tracing: Dynamically allocate the per-elt hist_elt_data array

Setting the hist_elt_data.field_var_str[] array unconditionally to a
size of SYNTH_FIELD_MAX elements wastes space unnecessarily. The
actual number of elements needed can be calculated at run-time
instead.

In most cases, this will save a lot of space since it's a per-elt
array which isn't normally close to being full. It also allows us to
increase SYNTH_FIELD_MAX without worrying about even more wastage when
we do that.

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

Signed-off-by: Tom Zanussi <[e-mail]>
Tested-by: Artem Bityutskiy <[e-mail]>
Signed-off-by: Steven Rostedt (VMware) <[e-mail]>

kernel/trace/trace_events_hist.c

···
 struct hist_elt_data {
 	char *comm;
 	u64 *var_ref_vals;
-	char *field_var_str[SYNTH_FIELDS_MAX];
+	char **field_var_str;
+	int n_field_var_str;
 };
 
 struct snapshot_context {
···
 {
 	unsigned int i;
 
-	for (i = 0; i < SYNTH_FIELDS_MAX; i++)
+	for (i = 0; i < elt_data->n_field_var_str; i++)
 		kfree(elt_data->field_var_str[i]);
+
+	kfree(elt_data->field_var_str);
 
 	kfree(elt_data->comm);
 	kfree(elt_data);
···
 	BUILD_BUG_ON(STR_VAR_LEN_MAX & (sizeof(u64) - 1));
 
 	size = STR_VAR_LEN_MAX;
+
+	elt_data->field_var_str = kcalloc(n_str, sizeof(char *), GFP_KERNEL);
+	if (!elt_data->field_var_str) {
+		hist_elt_data_free(elt_data);
+		return -EINVAL;
+	}
+	elt_data->n_field_var_str = n_str;
 
 	for (i = 0; i < n_str; i++) {
 		elt_data->field_var_str[i] = kzalloc(size, GFP_KERNEL);