tracing: make sched_switch stop/start light weight

The stopping and starting of a tracer should be light weight and
be able to be called in all contexts. The sched_switch grabbed
mutexes in the start/stop functions. This patch changes it to a
simple variable, on/off.
Signed-off-by: Steven Rostedt <srostedt@redhat.com>

kernel/trace/trace_sched_switch.c

@@ -18,6 +18,7 @@ static struct trace_array	*ctx_trace;
 static int __read_mostly	tracer_enabled;
 static int			sched_ref;
 static DEFINE_MUTEX(sched_register_mutex);
+static int			sched_stopped;
 
 static void
 probe_sched_switch(struct rq *__rq, struct task_struct *prev,
@@ -28,7 +29,7 @@ probe_sched_switch(struct rq *__rq, struct task_struct *prev,
 	int cpu;
 	int pc;
 
-	if (!sched_ref)
+	if (!sched_ref || sched_stopped)
 		return;
 
 	tracing_record_cmdline(prev);
@@ -193,6 +194,7 @@ static void stop_sched_trace(struct trace_array *tr)
 static int sched_switch_trace_init(struct trace_array *tr)
 {
 	ctx_trace = tr;
+	tracing_reset_online_cpus(tr);
 	tracing_start_sched_switch_record();
 	return 0;
 }
@@ -205,13 +207,12 @@ static void sched_switch_trace_reset(struct trace_array *tr)
 
 static void sched_switch_trace_start(struct trace_array *tr)
 {
-	tracing_reset_online_cpus(tr);
-	tracing_start_sched_switch();
+	sched_stopped = 0;
 }
 
 static void sched_switch_trace_stop(struct trace_array *tr)
 {
-	tracing_stop_sched_switch();
+	sched_stopped = 1;
 }
 
 static struct tracer sched_switch_trace __read_mostly =