[PATCH v2] ppc64/numa: consider the max numa node for migratable LPAR

Laurent Dufour ldufour at linux.ibm.com
Tue May 11 17:31:36 AEST 2021

When a LPAR is migratable, we should consider the maximum possible NUMA
node instead the number of NUMA node from the actual system.

The DT property 'ibm,current-associativity-domains' is defining the maximum
number of nodes the LPAR can see when running on that box. But if the LPAR
is being migrated on another box, it may seen up to the nodes defined by
'ibm,max-associativity-domains'. So if a LPAR is migratable, that value
should be used.

Unfortunately, there is no easy way to know if a LPAR is migratable or
not. The hypervisor is exporting the property 'ibm,migratable-partition' in
the case it set to migrate partition, but that would not mean that the
current partition is migratable.

Without this patch, when a LPAR is started on a 2 nodes box and then
migrated to a 3 nodes box, the hypervisor may spread the LPAR's CPUs on the
3rd node. In that case if a CPU from that 3rd node is added to the LPAR, it
will be wrongly assigned to the node because the kernel has been set to use
up to 2 nodes (the configuration of the departure node). With this patch
applies, the CPU is correctly added to the 3rd node.

Fixes: f9f130ff2ec9 ("powerpc/numa: Detect support for coregroup")
Reviewed-by: Srikar Dronamraju <srikar at linux.vnet.ibm.com>
Signed-off-by: Laurent Dufour <ldufour at linux.ibm.com>
V2: Address Srikar's comments
 - Fix the commit message
 - Use pr_info instead printk(KERN_INFO..)
 arch/powerpc/mm/numa.c | 13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)

diff --git a/arch/powerpc/mm/numa.c b/arch/powerpc/mm/numa.c
index f2bf98bdcea2..094a1076fd1f 100644
--- a/arch/powerpc/mm/numa.c
+++ b/arch/powerpc/mm/numa.c
@@ -893,7 +893,7 @@ static void __init setup_node_data(int nid, u64 start_pfn, u64 end_pfn)
 static void __init find_possible_nodes(void)
 	struct device_node *rtas;
-	const __be32 *domains;
+	const __be32 *domains = NULL;
 	int prop_length, max_nodes;
 	u32 i;
@@ -909,9 +909,14 @@ static void __init find_possible_nodes(void)
 	 * 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 the LPAR is migratable, new nodes might be activated after a LPM,
+	 * so we should consider the max number in that case.
-	domains = of_get_property(rtas, "ibm,current-associativity-domains",
-					&prop_length);
+	if (!of_get_property(of_root, "ibm,migratable-partition", NULL))
+		domains = of_get_property(rtas,
+					  "ibm,current-associativity-domains",
+					  &prop_length);
 	if (!domains) {
 		domains = of_get_property(rtas, "ibm,max-associativity-domains",
@@ -920,6 +925,8 @@ static void __init find_possible_nodes(void)
 	max_nodes = of_read_number(&domains[min_common_depth], 1);
+	pr_info("Partition configured for %d NUMA nodes.\n", max_nodes);
 	for (i = 0; i < max_nodes; i++) {
 		if (!node_possible(i))
 			node_set(i, node_possible_map);

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