f2fs: expose discard related parameters in sysfs

This patch exposes max_discard_request, min_discard_issue_time,
mid_discard_issue_time, and max_discard_issue_time in sysfs. This will
allow the user to fine tune discard operations.
Signed-off-by: Konstantin Vyshetsky <vkon@google.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

Documentation/ABI/testing/sysfs-fs-f2fs

@@ -98,6 +98,33 @@ Description:	Controls the issue rate of discard commands that consist of small
 		checkpoint is triggered, and issued during the checkpoint.
 		By default, it is disabled with 0.
 
+What:		/sys/fs/f2fs/<disk>/max_discard_request
+Date:		December 2021
+Contact:	"Konstantin Vyshetsky" <vkon@google.com>
+Description:	Controls the number of discards a thread will issue at a time.
+		Higher number will allow the discard thread to finish its work
+		faster, at the cost of higher latency for incomming I/O.
+
+What:		/sys/fs/f2fs/<disk>/min_discard_issue_time
+Date:		December 2021
+Contact:	"Konstantin Vyshetsky" <vkon@google.com>
+Description:	Controls the interval the discard thread will wait between
+		issuing discard requests when there are discards to be issued and
+		no I/O aware interruptions occur.
+
+What:		/sys/fs/f2fs/<disk>/mid_discard_issue_time
+Date:		December 2021
+Contact:	"Konstantin Vyshetsky" <vkon@google.com>
+Description:	Controls the interval the discard thread will wait between
+		issuing discard requests when there are discards to be issued and
+		an I/O aware interruption occurs.
+
+What:		/sys/fs/f2fs/<disk>/max_discard_issue_time
+Date:		December 2021
+Contact:	"Konstantin Vyshetsky" <vkon@google.com>
+Description:	Controls the interval the discard thread will wait when there are
+		no discard operations to be issued.
+
 What:		/sys/fs/f2fs/<disk>/discard_granularity
 Date:		July 2017
 Contact:	"Chao Yu" <yuchao0@huawei.com>

fs/f2fs/sysfs.c

@@ -716,6 +716,10 @@ F2FS_RW_ATTR(F2FS_SBI, f2fs_sb_info, gc_idle, gc_mode);
 F2FS_RW_ATTR(F2FS_SBI, f2fs_sb_info, gc_urgent, gc_mode);
 F2FS_RW_ATTR(SM_INFO, f2fs_sm_info, reclaim_segments, rec_prefree_segments);
 F2FS_RW_ATTR(DCC_INFO, discard_cmd_control, max_small_discards, max_discards);
+F2FS_RW_ATTR(DCC_INFO, discard_cmd_control, max_discard_request, max_discard_request);
+F2FS_RW_ATTR(DCC_INFO, discard_cmd_control, min_discard_issue_time, min_discard_issue_time);
+F2FS_RW_ATTR(DCC_INFO, discard_cmd_control, mid_discard_issue_time, mid_discard_issue_time);
+F2FS_RW_ATTR(DCC_INFO, discard_cmd_control, max_discard_issue_time, max_discard_issue_time);
 F2FS_RW_ATTR(DCC_INFO, discard_cmd_control, discard_granularity, discard_granularity);
 F2FS_RW_ATTR(RESERVED_BLOCKS, f2fs_sb_info, reserved_blocks, reserved_blocks);
 F2FS_RW_ATTR(SM_INFO, f2fs_sm_info, batched_trim_sections, trim_sections);
@@ -832,6 +836,10 @@ static struct attribute *f2fs_attrs[] = {
 	ATTR_LIST(reclaim_segments),
 	ATTR_LIST(main_blkaddr),
 	ATTR_LIST(max_small_discards),
+	ATTR_LIST(max_discard_request),
+	ATTR_LIST(min_discard_issue_time),
+	ATTR_LIST(mid_discard_issue_time),
+	ATTR_LIST(max_discard_issue_time),
 	ATTR_LIST(discard_granularity),
 	ATTR_LIST(pending_discard),
 	ATTR_LIST(batched_trim_sections),