tracing/timerlat: Notify IRQ new max latency only if stop tracing is set

Currently, the notification of a new max latency is sent from
timerlat's IRQ handler anytime a new max latency is found.

While this behavior is not wrong, the send IPI overhead itself
will increase the thread latency and that is not the desired
effect (tracing overhead).

Moreover, the thread will notify a new max latency again because
the thread latency as it is always higher than the IRQ latency
that woke it up.

The only case in which it is helpful to notify a new max latency
from IRQ is when stop tracing (for the IRQ) is set, as in this
case, the thread will not be dispatched.

Notify a new max latency from the IRQ handler only if stop tracing is
set for the IRQ handler.

Link: https://lkml.kernel.org/r/2c2d9a56c0886c8402ba320de32856cbbb10c2bb.1652175637.git.bristot@kernel.org

Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: Ingo Molnar <mingo@redhat.com>
Reported-by: Clark Williams <williams@redhat.com>
Fixes: a955d7eac1 ("trace: Add timerlat tracer")
Signed-off-by: Daniel Bristot de Oliveira <bristot@kernel.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

kernel/trace/trace_osnoise.c

@@ -1578,11 +1578,12 @@ static enum hrtimer_restart timerlat_irq(struct hrtimer *timer)
 
 	trace_timerlat_sample(&s);
 
-	notify_new_max_latency(diff);
+	if (osnoise_data.stop_tracing) {
-
+		if (time_to_us(diff) >= osnoise_data.stop_tracing) {
-	if (osnoise_data.stop_tracing)
-		if (time_to_us(diff) >= osnoise_data.stop_tracing)
 			osnoise_stop_tracing();
+			notify_new_max_latency(diff);
+		}
+	}
 
 	wake_up_process(tlat->kthread);