[f2fs-dev][PATCH]f2fs: avoid congestion_wait when do_checkpoint for better performance

[Yuan Zhong <yuan.mark.zhong@xxxxxxxxxxx>]

Previously,  do_checkpoint() will call congestion_wait() for waiting the pages (previous submitted node/meta/data pages) to be written back.
Because congestion_wait() will set a regular period (e.g. HZ / 50 ) for waiting.
For this reason, there is a situation that after the pages have been written back, but the checkpoint thread still wait for congestion_wait to exit.
This is a problem here, especially, when sync a large number of small files or dirs.
In order to avoid this, a wait_list is introduced, the checkpoint thread will be dropped into the wait_list if the pages have not been written back, and will be waked up by contrast.

Signed-off-by: Yuan Zhong <yuan.mark.zhong@xxxxxxxxxxx>
---
 fs/f2fs/checkpoint.c |  3 +--
 fs/f2fs/f2fs.h       | 19 +++++++++++++++++++
 fs/f2fs/segment.c    |  1 +
 fs/f2fs/super.c      |  1 +
 4 files changed, 22 insertions(+), 2 deletions(-)

diff --git a/fs/f2fs/checkpoint.c b/fs/f2fs/checkpoint.c
index bb31220..cf6b4a5 100644
--- a/fs/f2fs/checkpoint.c
+++ b/fs/f2fs/checkpoint.c
@@ -756,8 +756,7 @@ static void do_checkpoint(struct f2fs_sb_info *sbi, bool is_umount)
 	f2fs_put_page(cp_page, 1);
 
 	/* wait for previous submitted node/meta pages writeback */
-	while (get_pages(sbi, F2FS_WRITEBACK))
-		congestion_wait(BLK_RW_ASYNC, HZ / 50);
+	f2fs_writeback_wait(sbi);
 
 	filemap_fdatawait_range(sbi->node_inode->i_mapping, 0, LONG_MAX);
 	filemap_fdatawait_range(sbi->meta_inode->i_mapping, 0, LONG_MAX);
diff --git a/fs/f2fs/f2fs.h b/fs/f2fs/f2fs.h
index 608f0df..f8b62cc 100644
--- a/fs/f2fs/f2fs.h
+++ b/fs/f2fs/f2fs.h
@@ -18,6 +18,7 @@
 #include <linux/crc32.h>
 #include <linux/magic.h>
 #include <linux/kobject.h>
+#include <linux/wait.h>
 
 /*
  * For mount options
@@ -430,6 +431,8 @@ struct f2fs_sb_info {
 	/* For sysfs suppport */
 	struct kobject s_kobj;
 	struct completion s_kobj_unregister;
+	
+	wait_queue_head_t writeback_wqh;
 };
 
 /*
@@ -961,6 +964,22 @@ static inline int f2fs_readonly(struct super_block *sb)
 	return sb->s_flags & MS_RDONLY;
 }
 
+static inline void f2fs_writeback_wait(struct f2fs_sb_info *sbi)
+{
+	DEFINE_WAIT(wait);
+
+	prepare_to_wait(&sbi->writeback_wqh, &wait, TASK_UNINTERRUPTIBLE);
+	if (get_pages(sbi, F2FS_WRITEBACK))
+		io_schedule();
+	finish_wait(&sbi->writeback_wqh, &wait);
+}
+
+static inline void f2fs_writeback_wake(struct f2fs_sb_info *sbi)
+{
+	if (!get_pages(sbi, F2FS_WRITEBACK))
+		wake_up_all(&sbi->writeback_wqh);
+}
+
 /*
  * file.c
  */
diff --git a/fs/f2fs/segment.c b/fs/f2fs/segment.c
index 09af9c7..79293fe 100644
--- a/fs/f2fs/segment.c
+++ b/fs/f2fs/segment.c
@@ -597,6 +597,7 @@ static void f2fs_end_io_write(struct bio *bio, int err)
 
 	if (p->is_sync)
 		complete(p->wait);
+	f2fs_writeback_wake(p->sbi);	
 	kfree(p);
 	bio_put(bio);
 }
diff --git a/fs/f2fs/super.c b/fs/f2fs/super.c
index 13d0a0f..b31f686 100644
--- a/fs/f2fs/super.c
+++ b/fs/f2fs/super.c
@@ -818,6 +818,7 @@ static int f2fs_fill_super(struct super_block *sb, void *data, int silent)
 	mutex_init(&sbi->gc_mutex);
 	mutex_init(&sbi->writepages);
 	mutex_init(&sbi->cp_mutex);
+	init_waitqueue_head(&sbi->writeback_wqh);
 	for (i = 0; i < NR_GLOBAL_LOCKS; i++)
 		mutex_init(&sbi->fs_lock[i]);
 	mutex_init(&sbi->node_write);
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