trace: set max latency variable to zero on default

Impact: trace max latencies on start of latency tracing

This patch sets the max latency to zero whenever one of the
irq variant tracers or the wakeup tracer is set to current tracer.

Most developers expect to see output when starting up a latency
tracer. But since the max_latency is already set to max, and
it takes a latency greater than max_latency to be recorded, there
is no trace. This is not the expected behavior and has even confused
myself.

Signed-off-by: Steven Rostedt <srostedt@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/trace/trace.c

@@ -40,7 +40,7 @@
 
 #define TRACE_BUFFER_FLAGS	(RB_FL_OVERWRITE)
 
-unsigned long __read_mostly	tracing_max_latency = (cycle_t)ULONG_MAX;
+unsigned long __read_mostly	tracing_max_latency;
 unsigned long __read_mostly	tracing_thresh;
 
 /*

kernel/trace/trace_irqsoff.c

@@ -380,6 +380,7 @@ static void stop_irqsoff_tracer(struct trace_array *tr)
 
 static void __irqsoff_tracer_init(struct trace_array *tr)
 {
+	tracing_max_latency = 0;
 	irqsoff_trace = tr;
 	/* make sure that the tracer is visible */
 	smp_wmb();

kernel/trace/trace_sched_wakeup.c

@@ -333,6 +333,7 @@ static void stop_wakeup_tracer(struct trace_array *tr)
 
 static int wakeup_tracer_init(struct trace_array *tr)
 {
+	tracing_max_latency = 0;
 	wakeup_trace = tr;
 	start_wakeup_tracer(tr);
 	return 0;