Modifying the file position is done on a per-file basis. This renders
holding the inode lock useless and currently holding it makes the
performance of concurrent llseek's abysmal.
Fix this by removing calls to inode_lock. This is correct because
find_desired_extent already includes proper extent locking for the
range it's going to interrogate for finding a DATA/HOLE region. The
other two cases SEEK_END/SEEK_CUR are also safe. The latter is
synchronized by file::f_lock spinlock. SEEK_END is not synchronized
but atomic, but that's OK since there is not guarantee that SEEK_END
will always be at the end of the file in the face of tail modifications. For
more information on locking requirements see ef3d0fd27e90 ("vfs: do (nearly)
lockless generic_file_llseek")
This change brings ~85% performance improvement when doing a lot of
parallel fseeks. The workload essentially does:
for (d=0; d<num_seek_read; d++)
{
/* offset %= 16777216; */
fseek (f, 256 * d % 16777216, SEEK_SET);
fread (buffer, 64, 1, f);
}
Without patch:
num workprocesses = 16
num fseek/fread = 8000000
step = 256
fork 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
real 0m41.412s
user 0m28.777s
sys 2m16.510s
With patch:
num workprocesses = 16
num fseek/fread = 8000000
step = 256
fork 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
real 0m11.479s
user 0m27.629s
sys 0m18.187s
Signed-off-by: Nikolay Borisov <[email protected]>
---
fs/btrfs/file.c | 22 ++++++++--------------
1 file changed, 8 insertions(+), 14 deletions(-)
diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
index 12688ae6e6f2..31111a94251a 100644
--- a/fs/btrfs/file.c
+++ b/fs/btrfs/file.c
@@ -3347,13 +3347,14 @@ static int find_desired_extent(struct inode *inode,
loff_t *offset, int whence)
struct btrfs_fs_info *fs_info = btrfs_sb(inode->i_sb);
struct extent_map *em = NULL;
struct extent_state *cached_state = NULL;
+ loff_t i_size = i_size_read(inode);
u64 lockstart;
u64 lockend;
u64 start;
u64 len;
int ret = 0;
- if (inode->i_size == 0)
+ if (i_size == 0)
return -ENXIO;
/*
@@ -3363,8 +3364,7 @@ static int find_desired_extent(struct inode *inode,
loff_t *offset, int whence)
start = max_t(loff_t, 0, *offset);
lockstart = round_down(start, fs_info->sectorsize);
- lockend = round_up(i_size_read(inode),
- fs_info->sectorsize);
+ lockend = round_up(i_size, fs_info->sectorsize);
if (lockend <= lockstart)
lockend = lockstart + fs_info->sectorsize;
lockend--;
@@ -3373,7 +3373,7 @@ static int find_desired_extent(struct inode *inode,
loff_t *offset, int whence)
lock_extent_bits(&BTRFS_I(inode)->io_tree, lockstart, lockend,
&cached_state);
- while (start < inode->i_size) {
+ while (start < i_size) {
em = btrfs_get_extent_fiemap(BTRFS_I(inode), start, len);
if (IS_ERR(em)) {
ret = PTR_ERR(em);
@@ -3397,10 +3397,10 @@ static int find_desired_extent(struct inode *inode,
loff_t *offset, int whence)
}
free_extent_map(em);
if (!ret) {
- if (whence == SEEK_DATA && start >= inode->i_size)
+ if (whence == SEEK_DATA && start >= i_size)
ret = -ENXIO;
else
- *offset = min_t(loff_t, start, inode->i_size);
+ *offset = min_t(loff_t, start, i_size);
}
unlock_extent_cached(&BTRFS_I(inode)->io_tree, lockstart, lockend,
&cached_state);
@@ -3412,7 +3412,6 @@ static loff_t btrfs_file_llseek(struct file *file, loff_t
offset, int whence)
struct inode *inode = file->f_mapping->host;
int ret;
- inode_lock(inode);
switch (whence) {
case SEEK_END:
case SEEK_CUR:
@@ -3420,21 +3419,16 @@ static loff_t btrfs_file_llseek(struct file *file,
loff_t offset, int whence)
goto out;
case SEEK_DATA:
case SEEK_HOLE:
- if (offset >= i_size_read(inode)) {
- inode_unlock(inode);
+ if (offset >= i_size_read(inode))
return -ENXIO;
- }
ret = find_desired_extent(inode, &offset, whence);
- if (ret) {
- inode_unlock(inode);
+ if (ret)
return ret;
- }
}
offset = vfs_setpos(file, offset, inode->i_sb->s_maxbytes);
out:
- inode_unlock(inode);
return offset;
}
--
2.17.1
