[PATCH] f2fs: flush plug periodically during GC to maximize readahead effect

[Daeho Jeong <daeho43@gmail.com>]

From: Daeho Jeong <daehojeong@google.com>

During the garbage collection process, F2FS submits readahead I/Os for
valid blocks. However, since the GC loop runs within a single plug scope
without intermediate flushing, these readahead I/Os often accumulate in
the block layer's plug list instead of being dispatched to the device
immediately.

Consequently, when the GC thread attempts to lock the page later, the
I/O might not have completed (or even started), leading to a "read try
and wait" scenario. This negates the benefit of readahead and causes
unnecessary delays in GC latency.

This patch addresses this issue by introducing an intermediate
blk_finish_plug() and blk_start_plug() pair within the GC loop. This
forces the dispatch of pending I/Os, ensuring that readahead pages are
fetched in time, thereby reducing GC latency.

Signed-off-by: Daeho Jeong <daehojeong@google.com>
---
 fs/f2fs/gc.c | 19 +++++++++++++------
 1 file changed, 13 insertions(+), 6 deletions(-)

diff --git a/fs/f2fs/gc.c b/fs/f2fs/gc.c
index 384fa7e2085b..8ffc3d4f7989 100644
--- a/fs/f2fs/gc.c
+++ b/fs/f2fs/gc.c
@@ -1031,7 +1031,8 @@ static int check_valid_map(struct f2fs_sb_info *sbi,
  * ignore that.
  */
 static int gc_node_segment(struct f2fs_sb_info *sbi,
-		struct f2fs_summary *sum, unsigned int segno, int gc_type)
+		struct f2fs_summary *sum, unsigned int segno, int gc_type,
+		struct blk_plug *plug)
 {
 	struct f2fs_summary *entry;
 	block_t start_addr;
@@ -1100,8 +1101,11 @@ static int gc_node_segment(struct f2fs_sb_info *sbi,
 		stat_inc_node_blk_count(sbi, 1, gc_type);
 	}
 
-	if (++phase < 3)
+	if (++phase < 3) {
+		blk_finish_plug(plug);
+		blk_start_plug(plug);
 		goto next_step;
+	}
 
 	if (fggc)
 		atomic_dec(&sbi->wb_sync_req[NODE]);
@@ -1535,7 +1539,7 @@ static int move_data_page(struct inode *inode, block_t bidx, int gc_type,
  */
 static int gc_data_segment(struct f2fs_sb_info *sbi, struct f2fs_summary *sum,
 		struct gc_inode_list *gc_list, unsigned int segno, int gc_type,
-		bool force_migrate)
+		bool force_migrate, struct blk_plug *plug)
 {
 	struct super_block *sb = sbi->sb;
 	struct f2fs_summary *entry;
@@ -1703,8 +1707,11 @@ static int gc_data_segment(struct f2fs_sb_info *sbi, struct f2fs_summary *sum,
 		}
 	}
 
-	if (++phase < 5)
+	if (++phase < 5) {
+		blk_finish_plug(plug);
+		blk_start_plug(plug);
 		goto next_step;
+	}
 
 	return submitted;
 }
@@ -1853,11 +1860,11 @@ static int do_garbage_collect(struct f2fs_sb_info *sbi,
 			 */
 			if (type == SUM_TYPE_NODE)
 				submitted += gc_node_segment(sbi, sum->entries,
-						cur_segno, gc_type);
+						cur_segno, gc_type, &plug);
 			else
 				submitted += gc_data_segment(sbi, sum->entries,
 						gc_list, cur_segno,
-						gc_type, force_migrate);
+						gc_type, force_migrate, &plug);
 
 			stat_inc_gc_seg_count(sbi, data_type, gc_type);
 			sbi->gc_reclaimed_segs[sbi->gc_mode]++;
-- 
2.52.0.351.gbe84eed79e-goog