {
> >> +		*primary = of_read_number(&associativity[primary_domain_index], 1);
> >> +		secondary_index = of_read_number(&distance_ref_points[1], 1);
> >
> > Secondary ID is always the second reference point, but primary depends
> > on the length of resources?  That seems very weird.
> 
> primary_domain_index is distance_ref_point[0]. With Form2 we would find
> both primary and secondary domain ID same for all resources other than
> persistent memory device. The usage w.r.t. persistent memory is
> explained in patch 7.

Right, I misunderstood

> 
> With Form2 the primary domainID and secondary domainID are used to identify the NUMA nodes
> the kernel should use when using persistent memory devices.

This seems kind of bogus.  With Form1, the primary/secondary ID are a
sort of heirarchy of distance (things with same primary ID are very
close, things with same secondary are kinda-close, etc.).  With Form2,
it's referring to their effective node for different purposes.

Using the same terms for different meanings seems unnecessarily
confusing.

> Persistent memory devices
> can also be used as regular memory using DAX KMEM driver and primary domainID indicates
> the numa node number OS should use when using these devices as regular memory. Secondary
> domainID is the numa node number that should be used when using this device as
> persistent memory.

It's weird to me that you'd want to consider them in different nodes
for those different purposes.

> In the later case, we are interested in the locality of the
> device to an established numa node. In the above example, if the last row represents a
> persistent memory device/resource, NUMA node number 40 will be used when using the device
> as regular memory and NUMA node number 0 will be the device numa node when using it as
> a persistent memory device.

I don't really get what you mean by "locality of the device to an
established numa node".  Or at least how that's different from
anything else we're handling here.

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson