[RFC PATCH 2/2] f2fs: export a threshold in sysfs for controlling dio serialization

[Chao Yu <chao2.yu@samsung.com>]

As Yunlei He reported when he test with the patch ("f2fs: enhance
multithread dio write performance"):
"Does share writepages mutex lock have an effect on cache write?
Here is AndroBench result on my phone:

Before patch:
			1R1W		8R8W		16R16W
Sequential Write	161.31		163.85		154.67
Random  Write		9.48		17.66		18.09

After patch:
			1R1W		8R8W		16R16W
Sequential Write	159.61		157.24		160.11
Random  Write		9.17		8.51		8.8

Unit:Mb/s, File size: 64M, Buffer size: 4k"

The turth is androidbench uses single thread with dio write to test performance
of sequential write, and use multi-threads with dio write to test performance
of random write. so we can not see any improvement in sequentail write test
since serializing dio page allocation can only improve performance in
multi-thread scenario, and there is a regression in multi-thread test with 4k
dio write, this is because grabbing sbi->writepages lock for serializing block
allocation stop the concurrency, so that less small dio bios could be merged,
moreover, when there are huge number of small dio writes, grabbing mutex lock
per dio increases the overhead.

After all, serializing dio could only be used for concurrent scenario of
big dio, so this patch introduces a threshold in sysfs to provide user the
interface of defining 'a big dio' with specified page number, which could
be used to control wthether serialize or not that kind of dio with specified
page number.

Though, this is only RFC patch since the optimization works in rare scenario.

Signed-off-by: Chao Yu <chao2.yu@samsung.com>
---
 Documentation/ABI/testing/sysfs-fs-f2fs | 12 ++++++++++++
 fs/f2fs/data.c                          |  3 ++-
 fs/f2fs/f2fs.h                          |  3 +++
 fs/f2fs/super.c                         |  3 +++
 4 files changed, 20 insertions(+), 1 deletion(-)

diff --git a/Documentation/ABI/testing/sysfs-fs-f2fs b/Documentation/ABI/testing/sysfs-fs-f2fs
index 0345f2d..560a4f1 100644
--- a/Documentation/ABI/testing/sysfs-fs-f2fs
+++ b/Documentation/ABI/testing/sysfs-fs-f2fs
@@ -92,3 +92,15 @@ Date:		October 2015
 Contact:	"Chao Yu" <chao2.yu@samsung.com>
 Description:
 		 Controls the count of nid pages to be readaheaded.
+
+What:		/sys/fs/f2fs/<disk>/serialized_dio_pages
+Date:		December 2015
+Contact:	"Chao Yu" <chao2.yu@samsung.com>
+Description:
+		 It is a threshold with the unit of page size.
+                 If DIO page count is equal or big than the threshold,
+                 whole process of block address allocation of dio pages
+                 will become atomic like buffered write.
+                 It is used to maximize bandwidth utilization in the
+                 scenario of concurrent write with dio vs buffered or
+                 dio vs dio.
diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c
index 6b24446..abcd100 100644
--- a/fs/f2fs/data.c
+++ b/fs/f2fs/data.c
@@ -1660,7 +1660,8 @@ static ssize_t f2fs_direct_IO(struct kiocb *iocb, struct iov_iter *iter,
 	trace_f2fs_direct_IO_enter(inode, offset, count, rw);
 
 	if (rw == WRITE) {
-		bool serialized = (F2FS_BYTES_TO_BLK(count) >= 64);
+		bool serialized = (F2FS_BYTES_TO_BLK(count) >=
+						sbi->serialized_dio_pages);
 
 		if (serialized)
 			mutex_lock(&sbi->writepages);
diff --git a/fs/f2fs/f2fs.h b/fs/f2fs/f2fs.h
index 3406e99..8f35dd7 100644
--- a/fs/f2fs/f2fs.h
+++ b/fs/f2fs/f2fs.h
@@ -333,6 +333,8 @@ enum {
 
 #define MAX_DIR_RA_PAGES	4	/* maximum ra pages of dir */
 
+#define DEF_SERIALIZED_DIO_PAGES	64	/* default serialized dio pages */
+
 /* vector size for gang look-up from extent cache that consists of radix tree */
 #define EXT_TREE_VEC_SIZE	64
 
@@ -784,6 +786,7 @@ struct f2fs_sb_info {
 	unsigned int total_valid_inode_count;	/* valid inode count */
 	int active_logs;			/* # of active logs */
 	int dir_level;				/* directory level */
+	int serialized_dio_pages;		/* serialized direct IO pages */
 
 	block_t user_block_count;		/* # of user blocks */
 	block_t total_valid_block_count;	/* # of valid blocks */
diff --git a/fs/f2fs/super.c b/fs/f2fs/super.c
index 75704d9..ebe9bd4 100644
--- a/fs/f2fs/super.c
+++ b/fs/f2fs/super.c
@@ -218,6 +218,7 @@ F2FS_RW_ATTR(NM_INFO, f2fs_nm_info, ram_thresh, ram_thresh);
 F2FS_RW_ATTR(NM_INFO, f2fs_nm_info, ra_nid_pages, ra_nid_pages);
 F2FS_RW_ATTR(F2FS_SBI, f2fs_sb_info, max_victim_search, max_victim_search);
 F2FS_RW_ATTR(F2FS_SBI, f2fs_sb_info, dir_level, dir_level);
+F2FS_RW_ATTR(F2FS_SBI, f2fs_sb_info, serialized_dio_pages, serialized_dio_pages);
 F2FS_RW_ATTR(F2FS_SBI, f2fs_sb_info, cp_interval, cp_interval);
 
 #define ATTR_LIST(name) (&f2fs_attr_##name.attr)
@@ -234,6 +235,7 @@ static struct attribute *f2fs_attrs[] = {
 	ATTR_LIST(min_fsync_blocks),
 	ATTR_LIST(max_victim_search),
 	ATTR_LIST(dir_level),
+	ATTR_LIST(serialized_dio_pages),
 	ATTR_LIST(ram_thresh),
 	ATTR_LIST(ra_nid_pages),
 	ATTR_LIST(cp_interval),
@@ -1125,6 +1127,7 @@ static void init_sb_info(struct f2fs_sb_info *sbi)
 		atomic_set(&sbi->nr_pages[i], 0);
 
 	sbi->dir_level = DEF_DIR_LEVEL;
+	sbi->serialized_dio_pages = DEF_SERIALIZED_DIO_PAGES;
 	sbi->cp_interval = DEF_CP_INTERVAL;
 	clear_sbi_flag(sbi, SBI_NEED_FSCK);
 
-- 
2.6.3