md: expose max value of behind writes counter

Keep track of the maximum number of concurrent write-behind requests
for an md array and exposed this number in sysfs at
   md/bitmap/max_backlog_used

Writing any value to this file will clear it.

This allows userspace to be involved in tuning bitmap/backlog.

Signed-off-by: Paul Clements <paul.clements@steeleye.com>
Signed-off-by: NeilBrown <neilb@suse.de>

2 files changed, 29 insertions, 1 deletions

diff --git a/drivers/md/bitmap.c b/drivers/md/bitmap.c
index 26ac8aad0b19..6279393db64d 100644
--- a/drivers/md/bitmap.c
+++ b/drivers/md/bitmap.c
@@ -1292,9 +1292,14 @@ int bitmap_startwrite(struct bitmap *bitmap, sector_t offset, unsigned long sect
         if (!bitmap) return 0;
 
         if (behind) {
+                int bw;
                 atomic_inc(&bitmap->behind_writes);
+                bw = atomic_read(&bitmap->behind_writes);
+                if (bw > bitmap->behind_writes_used)
+                        bitmap->behind_writes_used = bw;
+
                 PRINTK(KERN_DEBUG "inc write-behind count %d/%d\n",
-                  atomic_read(&bitmap->behind_writes), bitmap->max_write_behind);
+                       bw, bitmap->max_write_behind);
         }
 
         while (sectors) {
@@ -2006,6 +2011,27 @@ static ssize_t can_clear_store(mddev_t *mddev, const char *buf, size_t len)
 static struct md_sysfs_entry bitmap_can_clear =
 __ATTR(can_clear, S_IRUGO|S_IWUSR, can_clear_show, can_clear_store);
 
+static ssize_t
+behind_writes_used_show(mddev_t *mddev, char *page)
+{
+        if (mddev->bitmap == NULL)
+                return sprintf(page, "0\n");
+        return sprintf(page, "%lu\n",
+                       mddev->bitmap->behind_writes_used);
+}
+
+static ssize_t
+behind_writes_used_reset(mddev_t *mddev, const char *buf, size_t len)
+{
+        if (mddev->bitmap)
+                mddev->bitmap->behind_writes_used = 0;
+        return len;
+}
+
+static struct md_sysfs_entry max_backlog_used =
+__ATTR(max_backlog_used, S_IRUGO | S_IWUSR,
+       behind_writes_used_show, behind_writes_used_reset);
+
 static struct attribute *md_bitmap_attrs[] = {
         &bitmap_location.attr,
         &bitmap_timeout.attr,
@@ -2013,6 +2039,7 @@ static struct attribute *md_bitmap_attrs[] = {
         &bitmap_chunksize.attr,
         &bitmap_metadata.attr,
         &bitmap_can_clear.attr,
+        &max_backlog_used.attr,
         NULL
 };
 struct attribute_group md_bitmap_group = {
diff --git a/drivers/md/bitmap.h b/drivers/md/bitmap.h
index cb821d76d1b4..aa82b7caa85f 100644
--- a/drivers/md/bitmap.h
+++ b/drivers/md/bitmap.h
@@ -227,6 +227,7 @@ struct bitmap {
         int allclean;
 
         atomic_t behind_writes;
+        unsigned long behind_writes_used; /* highest actual value at runtime */
 
         /*
          * the bitmap daemon - periodically wakes up and sweeps the bitmap