[PATCH v7 2/4] libtraceevent: Optimize tep_print_fields()

["Yordan Karadzhov (VMware)" <y.karadz@gmail.com>]

The current implementation of tep_print_fields() loops over all
individual fields of the event and calls tep_print_field() for
each one. In the same time inside tep_print_field() we loop over
the list of all tokens of the printing format descriptor in order
to determine how the current field must be printed (its own
appropriate printing format). The problem is that in this second
loop over the tokens we always start from the very first token
and this can be quite inefficient for example in a case of a
kprobe that has a large number of fields. This patch optimizes
tep_print_fields(), allowing the traverse of the list of tokens
to continue from the place reached when we searched for the
format of the previous field. For most of the tracing events
the order of fields matches the order of the corresponding format
tokens, however this is not strictly guaranteed. This problem is
addressed by making the loop over the tokens circular.

Signed-off-by: Yordan Karadzhov (VMware) <y.karadz@gmail.com>
---
 src/event-parse.c | 36 +++++++++++++++++++++++++++---------
 1 file changed, 27 insertions(+), 9 deletions(-)

diff --git a/src/event-parse.c b/src/event-parse.c
index 6129a0c..51cb30b 100644
--- a/src/event-parse.c
+++ b/src/event-parse.c
@@ -5391,22 +5391,25 @@ static void print_field_raw(struct trace_seq *s, void *data,
 static int print_parse_data(struct tep_print_parse *parse, struct trace_seq *s,
 			    void *data, int size, struct tep_event *event);
 
-void tep_print_field(struct trace_seq *s, void *data,
-		     struct tep_format_field *field)
+void static inline print_field(struct trace_seq *s, void *data,
+				    struct tep_format_field *field,
+				    struct tep_print_parse **parse_ptr)
 {
 	struct tep_event *event = field->event;
+	struct tep_print_parse *start_parse;
 	struct tep_print_parse *parse;
 	bool has_0x;
 
-	parse = event->print_fmt.print_cache;
+	parse = parse_ptr ? *parse_ptr : event->print_fmt.print_cache;
 
-	if (event->flags & TEP_EVENT_FL_FAILED)
+	if (!parse || event->flags & TEP_EVENT_FL_FAILED)
 		goto out;
 
 	if (field->flags & (TEP_FIELD_IS_ARRAY || TEP_FIELD_IS_STRING))
 		goto out;
 
-	for (;parse; parse = parse->next) {
+	start_parse = parse;
+	do {
 		if (parse->type == PRINT_FMT_STRING) {
 			int len = strlen(parse->format);
 
@@ -5416,37 +5419,52 @@ void tep_print_field(struct trace_seq *s, void *data,
 			else
 				has_0x = false;
 
-			continue;
+			goto next;
 		}
 
 		if (!parse->arg ||
 		    parse->arg->type != TEP_PRINT_FIELD ||
 		    parse->arg->field.field != field) {
 			has_0x = false;
-			continue;
+			goto next;
 		}
 
 		if (has_0x)
 			trace_seq_puts(s, "0x");
 
 		print_parse_data(parse, s, data, field->size, event);
+
+		if (parse_ptr)
+			*parse_ptr = parse->next;
+
 		return;
-	}
+
+ next:
+		parse = parse->next ? parse->next :
+				      event->print_fmt.print_cache;
+	} while (parse != start_parse);
 
  out:
 	/* Not found. */
 	print_field_raw(s, data, field);
 }
 
+void tep_print_field(struct trace_seq *s, void *data,
+		     struct tep_format_field *field)
+{
+	print_field(s, data, field, NULL);
+}
+
 void tep_print_fields(struct trace_seq *s, void *data,
 		      int size __maybe_unused, struct tep_event *event)
 {
+	struct tep_print_parse *parse = event->print_fmt.print_cache;
 	struct tep_format_field *field;
 
 	field = event->format.fields;
 	while (field) {
 		trace_seq_printf(s, " %s=", field->name);
-		tep_print_field(s, data, field);
+		print_field(s, data, field, &parse);
 		field = field->next;
 	}
 }
-- 
2.30.2