documentation: Long-running irq handlers can stall RCU grace periods

If a periodic interrupt's handler takes longer to execute than the period
between successive interrupts, RCU's kthreads and softirq handlers can
be prevented from executing, resulting in otherwise inexplicable RCU
CPU stall warnings.  This commit therefore calls out this possibility
in Documentation/RCU/stallwarn.txt.

Reported-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

Documentation/RCU/stallwarn.txt

@@ -40,7 +40,9 @@ o	Booting Linux using a console connection that is too slow to
 o	Anything that prevents RCU's grace-period kthreads from running.
 	This can result in the "All QSes seen" console-log message.
 	This message will include information on when the kthread last
-	ran and how often it should be expected to run.
+	ran and how often it should be expected to run.  It can also
+	result in the "rcu_.*kthread starved for" console-log message,
+	which will include additional debugging information.
 
 o	A CPU-bound real-time task in a CONFIG_PREEMPT kernel, which might
 	happen to preempt a low-priority task in the middle of an RCU
@@ -60,6 +62,14 @@ o	A CPU-bound real-time task in a CONFIG_PREEMPT_RT kernel that
 	CONFIG_PREEMPT_RCU case, you might see stall-warning
 	messages.
 
+o	A periodic interrupt whose handler takes longer than the time
+	interval between successive pairs of interrupts.  This can
+	prevent RCU's kthreads and softirq handlers from running.
+	Note that certain high-overhead debugging options, for example
+	the function_graph tracer, can result in interrupt handler taking
+	considerably longer than normal, which can in turn result in
+	RCU CPU stall warnings.
+
 o	A hardware or software issue shuts off the scheduler-clock
 	interrupt on a CPU that is not in dyntick-idle mode.  This
 	problem really has happened, and seems to be most likely to