As per PAPR, there are 2 device tree property
ibm,max-associativity-domains (which defines the maximum number of
domains that the firmware i.e PowerVM can support) and
ibm,current-associativity-domains (which defines the maximum number of
domains that the platform can support). Value of
ibm,max-associativity-domains property is always greater than or equal
to ibm,current-associativity-domains property.

Powerpc currently uses ibm,max-associativity-domains  property while
setting the possible number of nodes. This is currently set at 32.
However the possible number of nodes for a platform may be significantly
less. Hence set the possible number of nodes based on
ibm,current-associativity-domains property.

$ lsprop /proc/device-tree/rtas/ibm,*associ*-domains
/proc/device-tree/rtas/ibm,current-associativity-domains
		 00000005 00000001 00000002 00000002 00000002 00000010
/proc/device-tree/rtas/ibm,max-associativity-domains
		 00000005 00000001 00000008 00000020 00000020 00000100

$ cat /sys/devices/system/node/possible ##Before patch
0-31

$ cat /sys/devices/system/node/possible ##After patch
0-1

Note the maximum nodes this platform can support is only 2 but the
possible nodes is set to 32.

This is important because lot of kernel and user space code allocate
structures for all possible nodes leading to a lot of memory that is
allocated but not used.

I ran a simple experiment to create and destroy 100 memory cgroups on
boot on a 8 node machine (Power8 Alpine).

Before patch
free -k at boot
              total        used        free      shared  buff/cache   available
Mem:      523498176     4106816   518820608       22272      570752   516606720
Swap:       4194240           0     4194240

free -k after creating 100 memory cgroups
              total        used        free      shared  buff/cache   available
Mem:      523498176     4628416   518246464       22336      623296   516058688
Swap:       4194240           0     4194240

free -k after destroying 100 memory cgroups
              total        used        free      shared  buff/cache   available
Mem:      523498176     4697408   518173760       22400      627008   515987904
Swap:       4194240           0     4194240

After patch
free -k at boot
              total        used        free      shared  buff/cache   available
Mem:      523498176     3969472   518933888       22272      594816   516731776
Swap:       4194240           0     4194240

free -k after creating 100 memory cgroups
              total        used        free      shared  buff/cache   available
Mem:      523498176     4181888   518676096       22208      640192   516496448
Swap:       4194240           0     4194240

free -k after destroying 100 memory cgroups
              total        used        free      shared  buff/cache   available
Mem:      523498176     4232320   518619904       22272      645952   516443264
Swap:       4194240           0     4194240

Observations:
Fixed kernel takes 137344 kb (4106816-3969472) less to boot.
Fixed kernel takes 309184 kb (4628416-4181888-137344) less to create 100 memcgs.

Cc: Nathan Lynch <redacted>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: linuxppc-dev@lists.ozlabs.org
Cc: Anton Blanchard <redacted>
Cc: Bharata B Rao <redacted>
Cc: Tyrel Datwyler <tyreld@linux.ibm.com>
Signed-off-by: Srikar Dronamraju <redacted>
---
v1: https://lore.kernel.org/linuxppc-dev/20200706064002.14848-1-srikar@linux.vnet.ibm.com/t/#u (local)
Changelog v1->v2:
Fallback to ibm,max-associativity-domains if ibm,current-associativity
is not available. Suggested by Tyrel Datwyler.

 arch/powerpc/mm/numa.c | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

diff --git a/arch/powerpc/mm/numa.c b/arch/powerpc/mm/numa.c
index 9fcf2d195830..fc7b0505bdd8 100644
--- a/arch/powerpc/mm/numa.c
+++ b/arch/powerpc/mm/numa.c

@@ -896,10 +896,19 @@ static void __init find_possible_nodes(void)
 	if (!rtas)
 		return;
 
-	if (of_property_read_u32_index(rtas,
-				"ibm,max-associativity-domains",
+	if (of_property_read_u32_index(rtas, "ibm,current-associativity-domains",
+				min_common_depth, &numnodes)) {
+		/*
+		 * ibm,current-associativity-domains is a fairly recent
+		 * property. If it doesn't exist, then fallback on
+		 * ibm,max-associativity-domains. Current denotes what the
+		 * platform can support compared to max which denotes what the
+		 * Hypervisor can support.
+		 */
+		if (of_property_read_u32_index(rtas, "ibm,max-associativity-domains",
 				min_common_depth, &numnodes))
-		goto out;
+			goto out;
+	}
 
 	for (i = 0; i < numnodes; i++) {
 		if (!node_possible(i))

-- 
2.18.2