powerpc/smp: Cache CPU to chip lookup

On systems with large CPUs per node, even with the filtered matching of
related CPUs, there can be large number of calls to cpu_to_chip_id for
the same CPU. For example with 4096 vCPU, 1 node QEMU configuration,
with 4 threads per core, system could be see upto 1024 calls to
cpu_to_chip_id() for the same CPU. On a given system, cpu_to_chip_id()
for a given CPU would always return the same. Hence cache the result in
a lookup table for use in subsequent calls.

Since all CPUs sharing the same core will belong to the same chip, the
lookup_table has an entry for one CPU per core.  chip_id_lookup_table is
not being freed and would be used on subsequent CPU online post CPU
offline.

Reported-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Suggested-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Tested-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210415120934.232271-4-srikar@linux.vnet.ibm.com

1 files changed, 15 insertions, 4 deletions

diff --git a/arch/powerpc/kernel/prom.c b/arch/powerpc/kernel/prom.c
index a8b2d6bfc1ca..fbe9deebc8e1 100644
--- a/arch/powerpc/kernel/prom.c
+++ b/arch/powerpc/kernel/prom.c
@@ -65,6 +65,8 @@
 #define DBG(fmt...)
 #endif
 
+int *chip_id_lookup_table;
+
 #ifdef CONFIG_PPC64
 int __initdata iommu_is_off;
 int __initdata iommu_force_on;
@@ -914,13 +916,22 @@ EXPORT_SYMBOL(of_get_ibm_chip_id);
 int cpu_to_chip_id(int cpu)
 {
 	struct device_node *np;
+	int ret = -1, idx;
+
+	idx = cpu / threads_per_core;
+	if (chip_id_lookup_table && chip_id_lookup_table[idx] != -1)
+		return chip_id_lookup_table[idx];
 
 	np = of_get_cpu_node(cpu, NULL);
-	if (!np)
-		return -1;
+	if (np) {
+		ret = of_get_ibm_chip_id(np);
+		of_node_put(np);
+
+		if (chip_id_lookup_table)
+			chip_id_lookup_table[idx] = ret;
+	}
 
-	of_node_put(np);
-	return of_get_ibm_chip_id(np);
+	return ret;
 }
 EXPORT_SYMBOL(cpu_to_chip_id);