[PATCH AUTOSEL 3.18 58/98] Btrfs: avoid syncing log in the fast fsync path when not necessary

Sasha Levin
2018-10-25T14:18:13Z

From: Filipe Manana <fdmanana@suse.com>

[ Upstream commit b659ef027792219b590d67a2baf1643a93727d29 ]

Commit 3a8b36f37806 ("Btrfs: fix data loss in the fast fsync path") added
a performance regression for that causes an unnecessary sync of the log
trees (fs/subvol and root log trees) when 2 consecutive fsyncs are done
against a file, without no writes or any metadata updates to the inode in
between them and if a transaction is committed before the second fsync is
called.

Huang Ying reported this to lkml (https://lkml.org/lkml/2015/3/18/99)
after a test sysbench test that measured a -62% decrease of file io
requests per second for that tests' workload.

The test is:

echo performance > /sys/devices/system/cpu/cpu0/cpufreq/scaling_governor
echo performance > /sys/devices/system/cpu/cpu1/cpufreq/scaling_governor
echo performance > /sys/devices/system/cpu/cpu2/cpufreq/scaling_governor
echo performance > /sys/devices/system/cpu/cpu3/cpufreq/scaling_governor
mkfs -t btrfs /dev/sda2
mount -t btrfs /dev/sda2 /fs/sda2
cd /fs/sda2
for ((i = 0; i < 1024; i++)); do fallocate -l 67108864 testfile.$i; done
sysbench --test=fileio --max-requests=0 --num-threads=4 --max-time=600 \
--file-test-mode=rndwr --file-total-size=68719476736 --file-io-mode=sync \
--file-num=1024 run

A test on kvm guest, running a debug kernel gave me the following results:

Without 3a8b36f378060d: 16.01 reqs/sec
With 3a8b36f378060d: 3.39 reqs/sec
With 3a8b36f378060d and this patch: 16.04 reqs/sec

Reported-by: Huang Ying <ying.huang@intel.com>
Tested-by: Huang, Ying <ying.huang@intel.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Chris Mason <clm@fb.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
fs/btrfs/file.c | 9 ++++++---
fs/btrfs/ordered-data.c | 14 ++++++++++++++
fs/btrfs/ordered-data.h | 3 +++
3 files changed, 23 insertions(+), 3 deletions(-)

diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
index 2ad4cb3da8f6..ba37ec6263ca 100644
--- a/fs/btrfs/file.c
+++ b/fs/btrfs/file.c
@@ -1879,6 +1879,7 @@ int btrfs_sync_file(struct file *file, loff_t start, loff_t end, int datasync)
struct btrfs_log_ctx ctx;
int ret = 0;
bool full_sync = 0;
+ const u64 len = end - start + 1;

trace_btrfs_sync_file(file, datasync);

@@ -1907,7 +1908,7 @@ int btrfs_sync_file(struct file *file, loff_t start, loff_t end, int datasync)
* all extents are persisted and the respective file extent
* items are in the fs/subvol btree.
*/
- ret = btrfs_wait_ordered_range(inode, start, end - start + 1);
+ ret = btrfs_wait_ordered_range(inode, start, len);
} else {
/*
* Start any new ordered operations before starting to log the
@@ -1979,8 +1980,10 @@ int btrfs_sync_file(struct file *file, loff_t start, loff_t end, int datasync)
*/
smp_mb();
if (btrfs_inode_in_log(inode, root->fs_info->generation) ||
- (full_sync && BTRFS_I(inode)->last_trans <=
- root->fs_info->last_trans_committed)) {
+ (BTRFS_I(inode)->last_trans <=
+ root->fs_info->last_trans_committed &&
+ (full_sync ||
+ !btrfs_have_ordered_extents_in_range(inode, start, len)))) {
/*
* We'v had everything committed since the last time we were
* modified so clear this flag in case it was set for whatever
diff --git a/fs/btrfs/ordered-data.c b/fs/btrfs/ordered-data.c
index b23d024c0234..4c20199cef62 100644
--- a/fs/btrfs/ordered-data.c
+++ b/fs/btrfs/ordered-data.c
@@ -848,6 +848,20 @@ struct btrfs_ordered_extent *btrfs_lookup_ordered_range(struct inode *inode,
return entry;
}

+bool btrfs_have_ordered_extents_in_range(struct inode *inode,
+ u64 file_offset,
+ u64 len)
+{
+ struct btrfs_ordered_extent *oe;
+
+ oe = btrfs_lookup_ordered_range(inode, file_offset, len);
+ if (oe) {
+ btrfs_put_ordered_extent(oe);
+ return true;
+ }
+ return false;
+}
+
/*
* lookup and return any extent before 'file_offset'. NULL is returned
* if none is found
diff --git a/fs/btrfs/ordered-data.h b/fs/btrfs/ordered-data.h
index 0124bffc775f..a1bce0a5cccc 100644
--- a/fs/btrfs/ordered-data.h
+++ b/fs/btrfs/ordered-data.h
@@ -191,6 +191,9 @@ btrfs_lookup_first_ordered_extent(struct inode * inode, u64 file_offset);
struct btrfs_ordered_extent *btrfs_lookup_ordered_range(struct inode *inode,
u64 file_offset,
u64 len);
+bool btrfs_have_ordered_extents_in_range(struct inode *inode,
+ u64 file_offset,
+ u64 len);
int btrfs_ordered_update_i_size(struct inode *inode, u64 offset,
struct btrfs_ordered_extent *ordered);
int btrfs_find_ordered_sum(struct inode *inode, u64 offset, u64 disk_bytenr,
--
2.17.1