[PATCH v3] powerpc/numa: Restrict possible nodes based on platform

[Srikar Dronamraju <srikar@linux.vnet.ibm.com>]

As per draft LoPAPR (Revision 2.9_pre7), section B.5.3 "Run Time Abstaction
Services (RTAS) Node at
https://openpowerfoundation.org/wp-content/uploads/2020/07/LoPAR-20200611.pdf,
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. If the said property is not
available, use ibm,max-associativity-domain as fallback. In this yet to be
released LoPAPR, ibm,current-associativity-domains is mentioned in page 833
/ B.5.3 which is covered under under "Appendix B. System Binding" section

Currently Powerpc 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.

Nathan Lynch had raised a valid concern that post LPM, a user could DLPAR
add processors and memory after LPM with "new" associativity properties.
https://lore.kernel.org/linuxppc-dev/871rljfet9.fsf@linux.ibm.com/t/#u

He also pointed out that ibm,max-associativity-domains has the same contents
on all currently available PowerVM systems, unlike
ibm,current-associativity-domains and hence may be better able to handle the
new numa associativity properties.

However with the recent commit dbce45628085 ("powerpc/numa: Limit possible
nodes to within num_possible_nodes"), all new numa  associativity properties
are capped to initially set nr_node_ids. Hence this commit should be safe
with any new dlpar add post LPM.

$ 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 <nathanl@linux.ibm.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: linuxppc-dev@lists.ozlabs.org
Cc: Anton Blanchard <anton@ozlabs.org>
Cc: Bharata B Rao <bharata@linux.ibm.com>
Cc: Tyrel Datwyler <tyreld@linux.ibm.com>
Signed-off-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
---
Changelog v2->v3:
v2: https://lore.kernel.org/linuxppc-dev/20200707140644.7241-1-srikar@linux.vnet.ibm.com/t/#u
Updated commit msg to mention LoPAPR reference section  and url details
					(Michael Ellerman)
Updated commit msg to discuss about possible new numa associativity properties.
					(Nathan Lynch)

Changelog v1->v2:
v1: https://lore.kernel.org/linuxppc-dev/20200706064002.14848-1-srikar@linux.vnet.ibm.com/t/#u
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