[PATCH v2] xfs: move inode flush to the sync workqueue

["Darrick J. Wong" <darrick.wong@oracle.com>]

From: Darrick J. Wong <darrick.wong@oracle.com>

Move the inode dirty data flushing to a workqueue so that multiple
threads can take advantage of a single thread's flushing work.  The
ratelimiting technique used in bdd4ee4 was not successful, because
threads that skipped the inode flush scan due to ratelimiting would
ENOSPC early, which caused occasional (but noticeable) changes in
behavior and sporadic fstest regressions.

Therfore, make all the writer threads wait on a single inode flush,
which eliminates both the stampeding hoards of flushers and the small
window in which a write could fail with ENOSPC because it lost the
ratelimit race after even another thread freed space.

Fixes: bdd4ee4f8407 ("xfs: ratelimit inode flush on buffered write ENOSPC")
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
---
v2: run it on the sync workqueue
---
 fs/xfs/xfs_mount.h |    6 +++++-
 fs/xfs/xfs_super.c |   40 ++++++++++++++++++++++------------------
 2 files changed, 27 insertions(+), 19 deletions(-)

diff --git a/fs/xfs/xfs_mount.h b/fs/xfs/xfs_mount.h
index 50c43422fa17..b2e4598fdf7d 100644
--- a/fs/xfs/xfs_mount.h
+++ b/fs/xfs/xfs_mount.h
@@ -167,8 +167,12 @@ typedef struct xfs_mount {
 	struct xfs_kobj		m_error_meta_kobj;
 	struct xfs_error_cfg	m_error_cfg[XFS_ERR_CLASS_MAX][XFS_ERR_ERRNO_MAX];
 	struct xstats		m_stats;	/* per-fs stats */
-	struct ratelimit_state	m_flush_inodes_ratelimit;
 
+	/*
+	 * Workqueue item so that we can coalesce multiple inode flush attempts
+	 * into a single flush.
+	 */
+	struct work_struct	m_flush_inodes_work;
 	struct workqueue_struct *m_buf_workqueue;
 	struct workqueue_struct	*m_unwritten_workqueue;
 	struct workqueue_struct	*m_cil_workqueue;
diff --git a/fs/xfs/xfs_super.c b/fs/xfs/xfs_super.c
index abf06bf9c3f3..424bb9a2d532 100644
--- a/fs/xfs/xfs_super.c
+++ b/fs/xfs/xfs_super.c
@@ -516,6 +516,20 @@ xfs_destroy_mount_workqueues(
 	destroy_workqueue(mp->m_buf_workqueue);
 }
 
+static void
+xfs_flush_inodes_worker(
+	struct work_struct	*work)
+{
+	struct xfs_mount	*mp = container_of(work, struct xfs_mount,
+						   m_flush_inodes_work);
+	struct super_block	*sb = mp->m_super;
+
+	if (down_read_trylock(&sb->s_umount)) {
+		sync_inodes_sb(sb);
+		up_read(&sb->s_umount);
+	}
+}
+
 /*
  * Flush all dirty data to disk. Must not be called while holding an XFS_ILOCK
  * or a page lock. We use sync_inodes_sb() here to ensure we block while waiting
@@ -526,15 +540,15 @@ void
 xfs_flush_inodes(
 	struct xfs_mount	*mp)
 {
-	struct super_block	*sb = mp->m_super;
-
-	if (!__ratelimit(&mp->m_flush_inodes_ratelimit))
+	/*
+	 * If flush_work() returns true then that means we waited for a flush
+	 * which was already in progress.  Don't bother running another scan.
+	 */
+	if (flush_work(&mp->m_flush_inodes_work))
 		return;
 
-	if (down_read_trylock(&sb->s_umount)) {
-		sync_inodes_sb(sb);
-		up_read(&sb->s_umount);
-	}
+	queue_work(mp->m_sync_workqueue, &mp->m_flush_inodes_work);
+	flush_work(&mp->m_flush_inodes_work);
 }
 
 /* Catch misguided souls that try to use this interface on XFS */
@@ -1369,17 +1383,6 @@ xfs_fc_fill_super(
 	if (error)
 		goto out_free_names;
 
-	/*
-	 * Cap the number of invocations of xfs_flush_inodes to 16 for every
-	 * quarter of a second.  The magic numbers here were determined by
-	 * observation neither to cause stalls in writeback when there are a
-	 * lot of IO threads and the fs is near ENOSPC, nor cause any fstest
-	 * regressions.  YMMV.
-	 */
-	ratelimit_state_init(&mp->m_flush_inodes_ratelimit, HZ / 4, 16);
-	ratelimit_set_flags(&mp->m_flush_inodes_ratelimit,
-			RATELIMIT_MSG_ON_RELEASE);
-
 	error = xfs_init_mount_workqueues(mp);
 	if (error)
 		goto out_close_devices;
@@ -1752,6 +1755,7 @@ static int xfs_init_fs_context(
 	spin_lock_init(&mp->m_perag_lock);
 	mutex_init(&mp->m_growlock);
 	atomic_set(&mp->m_active_trans, 0);
+	INIT_WORK(&mp->m_flush_inodes_work, xfs_flush_inodes_worker);
 	INIT_DELAYED_WORK(&mp->m_reclaim_work, xfs_reclaim_worker);
 	INIT_DELAYED_WORK(&mp->m_eofblocks_work, xfs_eofblocks_worker);
 	INIT_DELAYED_WORK(&mp->m_cowblocks_work, xfs_cowblocks_worker);