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>

kernel/trace/Makefile

@@ -20,6 +20,7 @@ obj-$(CONFIG_RING_BUFFER) += ring_buffer.o
 
 obj-$(CONFIG_TRACING) += trace.o
 obj-$(CONFIG_TRACING) += trace_output.o
+obj-$(CONFIG_TRACING) += trace_stat.o
 obj-$(CONFIG_CONTEXT_SWITCH_TRACER) += trace_sched_switch.o
 obj-$(CONFIG_SYSPROF_TRACER) += trace_sysprof.o
 obj-$(CONFIG_FUNCTION_TRACER) += trace_functions.o

kernel/trace/trace.c

@@ -2354,6 +2354,7 @@ static int tracing_set_tracer(char *buf)
 		if (ret)
 			goto out;
 	}
+	init_tracer_stat(t);
 
 	trace_branch_enable(tr);
  out:
@@ -3206,7 +3207,7 @@ __init static int tracer_alloc_buffers(void)
 #else
 	current_trace = &nop_trace;
 #endif
-
+	init_tracer_stat(current_trace);
 	/* All seems OK, enable tracing */
 	tracing_disabled = 0;
 

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;

kernel/trace/trace_stat.c

@@ -0,0 +1,251 @@
+/*
+ * Infrastructure for statistic tracing (histogram output).
+ *
+ * Copyright (C) 2008 Frederic Weisbecker <fweisbec@gmail.com>
+ *
+ * Based on the code from trace_branch.c which is
+ * Copyright (C) 2008 Steven Rostedt <srostedt@redhat.com>
+ *
+ */
+
+
+#include <linux/list.h>
+#include <linux/seq_file.h>
+#include <linux/debugfs.h>
+#include "trace.h"
+
+
+/* List of stat entries from a tracer */
+struct trace_stat_list {
+	struct list_head list;
+	void *stat;
+};
+
+static struct trace_stat_list stat_list;
+
+/*
+ * This is a copy of the current tracer to avoid racy
+ * and dangerous output while the current tracer is
+ * switched.
+ */
+static struct tracer current_tracer;
+
+/*
+ * Protect both the current tracer and the global
+ * stat list.
+ */
+static DEFINE_MUTEX(stat_list_mutex);
+
+
+static void reset_stat_list(void)
+{
+	struct trace_stat_list *node;
+	struct list_head *next;
+
+	if (list_empty(&stat_list.list))
+		return;
+
+	node = list_entry(stat_list.list.next, struct trace_stat_list, list);
+	next = node->list.next;
+
+	while (&node->list != next) {
+		kfree(node);
+		node = list_entry(next, struct trace_stat_list, list);
+	}
+	kfree(node);
+
+	INIT_LIST_HEAD(&stat_list.list);
+}
+
+void init_tracer_stat(struct tracer *trace)
+{
+	mutex_lock(&stat_list_mutex);
+	current_tracer = *trace;
+	mutex_unlock(&stat_list_mutex);
+}
+
+/*
+ * For tracers that don't provide a stat_cmp callback.
+ * This one will force an immediate insertion on tail of
+ * the list.
+ */
+static int dummy_cmp(void *p1, void *p2)
+{
+	return 1;
+}
+
+/*
+ * Initialize the stat list at each trace_stat file opening.
+ * All of these copies and sorting are required on all opening
+ * since the stats could have changed between two file sessions.
+ */
+static int stat_seq_init(void)
+{
+	struct trace_stat_list *iter_entry, *new_entry;
+	void *prev_stat;
+	int ret = 0;
+	int i;
+
+	mutex_lock(&stat_list_mutex);
+	reset_stat_list();
+
+	if (!current_tracer.stat_start || !current_tracer.stat_next ||
+					!current_tracer.stat_show)
+		goto exit;
+
+	if (!current_tracer.stat_cmp)
+		current_tracer.stat_cmp = dummy_cmp;
+
+	/*
+	 * The first entry. Actually this is the second, but the first
+	 * one (the stat_list head) is pointless.
+	 */
+	new_entry = kmalloc(sizeof(struct trace_stat_list), GFP_KERNEL);
+	if (!new_entry) {
+		ret = -ENOMEM;
+		goto exit;
+	}
+
+	INIT_LIST_HEAD(&new_entry->list);
+	list_add(&new_entry->list, &stat_list.list);
+	new_entry->stat = current_tracer.stat_start();
+
+	prev_stat = new_entry->stat;
+
+	/*
+	 * Iterate over the tracer stat entries and store them in a sorted
+	 * list.
+	 */
+	for (i = 1; ; i++) {
+		new_entry = kmalloc(sizeof(struct trace_stat_list), GFP_KERNEL);
+		if (!new_entry) {
+			ret = -ENOMEM;
+			goto exit_free_list;
+		}
+
+		INIT_LIST_HEAD(&new_entry->list);
+		new_entry->stat = current_tracer.stat_next(prev_stat, i);
+
+		/* End of insertion */
+		if (!new_entry->stat)
+			break;
+
+		list_for_each_entry(iter_entry, &stat_list.list, list) {
+			/* Insertion with a descendent sorting */
+			if (current_tracer.stat_cmp(new_entry->stat,
+						iter_entry->stat) > 0) {
+
+				list_add_tail(&new_entry->list,
+						&iter_entry->list);
+				break;
+
+			/* The current smaller value */
+			} else if (list_is_last(&iter_entry->list,
+						&stat_list.list)) {
+				list_add(&new_entry->list, &iter_entry->list);
+				break;
+			}
+		}
+
+		prev_stat = new_entry->stat;
+	}
+exit:
+	mutex_unlock(&stat_list_mutex);
+	return ret;
+
+exit_free_list:
+	reset_stat_list();
+	mutex_unlock(&stat_list_mutex);
+	return ret;
+}
+
+
+static void *stat_seq_start(struct seq_file *s, loff_t *pos)
+{
+	struct trace_stat_list *l = (struct trace_stat_list *)s->private;
+
+	/* Prevent from tracer switch or stat_list modification */
+	mutex_lock(&stat_list_mutex);
+
+	/* If we are in the beginning of the file, print the headers */
+	if (!*pos && current_tracer.stat_headers)
+		current_tracer.stat_headers(s);
+
+	return seq_list_start(&l->list, *pos);
+}
+
+static void *stat_seq_next(struct seq_file *s, void *p, loff_t *pos)
+{
+	struct trace_stat_list *l = (struct trace_stat_list *)s->private;
+
+	return seq_list_next(p, &l->list, pos);
+}
+
+static void stat_seq_stop(struct seq_file *m, void *p)
+{
+	mutex_unlock(&stat_list_mutex);
+}
+
+static int stat_seq_show(struct seq_file *s, void *v)
+{
+	struct trace_stat_list *l = list_entry(v, struct trace_stat_list, list);
+	return current_tracer.stat_show(s, l->stat);
+}
+
+static const struct seq_operations trace_stat_seq_ops = {
+	.start = stat_seq_start,
+	.next = stat_seq_next,
+	.stop = stat_seq_stop,
+	.show = stat_seq_show
+};
+
+static int tracing_stat_open(struct inode *inode, struct file *file)
+{
+	int ret;
+
+	ret = seq_open(file, &trace_stat_seq_ops);
+	if (!ret) {
+		struct seq_file *m = file->private_data;
+		m->private = &stat_list;
+		ret = stat_seq_init();
+	}
+
+	return ret;
+}
+
+
+/*
+ * Avoid consuming memory with our now useless list.
+ */
+static int tracing_stat_release(struct inode *i, struct file *f)
+{
+	mutex_lock(&stat_list_mutex);
+	reset_stat_list();
+	mutex_unlock(&stat_list_mutex);
+	return 0;
+}
+
+static const struct file_operations tracing_stat_fops = {
+	.open		= tracing_stat_open,
+	.read		= seq_read,
+	.llseek		= seq_lseek,
+	.release	= tracing_stat_release
+};
+
+static int __init tracing_stat_init(void)
+{
+	struct dentry *d_tracing;
+	struct dentry *entry;
+
+	INIT_LIST_HEAD(&stat_list.list);
+	d_tracing = tracing_init_dentry();
+
+	entry = debugfs_create_file("trace_stat", 0444, d_tracing,
+					NULL,
+				    &tracing_stat_fops);
+	if (!entry)
+		pr_warning("Could not create debugfs "
+			   "'trace_stat' entry\n");
+	return 0;
+}
+fs_initcall(tracing_stat_init);