context_tracking: Ground setup for static key use

Prepare for using a static key in the context tracking subsystem.
This will help optimizing the off case on its many users:

* user_enter, user_exit, exception_enter, exception_exit, guest_enter,
  guest_exit, vtime_*()

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Li Zhong <zhong@linux.vnet.ibm.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Kevin Hilman <khilman@linaro.org>

include/linux/context_tracking.h

@@ -4,6 +4,7 @@
 #include <linux/sched.h>
 #include <linux/percpu.h>
 #include <linux/vtime.h>
+#include <linux/static_key.h>
 #include <asm/ptrace.h>
 
 struct context_tracking {
@@ -22,6 +23,7 @@ struct context_tracking {
 
 
 #ifdef CONFIG_CONTEXT_TRACKING
+extern struct static_key context_tracking_enabled;
 DECLARE_PER_CPU(struct context_tracking, context_tracking);
 
 static inline bool context_tracking_in_user(void)
@@ -67,6 +69,14 @@ static inline void context_tracking_task_switch(struct task_struct *prev,
 						struct task_struct *next) { }
 #endif /* !CONFIG_CONTEXT_TRACKING */
 
+
+#ifdef CONFIG_CONTEXT_TRACKING_FORCE
+extern void context_tracking_init(void);
+#else
+static inline void context_tracking_init(void) { }
+#endif /* CONFIG_CONTEXT_TRACKING_FORCE */
+
+
 #ifdef CONFIG_VIRT_CPU_ACCOUNTING_GEN
 extern void guest_enter(void);
 extern void guest_exit(void);

init/main.c

@@ -75,6 +75,7 @@
 #include <linux/blkdev.h>
 #include <linux/elevator.h>
 #include <linux/sched_clock.h>
+#include <linux/context_tracking.h>
 
 #include <asm/io.h>
 #include <asm/bugs.h>
@@ -545,6 +546,7 @@ asmlinkage void __init start_kernel(void)
 	idr_init_cache();
 	rcu_init();
 	tick_nohz_init();
+	context_tracking_init();
 	radix_tree_init();
 	/* init some links before init_ISA_irqs() */
 	early_irq_init();

kernel/context_tracking.c

@@ -20,15 +20,16 @@
 #include <linux/hardirq.h>
 #include <linux/export.h>
 
-DEFINE_PER_CPU(struct context_tracking, context_tracking) = {
-#ifdef CONFIG_CONTEXT_TRACKING_FORCE
-	.active = true,
-#endif
-};
+struct static_key context_tracking_enabled = STATIC_KEY_INIT_FALSE;
+
+DEFINE_PER_CPU(struct context_tracking, context_tracking);
 
 void context_tracking_cpu_set(int cpu)
 {
-	per_cpu(context_tracking.active, cpu) = true;
+	if (!per_cpu(context_tracking.active, cpu)) {
+		per_cpu(context_tracking.active, cpu) = true;
+		static_key_slow_inc(&context_tracking_enabled);
+	}
 }
 
 /**
@@ -202,3 +203,13 @@ void context_tracking_task_switch(struct task_struct *prev,
 	clear_tsk_thread_flag(prev, TIF_NOHZ);
 	set_tsk_thread_flag(next, TIF_NOHZ);
 }
+
+#ifdef CONFIG_CONTEXT_TRACKING_FORCE
+void __init context_tracking_init(void)
+{
+	int cpu;
+
+	for_each_possible_cpu(cpu)
+		context_tracking_cpu_set(cpu);
+}
+#endif