context_tracking: Optimize context switch off case with static keys

No need for syscall slowpath if no CPU is full dynticks,
rather nop this in this case.

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

@@ -40,6 +40,8 @@ extern void context_tracking_cpu_set(int cpu);
 
 extern void context_tracking_user_enter(void);
 extern void context_tracking_user_exit(void);
+extern void __context_tracking_task_switch(struct task_struct *prev,
+					   struct task_struct *next);
 
 static inline void user_enter(void)
 {
@@ -74,8 +76,12 @@ static inline void exception_exit(enum ctx_state prev_ctx)
 	}
 }
 
-extern void context_tracking_task_switch(struct task_struct *prev,
-					 struct task_struct *next);
+static inline void context_tracking_task_switch(struct task_struct *prev,
+						struct task_struct *next)
+{
+	if (static_key_false(&context_tracking_enabled))
+		__context_tracking_task_switch(prev, next);
+}
 #else
 static inline bool context_tracking_in_user(void) { return false; }
 static inline void user_enter(void) { }
@@ -112,6 +118,7 @@ static inline void guest_exit(void)
 	else
 		current->flags &= ~PF_VCPU;
 }
+
 #else
 static inline void guest_enter(void)
 {

kernel/context_tracking.c

@@ -166,7 +166,7 @@ void context_tracking_user_exit(void)
 }
 
 /**
- * context_tracking_task_switch - context switch the syscall callbacks
+ * __context_tracking_task_switch - context switch the syscall callbacks
  * @prev: the task that is being switched out
  * @next: the task that is being switched in
  *
@@ -178,8 +178,8 @@ void context_tracking_user_exit(void)
  * migrate to some CPU that doesn't do the context tracking. As such the TIF
  * flag may not be desired there.
  */
-void context_tracking_task_switch(struct task_struct *prev,
-			     struct task_struct *next)
+void __context_tracking_task_switch(struct task_struct *prev,
+				    struct task_struct *next)
 {
 	clear_tsk_thread_flag(prev, TIF_NOHZ);
 	set_tsk_thread_flag(next, TIF_NOHZ);