rtla/timerlat_aa: Fix negative IRQ delay

When estimating the IRQ timer delay, we are dealing with two different
clock sources: the external clock source that timerlat uses as a reference
and the clock used by the tracer. There are also two moments: the time
reading the clock and the timer in which the event is placed in the
buffer (the trace event timestamp).

If the processor is slow or there is some hardware noise, the difference
between the timestamp and the external clock, read can be longer than the
IRQ handler delay, resulting in a negative time.

If so, set IRQ to start delay as 0. In the end, it is less near-zero and relevant
then the noise.

Link: https://lore.kernel.org/lkml/a066fb667c7136d86dcddb3c7ccd72587db3e7c7.1691162043.git.bristot@kernel.org

Fixes: 27e348b ("rtla/timerlat: Add auto-analysis core")
Signed-off-by: Daniel Bristot de Oliveira <[email protected]>

Author: Daniel Bristot de Oliveira

tools/tracing/rtla/src/timerlat_aa.c

@@ -338,7 +338,23 @@ static int timerlat_aa_irq_handler(struct trace_seq *s, struct tep_record *recor
 		taa_data->timer_irq_start_time = start;
 		taa_data->timer_irq_duration = duration;
 
-		taa_data->timer_irq_start_delay = taa_data->timer_irq_start_time - expected_start;
+		/*
+		 * We are dealing with two different clock sources: the
+		 * external clock source that timerlat uses as a reference
+		 * and the clock used by the tracer. There are also two
+		 * moments: the time reading the clock and the timer in
+		 * which the event is placed in the buffer (the trace
+		 * event timestamp). If the processor is slow or there
+		 * is some hardware noise, the difference between the
+		 * timestamp and the external clock read can be longer
+		 * than the IRQ handler delay, resulting in a negative
+		 * time. If so, set IRQ start delay as 0. In the end,
+		 * it is less relevant than the noise.
+		 */
+		if (expected_start < taa_data->timer_irq_start_time)
+			taa_data->timer_irq_start_delay = taa_data->timer_irq_start_time - expected_start;
+		else
+			taa_data->timer_irq_start_delay = 0;
 
 		/*
 		 * not exit from idle.