tracing: Add hist trigger support for clearing a trace

Allow users to append 'clear' to an existing trigger in order to have
the hash table cleared.

This expands the hist trigger syntax from this:
    # echo hist:keys=xxx:vals=yyy:sort=zzz.descending:pause/cont \
           [ if filter] >> event/trigger

to this:

    # echo hist:keys=xxx:vals=yyy:sort=zzz.descending:pause/cont/clear \
          [ if filter] >> event/trigger

Link: http://lkml.kernel.org/r/ae15dd0d9b2f7af07a37c1ff682063e2dbcdf160.1457029949.git.tom.zanussi@linux.intel.com

Signed-off-by: Tom Zanussi <tom.zanussi@linux.intel.com>
Tested-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Reviewed-by: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

kernel/trace/trace.c

@@ -3837,7 +3837,7 @@ static const char readme_msg[] =
 	"\t            [:values=<field1[,field2,...]>]\n"
 	"\t            [:sort=<field1[,field2,...]>]\n"
 	"\t            [:size=#entries]\n"
-	"\t            [:pause][:continue]\n"
+	"\t            [:pause][:continue][:clear]\n"
 	"\t            [if <filter>]\n\n"
 	"\t    When a matching event is hit, an entry is added to a hash\n"
 	"\t    table using the key(s) and value(s) named, and the value of a\n"
@@ -3857,6 +3857,9 @@ static const char readme_msg[] =
 	"\t    trigger or to start a hist trigger but not log any events\n"
 	"\t    until told to do so.  'continue' can be used to start or\n"
 	"\t    restart a paused hist trigger.\n\n"
+	"\t    The 'clear' parameter will clear the contents of a running\n"
+	"\t    hist trigger and leave its current paused/active state\n"
+	"\t    unchanged.\n\n"
 #endif
 ;
 

kernel/trace/trace_events_hist.c

@@ -88,6 +88,7 @@ struct hist_trigger_attrs {
 	char		*sort_key_str;
 	bool		pause;
 	bool		cont;
+	bool		clear;
 	unsigned int	map_bits;
 };
 
@@ -200,6 +201,8 @@ static struct hist_trigger_attrs *parse_hist_trigger_attrs(char *trigger_str)
 		else if ((strcmp(str, "cont") == 0) ||
 			 (strcmp(str, "continue") == 0))
 			attrs->cont = true;
+		else if (strcmp(str, "clear") == 0)
+			attrs->clear = true;
 		else if (strncmp(str, "size=", strlen("size=")) == 0) {
 			int map_bits = parse_map_size(str);
 
@@ -916,6 +919,21 @@ static struct event_trigger_ops *event_hist_get_trigger_ops(char *cmd,
 	return &event_hist_trigger_ops;
 }
 
+static void hist_clear(struct event_trigger_data *data)
+{
+	struct hist_trigger_data *hist_data = data->private_data;
+	bool paused;
+
+	paused = data->paused;
+	data->paused = true;
+
+	synchronize_sched();
+
+	tracing_map_clear(hist_data->map);
+
+	data->paused = paused;
+}
+
 static int hist_register_trigger(char *glob, struct event_trigger_ops *ops,
 				 struct event_trigger_data *data,
 				 struct trace_event_file *file)
@@ -930,13 +948,15 @@ static int hist_register_trigger(char *glob, struct event_trigger_ops *ops,
 				test->paused = true;
 			else if (hist_data->attrs->cont)
 				test->paused = false;
+			else if (hist_data->attrs->clear)
+				hist_clear(test);
 			else
 				ret = -EEXIST;
 			goto out;
 		}
 	}
 
-	if (hist_data->attrs->cont) {
+	if (hist_data->attrs->cont || hist_data->attrs->clear) {
 		ret = -ENOENT;
 		goto out;
 	}
@@ -1035,7 +1055,7 @@ static int event_hist_trigger_func(struct event_command *cmd_ops,
 	 * triggers registered a failure too.
 	 */
 	if (!ret) {
-		if (!(attrs->pause || attrs->cont))
+		if (!(attrs->pause || attrs->cont || attrs->clear))
 			ret = -ENOENT;
 		goto out_free;
 	} else if (ret < 0)