tracing/ftrace: provide the base infrastructure for histogram tracing

Impact: extend the tracing API

The goal of this patch is to normalize and make more easy the
implementation of statistical (histogram) tracing.

It implements a trace_stat file into the /debugfs/tracing directory where
one can print a one-shot output of statistics/histogram entries.

A tracer has to provide two basic iterator callbacks:

  stat_start() => the first entry
  stat_next(prev, idx) => the next one.

Note that it is adapted for arrays or hash tables or lists.... since it
provides a pointer to the previous entry and the current index of the
iterator.

These two callbacks are called to get a snapshot of the statistics at each
opening of the trace_stat file because. The values are so updated between
two "cat trace_stat". And the tracer is free to lock its datas during the
iteration to keep consistent values.

Since it is almost always interesting to sort statisticals values to
address the problems by priority, this infrastructure provides a "sorting"
of the stat entries too if desired. A tracer has just to provide a
stat_cmp callback to compare two entries and the stat tracing
infrastructure will build a sorted list of the given entries.

A last callback, called stat_headers, can be implemented by a tracer to
output headers on its trace.

If one of these callbacks is changed on runtime, it just have to signal it
to the stat tracing API by calling the init_tracer_stat() helper.

Changes in V2:

- Fix a memory leak if the user opens multiple times the trace_stat file
  without closing it. Now we always free our list before rebuilding it.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 17 insertions, 0 deletions

diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h
index 6bd71fa1e1c7..05fa804d1c16 100644
--- a/kernel/trace/trace.h
+++ b/kernel/trace/trace.h
@@ -336,6 +336,21 @@ struct tracer {
         struct tracer           *next;
         int                     print_max;
         struct tracer_flags     *flags;
+
+        /*
+         * If you change one of the following on tracing runtime, recall
+         * init_tracer_stat()
+         */
+
+        /* Iteration over statistic entries */
+        void                    *(*stat_start)(void);
+        void                    *(*stat_next)(void *prev, int idx);
+        /* Compare two entries for sorting (optional) for stats */
+        int                     (*stat_cmp)(void *p1, void *p2);
+        /* Print a stat entry */
+        int                     (*stat_show)(struct seq_file *s, void *p);
+        /* Print the headers of your stat entries */
+        int                     (*stat_headers)(struct seq_file *s);
 };
 
 struct trace_seq {
@@ -421,6 +436,8 @@ void tracing_start_sched_switch_record(void);
 int register_tracer(struct tracer *type);
 void unregister_tracer(struct tracer *type);
 
+void init_tracer_stat(struct tracer *trace);
+
 extern unsigned long nsecs_to_usecs(unsigned long nsecs);
 
 extern unsigned long tracing_max_latency;