tracing/timerlat: Print stacktrace in the IRQ handler if needed

If print_stack and stop_tracing_us are set, and stop_tracing_us is hit
with latency higher than or equal to print_stack, print the
stack at the IRQ handler as it is useful to define the root cause for
the IRQ latency.

Link: https://lkml.kernel.org/r/fd04530ce98ae9270e41bb124ee5bf67b05ecfed.1652175637.git.bristot@kernel.org

Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: Clark Williams <williams@redhat.com>
Cc: Ingo Molnar <mingo@redhat.com>
Signed-off-by: Daniel Bristot de Oliveira <bristot@kernel.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

Documentation/trace/timerlat-tracer.rst

@@ -74,8 +74,9 @@ directory. The timerlat configs are:
  - stop_tracing_total_us: stop the system tracing if a
    timer latency at the *thread* context is higher than the configured
    value happens. Writing 0 disables this option.
- - print_stack: save the stack of the IRQ occurrence, and print
-   it after the *thread context* event".
+ - print_stack: save the stack of the IRQ occurrence. The stack is printed
+   after the *thread context* event, or at the IRQ handler if *stop_tracing_us*
+   is hit.
 
 timerlat and osnoise
 ----------------------------

kernel/trace/trace_osnoise.c

@@ -1580,6 +1580,19 @@ static enum hrtimer_restart timerlat_irq(struct hrtimer *timer)
 
 	if (osnoise_data.stop_tracing) {
 		if (time_to_us(diff) >= osnoise_data.stop_tracing) {
+
+			/*
+			 * At this point, if stop_tracing is set and <= print_stack,
+			 * print_stack is set and would be printed in the thread handler.
+			 *
+			 * Thus, print the stack trace as it is helpful to define the
+			 * root cause of an IRQ latency.
+			 */
+			if (osnoise_data.stop_tracing <= osnoise_data.print_stack) {
+				timerlat_save_stack(0);
+				timerlat_dump_stack(time_to_us(diff));
+			}
+
 			osnoise_stop_tracing();
 			notify_new_max_latency(diff);
 		}