context_tracking: Introduce context_tracking_enabled_cpu()

This allows us to check if a remote CPU runs context tracking
(ie: is nohz_full). We'll need that to reliably support "nice"
accounting on kcpustat.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Jacek Anaszewski <jacek.anaszewski@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Pavel Machek <pavel@ucw.cz>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Rafael J . Wysocki <rjw@rjwysocki.net>
Cc: Rik van Riel <riel@surriel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Viresh Kumar <viresh.kumar@linaro.org>
Cc: Wanpeng Li <wanpengli@tencent.com>
Cc: Yauheni Kaliuta <yauheni.kaliuta@redhat.com>
Link: https://lkml.kernel.org/r/20191016025700.31277-8-frederic@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

include/linux/context_tracking_state.h

@@ -31,6 +31,11 @@ static inline bool context_tracking_enabled(void)
 	return static_branch_unlikely(&context_tracking_key);
 }
 
+static inline bool context_tracking_enabled_cpu(int cpu)
+{
+	return per_cpu(context_tracking.active, cpu);
+}
+
 static inline bool context_tracking_enabled_this_cpu(void)
 {
 	return __this_cpu_read(context_tracking.active);
@@ -43,6 +48,7 @@ static inline bool context_tracking_in_user(void)
 #else
 static inline bool context_tracking_in_user(void) { return false; }
 static inline bool context_tracking_enabled(void) { return false; }
+static inline bool context_tracking_enabled_cpu(int cpu) { return false; }
 static inline bool context_tracking_enabled_this_cpu(void) { return false; }
 #endif /* CONFIG_CONTEXT_TRACKING */