[PATCH v2 4/4] arm64: Dereference CPU operations indirectly

[Gavin Shan <gshan@redhat.com>]

One CPU operations is maintained through array @cpu_ops[NR_CPUS]. 2KB
memory is consumed when CONFIG_NR_CPUS is set to 256. It seems too
much memory has been used for this.

This introduces another array (@cpu_ops_index[NR_CPUS/4]), of which the
index to CPU operations array is stored. With this, we just need one byte
to be shared by 4 CPUs, 64 bytes for 256 CPUs, to dereference the CPU
operations indirectly. Note this optimization has the assumption: these
CPU operations aren't dereferenced in hot path.

Signed-off-by: Gavin Shan <gshan@redhat.com>
---
 arch/arm64/kernel/cpu_ops.c | 49 ++++++++++++++++++-------------------
 1 file changed, 24 insertions(+), 25 deletions(-)

diff --git a/arch/arm64/kernel/cpu_ops.c b/arch/arm64/kernel/cpu_ops.c
index e133011f64b5..d9103d5c9c6f 100644
--- a/arch/arm64/kernel/cpu_ops.c
+++ b/arch/arm64/kernel/cpu_ops.c
@@ -4,7 +4,6 @@
  *
  * Copyright (C) 2013 ARM Ltd.
  */
-
 #include <linux/acpi.h>
 #include <linux/cache.h>
 #include <linux/errno.h>
@@ -20,39 +19,32 @@ extern const struct cpu_operations acpi_parking_protocol_ops;
 #endif
 extern const struct cpu_operations cpu_psci_ops;
 
-static const struct cpu_operations *cpu_ops[NR_CPUS] __ro_after_init;
-
-static const struct cpu_operations *const dt_supported_cpu_ops[] __initconst = {
+/*
+ * Each element of the index array is shared by 4 CPUs. It means each
+ * CPU index uses 2 bits.
+ */
+static const struct cpu_operations *const cpu_ops[] = {
 	&smp_spin_table_ops,
-	&cpu_psci_ops,
-	NULL,
-};
-
-static const struct cpu_operations *const acpi_supported_cpu_ops[] __initconst = {
 #ifdef CONFIG_ARM64_ACPI_PARKING_PROTOCOL
 	&acpi_parking_protocol_ops,
 #endif
 	&cpu_psci_ops,
-	NULL,
 };
+static unsigned char cpu_ops_indexes[DIV_ROUND_UP(NR_CPUS, 4)] __ro_after_init;
 
-static const struct cpu_operations * __init cpu_get_ops(const char *name)
+static int __init get_cpu_ops_index(const char *name)
 {
-	const struct cpu_operations *const *ops;
-
-	ops = acpi_disabled ? dt_supported_cpu_ops : acpi_supported_cpu_ops;
-
-	while (*ops) {
-		if (!strcmp(name, (*ops)->name))
-			return *ops;
+	int index;
 
-		ops++;
+	for (index = 0; index < ARRAY_SIZE(cpu_ops); index++) {
+		if (!strcmp(cpu_ops[index]->name, name))
+			return index;
 	}
 
-	return NULL;
+	return -ERANGE;
 }
 
-static const char *__init cpu_read_enable_method(int cpu)
+static const char *__init get_cpu_method(int cpu)
 {
 	const char *enable_method;
 
@@ -98,21 +90,28 @@ static const char *__init cpu_read_enable_method(int cpu)
  */
 int __init init_cpu_ops(int cpu)
 {
-	const char *enable_method = cpu_read_enable_method(cpu);
+	const char *enable_method = get_cpu_method(cpu);
+	unsigned char *pindex = &cpu_ops_indexes[cpu / 4];
+	int index;
 
 	if (!enable_method)
 		return -ENODEV;
 
-	cpu_ops[cpu] = cpu_get_ops(enable_method);
-	if (!cpu_ops[cpu]) {
+	index = get_cpu_ops_index(enable_method);
+	if (index < 0) {
 		pr_warn("Unsupported enable-method: %s\n", enable_method);
 		return -EOPNOTSUPP;
 	}
 
+	*pindex &= ~(0x3 << (2 * (cpu % 4)));
+	*pindex |= ((index & 0x3) << (2 * (cpu % 4)));
+
 	return 0;
 }
 
 const struct cpu_operations *get_cpu_ops(int cpu)
 {
-	return cpu_ops[cpu];
+	int index = ((cpu_ops_indexes[cpu / 4] >> (2 * (cpu % 4))) & 0x3);
+
+	return cpu_ops[index];
 }
-- 
2.23.0


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