[PATCH v4 2/3] tracing, function_graph: add additional marks to signal very large function execution time

[byungchul.park@lge.com]

From: Byungchul Park <byungchul.park@lge.com>

Currently, function graph tracer prints "!" or "+" just before
function execution time to signal a function overhead, depending
on the time. Even it is usually enough to do that, we sometimes
need to be signaled for bigger execution time than 100 micro seconds.

For example, I used function graph tracer to detect if there is
any case that exit_mm() takes too much time. I did following steps
in /sys/kernel/debug/tracing. It was easier to detect very big
excution time with patched kernel than with original kernel.

$ echo exit_mm > set_graph_function
$ echo function_graph > current_tracer
$ echo > trace
$ cat trace_pipe > $LOGFILE
 ... (do something and terminate logging)
$ grep "\\$" $LOGFILE
 3) $ 22082032 us |                      } /* kernel_map_pages */
 3) $ 22082040 us |                    } /* free_pages_prepare */
 3) $ 22082113 us |                  } /* free_hot_cold_page */
 3) $ 22083455 us |                } /* free_hot_cold_page_list */
 3) $ 22083895 us |              } /* release_pages */
 3) $ 22177873 us |            } /* free_pages_and_swap_cache */
 3) $ 22178929 us |          } /* unmap_single_vma */
 3) $ 22198885 us |        } /* unmap_vmas */
 3) $ 22206949 us |      } /* exit_mmap */
 3) $ 22207659 us |    } /* mmput */
 3) $ 22207793 us |  } /* exit_mm */

And then, it was easy to find out that a schedule-out occured by
sub_preempt_count() within kernel_map_pages().

To detect very large function exection time caused by either problematic
function implementation or scheduling issues, this patch can be useful.

Signed-off-by: Byungchul Park <byungchul.park@lge.com>
---
 Documentation/trace/ftrace.txt       |    2 ++
 kernel/trace/trace.h                 |   21 +++++++++++++++++++++
 kernel/trace/trace_functions_graph.c |   23 +++++++++++++++++------
 3 files changed, 40 insertions(+), 6 deletions(-)

diff --git a/Documentation/trace/ftrace.txt b/Documentation/trace/ftrace.txt
index 4da4261..f827e2f 100644
--- a/Documentation/trace/ftrace.txt
+++ b/Documentation/trace/ftrace.txt
@@ -1951,6 +1951,8 @@ want, depending on your needs.
 
   + means that the function exceeded 10 usecs.
   ! means that the function exceeded 100 usecs.
+  # means that the function exceeded 1000 usecs.
+  $ means that the function exceeded 1 sec.
 
 
 - The task/pid field displays the thread cmdline and pid which
diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h
index 385391f..49ce26c 100644
--- a/kernel/trace/trace.h
+++ b/kernel/trace/trace.h
@@ -331,6 +331,27 @@ struct tracer_flags {
 /* Makes more easy to define a tracer opt */
 #define TRACER_OPT(s, b)	.name = #s, .bit = b
 
+/* trace overhead mark */
+struct trace_mark {
+	unsigned long long	val; /* unit: nsec */
+	char			sym;
+};
+
+static inline char trace_duration_mark(unsigned long long d,
+				       const struct trace_mark m[],
+				       int size)
+{
+	int i;
+
+	if (size <= 0) return ' ';
+
+	for (i = 0; i < size; i++) {
+		if (d >= m[i].val)
+			break;
+	}
+
+	return (i == size)? ' ' : m[i].sym;
+}
 
 /**
  * struct tracer - a specific tracer and its callbacks to interact with debugfs
diff --git a/kernel/trace/trace_functions_graph.c b/kernel/trace/trace_functions_graph.c
index cb33f46..c187c4a 100644
--- a/kernel/trace/trace_functions_graph.c
+++ b/kernel/trace/trace_functions_graph.c
@@ -797,6 +797,22 @@ trace_print_graph_duration(unsigned long long duration, struct trace_seq *s)
 	return TRACE_TYPE_HANDLED;
 }
 
+#undef MARK
+#define MARK(v, s) {.val = v, .sym = s}
+static const struct trace_mark mark[] = {
+	MARK(1000000000ULL	, '$'), /* 1 sec */
+	MARK(1000000ULL		, '#'), /* 1000 usecs */
+	MARK(100000ULL		, '!'), /* 100 usecs */
+	MARK(10000ULL		, '+'), /* 10 usecs */
+	MARK(0ULL		, ' '), /* 0 usecs */
+};
+#undef MARK
+
+static inline char find_trace_mark(unsigned long long d)
+{
+	return trace_duration_mark(d, mark, ARRAY_SIZE(mark));
+}
+
 static enum print_line_t
 print_graph_duration(unsigned long long duration, struct trace_seq *s,
 		     u32 flags)
@@ -822,12 +838,7 @@ print_graph_duration(unsigned long long duration, struct trace_seq *s,
 
 	/* Signal a overhead of time execution to the output */
 	if (flags & TRACE_GRAPH_PRINT_OVERHEAD) {
-		/* Duration exceeded 100 usecs */
-		if (duration > 100000ULL)
-			ret = trace_seq_puts(s, "! ");
-		/* Duration exceeded 10 usecs */
-		else if (duration > 10000ULL)
-			ret = trace_seq_puts(s, "+ ");
+		ret = trace_seq_printf(s, "%c ", find_trace_mark(duration));
 	}
 
 	/*
-- 
1.7.9.5