[PATCH -next v2 5/7] md/md-bitmap: add a new helper to unplug bitmap asynchrously

[Yu Kuai <yukuai1@xxxxxxxxxxxxxxx>]

From: Yu Kuai <yukuai3@xxxxxxxxxx>

If bitmap is enabled, bitmap must update before submitting write io, this
is why unplug callback must move these io to 'conf->pending_io_list' if
'current->bio_list' is not empty, which will suffer performance
degradation.

This patch add a new helper md_bitmap_unplug_async() to submit bitmap io
in a kworker, so that submit bitmap io in raid10_unplug() doesn't require
that 'current->bio_list' is empty.

This patch prepare to limit the number of plugged bio.

Signed-off-by: Yu Kuai <yukuai3@xxxxxxxxxx>
---
 drivers/md/md-bitmap.c | 51 +++++++++++++++++++++++++++++++++++++++---
 drivers/md/md-bitmap.h |  3 +++
 2 files changed, 51 insertions(+), 3 deletions(-)

diff --git a/drivers/md/md-bitmap.c b/drivers/md/md-bitmap.c
index 17e988f88303..1afee157b96a 100644
--- a/drivers/md/md-bitmap.c
+++ b/drivers/md/md-bitmap.c
@@ -1023,10 +1023,12 @@ static int md_bitmap_file_test_bit(struct bitmap *bitmap, sector_t block)
 	return set;
 }
 
-/* this gets called when the md device is ready to unplug its underlying
+/*
+ * This gets called when the md device is ready to unplug its underlying
  * (slave) device queues -- before we let any writes go down, we need to
- * sync the dirty pages of the bitmap file to disk */
-void md_bitmap_unplug(struct bitmap *bitmap)
+ * sync the dirty pages of the bitmap file to disk.
+ */
+static void md_do_bitmap_unplug(struct bitmap *bitmap)
 {
 	unsigned long i;
 	int dirty, need_write;
@@ -1059,8 +1061,42 @@ void md_bitmap_unplug(struct bitmap *bitmap)
 	if (test_bit(BITMAP_WRITE_ERROR, &bitmap->flags))
 		md_bitmap_file_kick(bitmap);
 }
+
+void md_bitmap_unplug(struct bitmap *bitmap)
+{
+	return md_do_bitmap_unplug(bitmap);
+}
 EXPORT_SYMBOL(md_bitmap_unplug);
 
+struct bitmap_unplug_work {
+	struct work_struct work;
+	struct bitmap *bitmap;
+	struct completion *done;
+};
+
+static void md_bitmap_unplug_fn(struct work_struct *work)
+{
+	struct bitmap_unplug_work *unplug_work =
+		container_of(work, struct bitmap_unplug_work, work);
+
+	md_do_bitmap_unplug(unplug_work->bitmap);
+	complete(unplug_work->done);
+}
+
+void md_bitmap_unplug_async(struct bitmap *bitmap)
+{
+	DECLARE_COMPLETION_ONSTACK(done);
+	struct bitmap_unplug_work unplug_work;
+
+	INIT_WORK(&unplug_work.work, md_bitmap_unplug_fn);
+	unplug_work.bitmap = bitmap;
+	unplug_work.done = &done;
+
+	queue_work(bitmap->unplug_wq, &unplug_work.work);
+	wait_for_completion(&done);
+}
+EXPORT_SYMBOL(md_bitmap_unplug_async);
+
 static void md_bitmap_set_memory_bits(struct bitmap *bitmap, sector_t offset, int needed);
 /* * bitmap_init_from_disk -- called at bitmap_create time to initialize
  * the in-memory bitmap from the on-disk bitmap -- also, sets up the
@@ -1776,6 +1812,9 @@ void md_bitmap_free(struct bitmap *bitmap)
 	if (!bitmap) /* there was no bitmap */
 		return;
 
+	if (bitmap->unplug_wq)
+		destroy_workqueue(bitmap->unplug_wq);
+
 	if (bitmap->sysfs_can_clear)
 		sysfs_put(bitmap->sysfs_can_clear);
 
@@ -1866,6 +1905,12 @@ struct bitmap *md_bitmap_create(struct mddev *mddev, int slot)
 	if (!bitmap)
 		return ERR_PTR(-ENOMEM);
 
+	bitmap->unplug_wq = create_workqueue("md_bitmap");
+	if (!bitmap->unplug_wq) {
+		err  = -ENOMEM;
+		goto error;
+	}
+
 	spin_lock_init(&bitmap->counts.lock);
 	atomic_set(&bitmap->pending_writes, 0);
 	init_waitqueue_head(&bitmap->write_wait);
diff --git a/drivers/md/md-bitmap.h b/drivers/md/md-bitmap.h
index 3a4750952b3a..55531669db24 100644
--- a/drivers/md/md-bitmap.h
+++ b/drivers/md/md-bitmap.h
@@ -231,6 +231,8 @@ struct bitmap {
 
 	struct kernfs_node *sysfs_can_clear;
 	int cluster_slot;		/* Slot offset for clustered env */
+
+	struct workqueue_struct *unplug_wq;
 };
 
 /* the bitmap API */
@@ -264,6 +266,7 @@ void md_bitmap_sync_with_cluster(struct mddev *mddev,
 				 sector_t new_lo, sector_t new_hi);
 
 void md_bitmap_unplug(struct bitmap *bitmap);
+void md_bitmap_unplug_async(struct bitmap *bitmap);
 void md_bitmap_daemon_work(struct mddev *mddev);
 
 int md_bitmap_resize(struct bitmap *bitmap, sector_t blocks,
-- 
2.39.2