sched: Define sched_clock_irqtime as static key

JIRA: https://issues.redhat.com/browse/RHEL-78821

commit 8722903
Author: Yafang Shao <laoar.shao@gmail.com>
Date:   Fri Jan 3 10:24:06 2025 +0800

    sched: Define sched_clock_irqtime as static key

    Since CPU time accounting is a performance-critical path, let's define
    sched_clock_irqtime as a static key to minimize potential overhead.

    Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
    Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
    Reviewed-by: Michal Koutný <mkoutny@suse.com>
    Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
    Link: https://lore.kernel.org/r/20250103022409.2544-2-laoar.shao@gmail.com

Signed-off-by: Phil Auld <pauld@redhat.com>

Author: auldp

kernel/sched/cputime.c

@@ -5,6 +5,8 @@
 
 #ifdef CONFIG_IRQ_TIME_ACCOUNTING
 
+DEFINE_STATIC_KEY_FALSE(sched_clock_irqtime);
+
 /*
  * There are no locks covering percpu hardirq/softirq time.
  * They are only modified in vtime_account, on corresponding CPU
@@ -18,16 +20,14 @@
  */
 DEFINE_PER_CPU(struct irqtime, cpu_irqtime);
 
-static int sched_clock_irqtime;
-
 void enable_sched_clock_irqtime(void)
 {
-	sched_clock_irqtime = 1;
+	static_branch_enable(&sched_clock_irqtime);
 }
 
 void disable_sched_clock_irqtime(void)
 {
-	sched_clock_irqtime = 0;
+	static_branch_disable(&sched_clock_irqtime);
 }
 
 static void irqtime_account_delta(struct irqtime *irqtime, u64 delta,
@@ -53,7 +53,7 @@ void irqtime_account_irq(struct task_struct *curr, unsigned int offset)
 	s64 delta;
 	int cpu;
 
-	if (!sched_clock_irqtime)
+	if (!irqtime_enabled())
 		return;
 
 	cpu = smp_processor_id();
@@ -86,8 +86,6 @@ static u64 irqtime_tick_accounted(u64 maxtime)
 
 #else /* CONFIG_IRQ_TIME_ACCOUNTING */
 
-#define sched_clock_irqtime	(0)
-
 static u64 irqtime_tick_accounted(u64 dummy)
 {
 	return 0;
@@ -487,7 +485,7 @@ void account_process_tick(struct task_struct *p, int user_tick)
 	if (vtime_accounting_enabled_this_cpu())
 		return;
 
-	if (sched_clock_irqtime) {
+	if (irqtime_enabled()) {
 		irqtime_account_process_tick(p, user_tick, 1);
 		return;
 	}
@@ -516,7 +514,7 @@ void account_idle_ticks(unsigned long ticks)
 {
 	u64 cputime, steal;
 
-	if (sched_clock_irqtime) {
+	if (irqtime_enabled()) {
 		irqtime_account_idle_ticks(ticks);
 		return;
 	}

kernel/sched/sched.h

@@ -3029,6 +3029,12 @@ struct irqtime {
 };
 
 DECLARE_PER_CPU(struct irqtime, cpu_irqtime);
+DECLARE_STATIC_KEY_FALSE(sched_clock_irqtime);
+
+static inline int irqtime_enabled(void)
+{
+	return static_branch_likely(&sched_clock_irqtime);
+}
 
 /*
  * Returns the irqtime minus the softirq time computed by ksoftirqd.
@@ -3049,6 +3055,13 @@ static inline u64 irq_time_read(int cpu)
 	return total;
 }
 
+#else
+
+static inline int irqtime_enabled(void)
+{
+	return 0;
+}
+
 #endif /* CONFIG_IRQ_TIME_ACCOUNTING */
 
 #ifdef CONFIG_CPU_FREQ