cpuidle: tracing: Warn about !rcu_is_watching()

When using noinstr, WARN when tracing hits when RCU is disabled.

Suggested-by: Steven Rostedt (Google) <[e-mail]>
Signed-off-by: Peter Zijlstra (Intel) <[e-mail]>
Signed-off-by: Ingo Molnar <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

include/linux/trace_recursion.h

···
 # define do_ftrace_record_recursion(ip, pip)	do { } while (0)
 #endif
 
+#ifdef CONFIG_ARCH_WANTS_NO_INSTR
+# define trace_warn_on_no_rcu(ip)					\
+	({								\
+		bool __ret = !rcu_is_watching();			\
+		if (__ret && !trace_recursion_test(TRACE_RECORD_RECURSION_BIT)) { \
+			trace_recursion_set(TRACE_RECORD_RECURSION_BIT); \
+			WARN_ONCE(true, "RCU not on for: %pS\n", (void *)ip); \
+			trace_recursion_clear(TRACE_RECORD_RECURSION_BIT); \
+		}							\
+		__ret;							\
+	})
+#else
+# define trace_warn_on_no_rcu(ip)	false
+#endif
+
 /*
  * Preemption is promised to be disabled when return bit >= 0.
  */
···
 {
 	unsigned int val = READ_ONCE(current->trace_recursion);
 	int bit;
+
+	if (trace_warn_on_no_rcu(ip))
+		return -1;
 
 	bit = trace_get_context_bit() + start;
 	if (unlikely(val & (1 << bit))) {