FS-Cache: Add use of /proc and presentation of statistics

Make FS-Cache create its /proc interface and present various statistical
information through it.  Also provide the functions for updating this
information.

These features are enabled by:

	CONFIG_FSCACHE_PROC
	CONFIG_FSCACHE_STATS
	CONFIG_FSCACHE_HISTOGRAM

The /proc directory for FS-Cache is also exported so that caching modules can
add their own statistics there too.

The FS-Cache module is loadable at this point, and the statistics files can be
examined by userspace:

	cat /proc/fs/fscache/stats
	cat /proc/fs/fscache/histogram

Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: Steve Dickson <steved@redhat.com>
Acked-by: Trond Myklebust <Trond.Myklebust@netapp.com>
Acked-by: Al Viro <viro@zeniv.linux.org.uk>
Tested-by: Daire Byrne <Daire.Byrne@framestore.com>

fs/fscache/histogram.c

@@ -0,0 +1,109 @@
+/* FS-Cache latency histogram
+ *
+ * Copyright (C) 2008 Red Hat, Inc. All Rights Reserved.
+ * Written by David Howells (dhowells@redhat.com)
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public Licence
+ * as published by the Free Software Foundation; either version
+ * 2 of the Licence, or (at your option) any later version.
+ */
+
+#define FSCACHE_DEBUG_LEVEL THREAD
+#include <linux/module.h>
+#include <linux/proc_fs.h>
+#include <linux/seq_file.h>
+#include "internal.h"
+
+atomic_t fscache_obj_instantiate_histogram[HZ];
+atomic_t fscache_objs_histogram[HZ];
+atomic_t fscache_ops_histogram[HZ];
+atomic_t fscache_retrieval_delay_histogram[HZ];
+atomic_t fscache_retrieval_histogram[HZ];
+
+/*
+ * display the time-taken histogram
+ */
+static int fscache_histogram_show(struct seq_file *m, void *v)
+{
+	unsigned long index;
+	unsigned n[5], t;
+
+	switch ((unsigned long) v) {
+	case 1:
+		seq_puts(m, "JIFS  SECS  OBJ INST  OP RUNS   OBJ RUNS "
+			 " RETRV DLY RETRIEVLS\n");
+		return 0;
+	case 2:
+		seq_puts(m, "===== ===== ========= ========= ========="
+			 " ========= =========\n");
+		return 0;
+	default:
+		index = (unsigned long) v - 3;
+		n[0] = atomic_read(&fscache_obj_instantiate_histogram[index]);
+		n[1] = atomic_read(&fscache_ops_histogram[index]);
+		n[2] = atomic_read(&fscache_objs_histogram[index]);
+		n[3] = atomic_read(&fscache_retrieval_delay_histogram[index]);
+		n[4] = atomic_read(&fscache_retrieval_histogram[index]);
+		if (!(n[0] | n[1] | n[2] | n[3] | n[4]))
+			return 0;
+
+		t = (index * 1000) / HZ;
+
+		seq_printf(m, "%4lu  0.%03u %9u %9u %9u %9u %9u\n",
+			   index, t, n[0], n[1], n[2], n[3], n[4]);
+		return 0;
+	}
+}
+
+/*
+ * set up the iterator to start reading from the first line
+ */
+static void *fscache_histogram_start(struct seq_file *m, loff_t *_pos)
+{
+	if ((unsigned long long)*_pos >= HZ + 2)
+		return NULL;
+	if (*_pos == 0)
+		*_pos = 1;
+	return (void *)(unsigned long) *_pos;
+}
+
+/*
+ * move to the next line
+ */
+static void *fscache_histogram_next(struct seq_file *m, void *v, loff_t *pos)
+{
+	(*pos)++;
+	return (unsigned long long)*pos > HZ + 2 ?
+		NULL : (void *)(unsigned long) *pos;
+}
+
+/*
+ * clean up after reading
+ */
+static void fscache_histogram_stop(struct seq_file *m, void *v)
+{
+}
+
+static const struct seq_operations fscache_histogram_ops = {
+	.start		= fscache_histogram_start,
+	.stop		= fscache_histogram_stop,
+	.next		= fscache_histogram_next,
+	.show		= fscache_histogram_show,
+};
+
+/*
+ * open "/proc/fs/fscache/histogram" to provide latency data
+ */
+static int fscache_histogram_open(struct inode *inode, struct file *file)
+{
+	return seq_open(file, &fscache_histogram_ops);
+}
+
+const struct file_operations fscache_histogram_fops = {
+	.owner		= THIS_MODULE,
+	.open		= fscache_histogram_open,
+	.read		= seq_read,
+	.llseek		= seq_lseek,
+	.release	= seq_release,
+};