[PATCH 2/2] btrfs: Expose the BTRFS commit stats through sysfs

[Ioannis Angelakopoulos <iangelak@fb.com>]

Create a new sysfs entry named "commit_stats" for each mounted BTRFS
filesystem. The entry exposes: 1) The number of commits so far, 2) The
duration of the last commit in ms, 3) The maximum commit duration seen
so far in ms and 4) The total duration for all commits so far in ms.

The function "btrfs_commit_stats_show" is responsible for exposing the
stats to user space.

The function "btrfs_commit_stats_store" is responsible for resetting the
above values to zero.

Signed-off-by: Ioannis Angelakopoulos <iangelak@fb.com>
---
 fs/btrfs/sysfs.c | 48 ++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 48 insertions(+)

diff --git a/fs/btrfs/sysfs.c b/fs/btrfs/sysfs.c
index b6cb5551050e..f68fc73006c0 100644
--- a/fs/btrfs/sysfs.c
+++ b/fs/btrfs/sysfs.c
@@ -991,6 +991,53 @@ static ssize_t btrfs_sectorsize_show(struct kobject *kobj,
 
 BTRFS_ATTR(, sectorsize, btrfs_sectorsize_show);
 
+static ssize_t btrfs_commit_stats_show(struct kobject *kobj,
+				struct kobj_attribute *a, char *buf)
+{
+	struct btrfs_fs_info *fs_info = to_fs_info(kobj);
+
+	/*
+	 * Expose the commits so far, the duration of the last commit, the
+	 * maximum duration of a commit so far and the total duration of
+	 * all the commits so far
+	 */
+	return sysfs_emit(buf, "Commits: %llu, Last: %llu ms, Max: %llu ms, Total: %llu ms\n",
+					  fs_info->commit_stats->commit_counter,
+					  fs_info->commit_stats->last_commit_dur,
+					  fs_info->commit_stats->max_commit_dur,
+					  fs_info->commit_stats->total_commit_dur);
+}
+
+static ssize_t btrfs_commit_stats_store(struct kobject *kobj,
+						struct kobj_attribute *a,
+						const char *buf, size_t len)
+{
+	struct btrfs_fs_info *fs_info = to_fs_info(kobj);
+
+	if (!fs_info)
+		return -EPERM;
+
+	if (!capable(CAP_SYS_RESOURCE))
+		return -EPERM;
+
+	/*
+	 * Just reset everything
+	 * Also take the trans_lock to avoid race conditions with the udpates
+	 * in btrfs_commit_transaction()
+	 */
+	spin_lock(&fs_info->trans_lock);
+	fs_info->commit_stats->commit_counter = 0;
+	fs_info->commit_stats->last_commit_dur = 0;
+	fs_info->commit_stats->max_commit_dur = 0;
+	fs_info->commit_stats->total_commit_dur = 0;
+	spin_unlock(&fs_info->trans_lock);
+
+	return len;
+}
+
+BTRFS_ATTR_RW(, commit_stats, btrfs_commit_stats_show,
+			  btrfs_commit_stats_store);
+
 static ssize_t btrfs_clone_alignment_show(struct kobject *kobj,
 				struct kobj_attribute *a, char *buf)
 {
@@ -1230,6 +1277,7 @@ static const struct attribute *btrfs_attrs[] = {
 	BTRFS_ATTR_PTR(, generation),
 	BTRFS_ATTR_PTR(, read_policy),
 	BTRFS_ATTR_PTR(, bg_reclaim_threshold),
+	BTRFS_ATTR_PTR(, commit_stats),
 	NULL,
 };
 
-- 
2.30.2