tracing: Call on_each_cpu() when adding or removing single pids from set_event_pid

For the case where pids are already in set_event_pid, and one is added or
removed then each CPU should be checked to make sure that the new or old pid
is on or not on a CPU.

 For example:

 # echo 123 >> set_event_pid

or

 # echo '!123' >> set_event_pid

Link: http://lkml.kernel.org/r/20151030061643.GA19480@cac

Suggested-by: Jiaxing Wang <hello.wjx@gmail.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

kernel/trace/trace_events.c

@@ -1729,14 +1729,15 @@ ftrace_event_pid_write(struct file *filp, const char __user *ubuf,
 						 tr, INT_MAX);
 		register_trace_prio_sched_wakeup(event_filter_pid_sched_wakeup_probe_post,
 						 tr, 0);
-
-		/*
-		 * Ignoring of pids is done at task switch. But we have to
-		 * check for those tasks that are currently running.
-		 */
-		on_each_cpu(ignore_task_cpu, tr, 1);
 	}
 
+	/*
+	 * Ignoring of pids is done at task switch. But we have to
+	 * check for those tasks that are currently running.
+	 * Always do this in case a pid was appended or removed.
+	 */
+	on_each_cpu(ignore_task_cpu, tr, 1);
+
 	mutex_unlock(&event_mutex);
 
 	ret = read;