linux-kernel - [PATCH] cpu/hotplug: Cache number of online CPUs

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Message-ID: <alpine.DEB.2.21.1907042237010.1802@nanos.tec.linutronix.de>
Date:   Thu, 4 Jul 2019 22:42:17 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     LKML <linux-kernel@...r.kernel.org>
cc:     x86@...nel.org, Nadav Amit <namit@...are.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
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@...utronix.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.
  */