tracing: Allow execnames to be passed as args for synthetic events

Allow common_pid.execname to be saved in a variable in one histogram to be
passed to another histogram that can pass it as a parameter to a synthetic
event.

># echo 'hist:keys=pid:__arg__1=common_timestamp.usecs:arg2=common_pid.execname' \
> events/sched/sched_waking/trigger
># echo 'wakeup_lat s32 pid; u64 delta; char wake_comm[]' > synthetic_events
># echo 'hist:keys=next_pid:pid=next_pid,delta=common_timestamp.usecs-$__arg__1,exec=$arg2'\
':onmatch(sched.sched_waking).trace(wakeup_lat,$pid,$delta,$exec)' \
> events/sched/sched_switch/trigger

The above is a wake up latency synthetic event setup that passes the execname
of the common_pid that woke the task to the scheduling of that task, which
triggers a synthetic event that passes the original execname as a
parameter to display it.

># echo 1 > events/synthetic/enable
># cat trace
<idle>-0 [006] d..4 186.863801: wakeup_lat: pid=1306 delta=65 wake_comm=kworker/u16:3
<idle>-0 [000] d..4 186.863858: wakeup_lat: pid=163 delta=27 wake_comm=<idle>
<idle>-0 [001] d..4 186.863903: wakeup_lat: pid=1307 delta=36 wake_comm=kworker/u16:4
<idle>-0 [000] d..4 186.863927: wakeup_lat: pid=163 delta=5 wake_comm=<idle>
<idle>-0 [006] d..4 186.863957: wakeup_lat: pid=1306 delta=24 wake_comm=kworker/u16:3
sshd-1306 [006] d..4 186.864051: wakeup_lat: pid=61 delta=62 wake_comm=<idle>
<idle>-0 [000] d..4 186.965030: wakeup_lat: pid=609 delta=18 wake_comm=<idle>
<idle>-0 [006] d..4 186.987582: wakeup_lat: pid=1306 delta=65 wake_comm=kworker/u16:3
<idle>-0 [000] d..4 186.987639: wakeup_lat: pid=163 delta=27 wake_comm=<idle>

Link: https://lkml.kernel.org/r/[e-mail]

Reviewed-by: Tom Zanussi <[e-mail]>
Reviewed-by: Masami Hiramatsu <[e-mail]>
Signed-off-by: Steven Rostedt (VMware) <[e-mail]>

kernel/trace/trace_events_hist.c

···
 	struct hist_trigger_data *hist_data = elt->map->private_data;
 	unsigned int size = TASK_COMM_LEN;
 	struct hist_elt_data *elt_data;
-	struct hist_field *key_field;
+	struct hist_field *hist_field;
 	unsigned int i, n_str;
 
 	elt_data = kzalloc(sizeof(*elt_data), GFP_KERNEL);
 	if (!elt_data)
 		return -ENOMEM;
 
-	for_each_hist_key_field(i, hist_data) {
-		key_field = hist_data->fields[i];
+	for_each_hist_field(i, hist_data) {
+		hist_field = hist_data->fields[i];
 
-		if (key_field->flags & HIST_FIELD_FL_EXECNAME) {
+		if (hist_field->flags & HIST_FIELD_FL_EXECNAME) {
 			elt_data->comm = kzalloc(size, GFP_KERNEL);
 			if (!elt_data->comm) {
 				kfree(elt_data);
···
 	return __create_val_field(hist_data, val_idx, file, NULL, field_str, 0);
 }
 
+static const char *no_comm = "(no comm)";
+
+static u64 hist_field_execname(struct hist_field *hist_field,
+			       struct tracing_map_elt *elt,
+			       struct trace_buffer *buffer,
+			       struct ring_buffer_event *rbe,
+			       void *event)
+{
+	struct hist_elt_data *elt_data;
+
+	if (WARN_ON_ONCE(!elt))
+		return (u64)(unsigned long)no_comm;
+
+	elt_data = elt->private_data;
+
+	if (WARN_ON_ONCE(!elt_data->comm))
+		return (u64)(unsigned long)no_comm;
+
+	return (u64)(unsigned long)(elt_data->comm);
+}
+
+/* Convert a var that points to common_pid.execname to a string */
+static void update_var_execname(struct hist_field *hist_field)
+{
+	hist_field->flags = HIST_FIELD_FL_STRING | HIST_FIELD_FL_VAR |
+		HIST_FIELD_FL_EXECNAME;
+	hist_field->size = MAX_FILTER_STR_VAL;
+	hist_field->is_signed = 0;
+
+	kfree_const(hist_field->type);
+	hist_field->type = "char[]";
+
+	hist_field->fn = hist_field_execname;
+}
+
 static int create_var_field(struct hist_trigger_data *hist_data,
 			    unsigned int val_idx,
 			    struct trace_event_file *file,
···
 		return -EINVAL;
 
 	ret = __create_val_field(hist_data, val_idx, file, var_name, expr_str, flags);
+
+	if (!ret && hist_data->fields[val_idx]->flags & HIST_FIELD_FL_EXECNAME)
+		update_var_execname(hist_data->fields[val_idx]);
 
 	if (!ret && hist_data->fields[val_idx]->flags & HIST_FIELD_FL_STRING)
 		hist_data->fields[val_idx]->var_str_idx = hist_data->n_var_str++;