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Subject[PATCH] cpu/hotplug: Cache number of online CPUs
Revaluating the bitmap wheight of the online cpus bitmap in every
invocation of num_online_cpus() over and over is a pretty useless
exercise. Especially when num_online_cpus() is used in code pathes like the
IPI delivery of x86 or the membarrier code.

Cache the number of online CPUs in the core and just return the cached
variable.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
---
include/linux/cpumask.h | 16 +++++++---------
kernel/cpu.c | 16 ++++++++++++++++
2 files changed, 23 insertions(+), 9 deletions(-)

--- a/include/linux/cpumask.h
+++ b/include/linux/cpumask.h
@@ -95,8 +95,13 @@ extern struct cpumask __cpu_active_mask;
#define cpu_present_mask ((const struct cpumask *)&__cpu_present_mask)
#define cpu_active_mask ((const struct cpumask *)&__cpu_active_mask)

+extern unsigned int __num_online_cpus;
+
#if NR_CPUS > 1
-#define num_online_cpus() cpumask_weight(cpu_online_mask)
+static inline unsigned int num_online_cpus(void)
+{
+ return __num_online_cpus;
+}
#define num_possible_cpus() cpumask_weight(cpu_possible_mask)
#define num_present_cpus() cpumask_weight(cpu_present_mask)
#define num_active_cpus() cpumask_weight(cpu_active_mask)
@@ -821,14 +826,7 @@ set_cpu_present(unsigned int cpu, bool p
cpumask_clear_cpu(cpu, &__cpu_present_mask);
}

-static inline void
-set_cpu_online(unsigned int cpu, bool online)
-{
- if (online)
- cpumask_set_cpu(cpu, &__cpu_online_mask);
- else
- cpumask_clear_cpu(cpu, &__cpu_online_mask);
-}
+void set_cpu_online(unsigned int cpu, bool online);

static inline void
set_cpu_active(unsigned int cpu, bool active)
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -2291,6 +2291,9 @@ EXPORT_SYMBOL(__cpu_present_mask);
struct cpumask __cpu_active_mask __read_mostly;
EXPORT_SYMBOL(__cpu_active_mask);

+unsigned int __num_online_cpus __read_mostly;
+EXPORT_SYMBOL(__num_online_cpus);
+
void init_cpu_present(const struct cpumask *src)
{
cpumask_copy(&__cpu_present_mask, src);
@@ -2306,6 +2309,19 @@ void init_cpu_online(const struct cpumas
cpumask_copy(&__cpu_online_mask, src);
}

+void set_cpu_online(unsigned int cpu, bool online)
+{
+ lockdep_assert_cpus_held();
+
+ if (online) {
+ if (!cpumask_test_and_set_cpu(cpu, &__cpu_online_mask))
+ __num_online_cpus++;
+ } else {
+ if (cpumask_test_and_clear_cpu(cpu, &__cpu_online_mask))
+ __num_online_cpus--;
+ }
+}
+
/*
* Activate the first processor.
*/
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