md/md-bitmap: add a new helper to unplug bitmap asynchrously

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.

A new helper md_bitmap_unplug_async() is introduced 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@huawei.com>
Signed-off-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/r/20230529131106.2123367-6-yukuai1@huaweicloud.com

drivers/md/md-bitmap.c

@@ -1054,6 +1054,35 @@ void md_bitmap_unplug(struct bitmap *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_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_ONSTACK(&unplug_work.work, md_bitmap_unplug_fn);
+	unplug_work.bitmap = bitmap;
+	unplug_work.done = &done;
+
+	queue_work(md_bitmap_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

drivers/md/md-bitmap.h

@@ -264,6 +264,7 @@ 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,

drivers/md/md.c

@@ -83,6 +83,7 @@ static struct module *md_cluster_mod;
 static DECLARE_WAIT_QUEUE_HEAD(resync_wait);
 static struct workqueue_struct *md_wq;
 static struct workqueue_struct *md_misc_wq;
+struct workqueue_struct *md_bitmap_wq;
 
 static int remove_and_add_spares(struct mddev *mddev,
 				 struct md_rdev *this);
@@ -9637,6 +9638,11 @@ static int __init md_init(void)
 	if (!md_misc_wq)
 		goto err_misc_wq;
 
+	md_bitmap_wq = alloc_workqueue("md_bitmap", WQ_MEM_RECLAIM | WQ_UNBOUND,
+				       0);
+	if (!md_bitmap_wq)
+		goto err_bitmap_wq;
+
 	ret = __register_blkdev(MD_MAJOR, "md", md_probe);
 	if (ret < 0)
 		goto err_md;
@@ -9655,6 +9661,8 @@ static int __init md_init(void)
 err_mdp:
 	unregister_blkdev(MD_MAJOR, "md");
 err_md:
+	destroy_workqueue(md_bitmap_wq);
+err_bitmap_wq:
 	destroy_workqueue(md_misc_wq);
 err_misc_wq:
 	destroy_workqueue(md_wq);
@@ -9951,6 +9959,7 @@ static __exit void md_exit(void)
 	spin_unlock(&all_mddevs_lock);
 
 	destroy_workqueue(md_misc_wq);
+	destroy_workqueue(md_bitmap_wq);
 	destroy_workqueue(md_wq);
 }
 

drivers/md/md.h

@@ -852,6 +852,7 @@ struct mdu_array_info_s;
 struct mdu_disk_info_s;
 
 extern int mdp_major;
+extern struct workqueue_struct *md_bitmap_wq;
 void md_autostart_arrays(int part);
 int md_set_array_info(struct mddev *mddev, struct mdu_array_info_s *info);
 int md_add_new_disk(struct mddev *mddev, struct mdu_disk_info_s *info);