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

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

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 <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>
Signed-off-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
---
 arch/powerpc/mm/numa.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/powerpc/mm/numa.c b/arch/powerpc/mm/numa.c
index 9fcf2d195830..3d55cef1a2dc 100644
--- a/arch/powerpc/mm/numa.c
+++ b/arch/powerpc/mm/numa.c
@@ -897,7 +897,7 @@ static void __init find_possible_nodes(void)
 		return;
 
 	if (of_property_read_u32_index(rtas,
-				"ibm,max-associativity-domains",
+				"ibm,current-associativity-domains",
 				min_common_depth, &numnodes))
 		goto out;
 
-- 
2.18.2