Re: [PATCH 3/3] 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.

Suggested-by: Michael Ellerman <mpe@ellerman.id.au>
Cc: linuxppc-dev@lists.ozlabs.org
Cc: qemu-ppc@nongnu.org
Cc: Cedric Le Goater <clg@kaod.org>
Cc: David Gibson <redacted>
Cc: Nathan Lynch <redacted>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Valentin Schneider <redacted>
Cc: Gautham R Shenoy <redacted>
Reported-by: Daniel Henrique Barboza <redacted>
Signed-off-by: Srikar Dronamraju <redacted>
---
 arch/powerpc/include/asm/smp.h |  1 +
 arch/powerpc/kernel/prom.c     | 19 +++++++++++++++----
 arch/powerpc/kernel/smp.c      | 21 +++++++++++++++++++--
 3 files changed, 35 insertions(+), 6 deletions(-)

diff --git a/arch/powerpc/include/asm/smp.h b/arch/powerpc/include/asm/smp.h
index 47081a9e13ca..03b3d010cbab 100644
--- a/arch/powerpc/include/asm/smp.h
+++ b/arch/powerpc/include/asm/smp.h

@@ -31,6 +31,7 @@ extern u32 *cpu_to_phys_id;
 extern bool coregroup_enabled;

 extern int cpu_to_chip_id(int cpu);
+extern int *chip_id_lookup_table;

 #ifdef CONFIG_SMP

diff --git a/arch/powerpc/kernel/prom.c b/arch/powerpc/kernel/prom.c
index 9a4797d1d40d..6d2e4a5bc471 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);

diff --git a/arch/powerpc/kernel/smp.c b/arch/powerpc/kernel/smp.c
index 5c7ce1d50631..50520fbea424 100644
--- a/arch/powerpc/kernel/smp.c
+++ b/arch/powerpc/kernel/smp.c

@@ -1073,6 +1073,20 @@ void __init smp_prepare_cpus(unsigned int max_cpus)
 				cpu_smallcore_mask(boot_cpuid));
 	}

+	if (cpu_to_chip_id(boot_cpuid) != -1) {
+		int idx = num_possible_cpus() / threads_per_core;
+
+		/*
+		 * All threads of a core will all belong to the same core,
+		 * chip_id_lookup_table will have one entry per core.
+		 * Assumption: if boot_cpuid doesn't have a chip-id, then no
+		 * other CPUs, will also not have chip-id.
+		 */
+		chip_id_lookup_table = kcalloc(idx, sizeof(int), GFP_KERNEL);
+		if (chip_id_lookup_table)
+			memset(chip_id_lookup_table, -1, sizeof(int) * idx);
+	}
+
 	if (smp_ops && smp_ops->probe)
 		smp_ops->probe();
 }

@@ -1468,8 +1482,8 @@ static void add_cpu_to_masks(int cpu)
 {
 	struct cpumask *(*submask_fn)(int) = cpu_sibling_mask;
 	int first_thread = cpu_first_thread_sibling(cpu);
-	int chip_id = cpu_to_chip_id(cpu);
 	cpumask_var_t mask;
+	int chip_id = -1;
 	bool ret;
 	int i;

@@ -1492,7 +1506,10 @@ static void add_cpu_to_masks(int cpu)
 	if (has_coregroup_support())
 		update_coregroup_mask(cpu, &mask);

-	if (chip_id == -1 || !ret) {
+	if (chip_id_lookup_table && ret)
+		chip_id = cpu_to_chip_id(cpu);
+
+	if (chip_id == -1) {
 		cpumask_copy(per_cpu(cpu_core_map, cpu), cpu_cpu_mask(cpu));
 		goto out;
 	}

-- 
2.25.1