[PATCH 07/13] btrfs: kick off async delayed ref flushing if we are over time budget

[Josef Bacik <josef@toxicpanda.com>]

For very large file systems we cannot rely on the space reservation
system to provide enough pressure to flush delayed refs in a timely
manner.  We have the infrastructure in place to keep track of how much
theoretical time it'll take to run our outstanding delayed refs, but
unfortunately I ripped all of that out when I added the delayed refs
rsv.  This code originally was added to address the problem of too many
delayed refs building up and thus causing transaction commits to take
several minutes to finish.

Fix this by adding back the ability to flush delayed refs based on the
time budget for them.  We want to limit to around 1 seconds worth of
delayed refs to be pending at any given time.  In order to keep up with
demand we will start the async flusher once we are at the 500ms mark,
and the async flusher will attempt to keep us in this ballpark.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
---
 fs/btrfs/ctree.h       |  4 ++++
 fs/btrfs/disk-io.c     |  3 +++
 fs/btrfs/extent-tree.c | 44 ++++++++++++++++++++++++++++++++++++++++++
 fs/btrfs/transaction.c |  8 ++++++++
 4 files changed, 59 insertions(+)

diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
index 992ce47977b8..2a6b2938f9ea 100644
--- a/fs/btrfs/ctree.h
+++ b/fs/btrfs/ctree.h
@@ -494,6 +494,7 @@ enum btrfs_orphan_cleanup_state {
 };
 
 void btrfs_init_async_reclaim_work(struct btrfs_fs_info *fs_info);
+void btrfs_init_async_delayed_ref_work(struct btrfs_fs_info *fs_info);
 
 /* fs_info */
 struct reloc_control;
@@ -924,6 +925,9 @@ struct btrfs_fs_info {
 	struct work_struct async_reclaim_work;
 	struct work_struct async_data_reclaim_work;
 
+	/* Used to run delayed refs in the background. */
+	struct work_struct async_delayed_ref_work;
+
 	spinlock_t unused_bgs_lock;
 	struct list_head unused_bgs;
 	struct mutex unused_bg_unpin_mutex;
diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index b5846552666e..b1a9fe5a639a 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -2754,6 +2754,7 @@ void btrfs_init_fs_info(struct btrfs_fs_info *fs_info)
 #endif
 	btrfs_init_balance(fs_info);
 	btrfs_init_async_reclaim_work(fs_info);
+	btrfs_init_async_delayed_ref_work(fs_info);
 
 	spin_lock_init(&fs_info->block_group_cache_lock);
 	fs_info->block_group_cache_tree = RB_ROOT;
@@ -3997,6 +3998,8 @@ void __cold close_ctree(struct btrfs_fs_info *fs_info)
 	 */
 	kthread_park(fs_info->cleaner_kthread);
 
+	cancel_work_sync(&fs_info->async_delayed_ref_work);
+
 	/* wait for the qgroup rescan worker to stop */
 	btrfs_qgroup_wait_for_completion(fs_info, false);
 
diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
index 645ae95f465e..0e81990b57e0 100644
--- a/fs/btrfs/extent-tree.c
+++ b/fs/btrfs/extent-tree.c
@@ -2249,6 +2249,50 @@ int btrfs_run_delayed_refs(struct btrfs_trans_handle *trans,
 	return 0;
 }
 
+static void btrfs_async_run_delayed_refs(struct work_struct *work)
+{
+	struct btrfs_fs_info *fs_info;
+	struct btrfs_trans_handle *trans;
+
+	fs_info = container_of(work, struct btrfs_fs_info,
+			       async_delayed_ref_work);
+
+	while (!btrfs_fs_closing(fs_info)) {
+		unsigned long count;
+		int ret;
+
+		trans = btrfs_attach_transaction(fs_info->extent_root);
+		if (IS_ERR(trans))
+			break;
+
+		smp_rmb();
+		if (trans->transaction->delayed_refs.flushing) {
+			btrfs_end_transaction(trans);
+			break;
+		}
+
+		/* No longer over our threshold, lets bail. */
+		if (!btrfs_should_throttle_delayed_refs(trans, true)) {
+			btrfs_end_transaction(trans);
+			break;
+		}
+
+		count = atomic_read(&trans->transaction->delayed_refs.num_entries);
+		count >>= 2;
+
+		ret = btrfs_run_delayed_refs(trans, count);
+		btrfs_end_transaction(trans);
+		if (ret < 0)
+			break;
+	}
+}
+
+void btrfs_init_async_delayed_ref_work(struct btrfs_fs_info *fs_info)
+{
+	INIT_WORK(&fs_info->async_delayed_ref_work,
+		  btrfs_async_run_delayed_refs);
+}
+
 int btrfs_set_disk_extent_flags(struct btrfs_trans_handle *trans,
 				u64 bytenr, u64 num_bytes, u64 flags,
 				int level, int is_data)
diff --git a/fs/btrfs/transaction.c b/fs/btrfs/transaction.c
index b0d82e1a6a6e..7f994ab73b0b 100644
--- a/fs/btrfs/transaction.c
+++ b/fs/btrfs/transaction.c
@@ -899,6 +899,7 @@ static int __btrfs_end_transaction(struct btrfs_trans_handle *trans,
 	struct btrfs_fs_info *info = trans->fs_info;
 	struct btrfs_transaction *cur_trans = trans->transaction;
 	int err = 0;
+	bool run_async = false;
 
 	if (refcount_read(&trans->use_count) > 1) {
 		refcount_dec(&trans->use_count);
@@ -906,6 +907,9 @@ static int __btrfs_end_transaction(struct btrfs_trans_handle *trans,
 		return 0;
 	}
 
+	if (btrfs_should_throttle_delayed_refs(trans, true))
+		run_async = true;
+
 	btrfs_trans_release_metadata(trans);
 	trans->block_rsv = NULL;
 
@@ -936,6 +940,10 @@ static int __btrfs_end_transaction(struct btrfs_trans_handle *trans,
 		err = -EIO;
 	}
 
+	if (run_async && !work_busy(&info->async_delayed_ref_work))
+		queue_work(system_unbound_wq,
+			   &info->async_delayed_ref_work);
+
 	kmem_cache_free(btrfs_trans_handle_cachep, trans);
 	return err;
 }
-- 
2.24.1