Btrfs: allow block group cache writeout outside critical section in commit

We loop through all of the dirty block groups during commit and write
the free space cache.  In order to make sure the cache is currect, we do
this while no other writers are allowed in the commit.

If a large number of block groups are dirty, this can introduce long
stalls during the final stages of the commit, which can block new procs
trying to change the filesystem.

This commit changes the block group cache writeout to take appropriate
locks and allow it to run earlier in the commit.  We'll still have to
redo some of the block groups, but it means we can get most of the work
out of the way without blocking the entire FS.

Signed-off-by: Chris Mason <clm@fb.com>

1 files changed, 37 insertions, 1 deletions

diff --git a/fs/btrfs/transaction.c b/fs/btrfs/transaction.c
index 234d6063bbf3..5628e25250c0 100644
--- a/fs/btrfs/transaction.c
+++ b/fs/btrfs/transaction.c
@@ -222,6 +222,7 @@ loop:
 	atomic_set(&cur_trans->use_count, 2);
 	cur_trans->have_free_bgs = 0;
 	cur_trans->start_time = get_seconds();
+	cur_trans->dirty_bg_run = 0;
 
 	cur_trans->delayed_refs.href_root = RB_ROOT;
 	atomic_set(&cur_trans->delayed_refs.num_entries, 0);
@@ -251,6 +252,8 @@ loop:
 	INIT_LIST_HEAD(&cur_trans->switch_commits);
 	INIT_LIST_HEAD(&cur_trans->pending_ordered);
 	INIT_LIST_HEAD(&cur_trans->dirty_bgs);
+	INIT_LIST_HEAD(&cur_trans->io_bgs);
+	mutex_init(&cur_trans->cache_write_mutex);
 	cur_trans->num_dirty_bgs = 0;
 	spin_lock_init(&cur_trans->dirty_bgs_lock);
 	list_add_tail(&cur_trans->list, &fs_info->trans_list);
@@ -1059,6 +1062,7 @@ static noinline int commit_cowonly_roots(struct btrfs_trans_handle *trans,
 {
 	struct btrfs_fs_info *fs_info = root->fs_info;
 	struct list_head *dirty_bgs = &trans->transaction->dirty_bgs;
+	struct list_head *io_bgs = &trans->transaction->io_bgs;
 	struct list_head *next;
 	struct extent_buffer *eb;
 	int ret;
@@ -1112,7 +1116,7 @@ again:
 			return ret;
 	}
 
-	while (!list_empty(dirty_bgs)) {
+	while (!list_empty(dirty_bgs) || !list_empty(io_bgs)) {
 		ret = btrfs_write_dirty_block_groups(trans, root);
 		if (ret)
 			return ret;
@@ -1812,6 +1816,37 @@ int btrfs_commit_transaction(struct btrfs_trans_handle *trans,
 		return ret;
 	}
 
+	if (!cur_trans->dirty_bg_run) {
+		int run_it = 0;
+
+		/* this mutex is also taken before trying to set
+		 * block groups readonly.  We need to make sure
+		 * that nobody has set a block group readonly
+		 * after a extents from that block group have been
+		 * allocated for cache files.  btrfs_set_block_group_ro
+		 * will wait for the transaction to commit if it
+		 * finds dirty_bg_run = 1
+		 *
+		 * The dirty_bg_run flag is also used to make sure only
+		 * one process starts all the block group IO.  It wouldn't
+		 * hurt to have more than one go through, but there's no
+		 * real advantage to it either.
+		 */
+		mutex_lock(&root->fs_info->ro_block_group_mutex);
+		if (!cur_trans->dirty_bg_run) {
+			run_it = 1;
+			cur_trans->dirty_bg_run = 1;
+		}
+		mutex_unlock(&root->fs_info->ro_block_group_mutex);
+
+		if (run_it)
+			ret = btrfs_start_dirty_block_groups(trans, root);
+	}
+	if (ret) {
+		btrfs_end_transaction(trans, root);
+		return ret;
+	}
+
 	spin_lock(&root->fs_info->trans_lock);
 	list_splice(&trans->ordered, &cur_trans->pending_ordered);
 	if (cur_trans->state >= TRANS_STATE_COMMIT_START) {
@@ -2005,6 +2040,7 @@ int btrfs_commit_transaction(struct btrfs_trans_handle *trans,
 
 	assert_qgroups_uptodate(trans);
 	ASSERT(list_empty(&cur_trans->dirty_bgs));
+	ASSERT(list_empty(&cur_trans->io_bgs));
 	update_super_roots(root);
 
 	btrfs_set_super_log_root(root->fs_info->super_copy, 0);