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Subject[PATCH 1/3] direct-io: only inc/dec inode->i_dio_count for file systems
Date
do_blockdev_direct_IO() increments and decrements the inode
->i_dio_count for each IO operation. It does this to protect against
truncate of a file. Block devices don't need this sort of protection.

For a capable multiqueue setup, this atomic int is the only shared
state between applications accessing the device for O_DIRECT, and it
presents a scaling wall for that. In my testing, as much as 30% of
system time is spent incrementing and decrementing this value. A mixed
read/write workload improved from ~2.5M IOPS to ~9.6M IOPS, with
better latencies too. Before:

clat percentiles (usec):
| 1.00th=[ 33], 5.00th=[ 34], 10.00th=[ 34], 20.00th=[ 34],
| 30.00th=[ 34], 40.00th=[ 34], 50.00th=[ 35], 60.00th=[ 35],
| 70.00th=[ 35], 80.00th=[ 35], 90.00th=[ 37], 95.00th=[ 80],
| 99.00th=[ 98], 99.50th=[ 151], 99.90th=[ 155], 99.95th=[ 155],
| 99.99th=[ 165]

After:

clat percentiles (usec):
| 1.00th=[ 95], 5.00th=[ 108], 10.00th=[ 129], 20.00th=[ 149],
| 30.00th=[ 155], 40.00th=[ 161], 50.00th=[ 167], 60.00th=[ 171],
| 70.00th=[ 177], 80.00th=[ 185], 90.00th=[ 201], 95.00th=[ 270],
| 99.00th=[ 390], 99.50th=[ 398], 99.90th=[ 418], 99.95th=[ 422],
| 99.99th=[ 438]

In other setups, Robert Elliott reported seeing good performance
improvements:

https://lkml.org/lkml/2015/4/3/557

The more applications accessing the device, the worse it gets.

Add a new direct-io flags, DIO_SKIP_DIO_COUNT, which tells
do_blockdev_direct_IO() that it need not worry about incrementing
or decrementing the inode i_dio_count for this caller.

Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Theodore Ts'o <tytso@mit.edu>
Cc: Elliott, Robert (Server Storage) <elliott@hp.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Jens Axboe <axboe@fb.com>
---
fs/block_dev.c | 2 +-
fs/btrfs/inode.c | 6 +++---
fs/dax.c | 4 ++--
fs/direct-io.c | 7 +++++--
fs/ext4/indirect.c | 6 +++---
fs/ext4/inode.c | 4 ++--
fs/inode.c | 19 ++++++++++++++++---
fs/nfs/direct.c | 10 +++++-----
include/linux/fs.h | 6 +++++-
9 files changed, 42 insertions(+), 22 deletions(-)

diff --git a/fs/block_dev.c b/fs/block_dev.c
index 975266be67d3..be1335dbd6be 100644
--- a/fs/block_dev.c
+++ b/fs/block_dev.c
@@ -155,7 +155,7 @@ blkdev_direct_IO(int rw, struct kiocb *iocb, struct iov_iter *iter,

return __blockdev_direct_IO(rw, iocb, inode, I_BDEV(inode), iter,
offset, blkdev_get_block,
- NULL, NULL, 0);
+ NULL, NULL, DIO_SKIP_DIO_COUNT);
}

int __sync_blockdev(struct block_device *bdev, int wait)
diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index d2e732d7af52..6fe341a66ed8 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -8129,7 +8129,7 @@ static ssize_t btrfs_direct_IO(int rw, struct kiocb *iocb,
if (check_direct_IO(BTRFS_I(inode)->root, rw, iocb, iter, offset))
return 0;

- atomic_inc(&inode->i_dio_count);
+ inode_dio_inc(inode);
smp_mb__after_atomic();

/*
@@ -8169,7 +8169,7 @@ static ssize_t btrfs_direct_IO(int rw, struct kiocb *iocb,
current->journal_info = &outstanding_extents;
} else if (test_bit(BTRFS_INODE_READDIO_NEED_LOCK,
&BTRFS_I(inode)->runtime_flags)) {
- inode_dio_done(inode);
+ inode_dio_dec(inode);
flags = DIO_LOCKING | DIO_SKIP_HOLES;
wakeup = false;
}
@@ -8188,7 +8188,7 @@ static ssize_t btrfs_direct_IO(int rw, struct kiocb *iocb,
}
out:
if (wakeup)
- inode_dio_done(inode);
+ inode_dio_dec(inode);
if (relock)
mutex_lock(&inode->i_mutex);

diff --git a/fs/dax.c b/fs/dax.c
index ed1619ec6537..1d4a9a54a938 100644
--- a/fs/dax.c
+++ b/fs/dax.c
@@ -210,7 +210,7 @@ ssize_t dax_do_io(int rw, struct kiocb *iocb, struct inode *inode,
}

/* Protects against truncate */
- atomic_inc(&inode->i_dio_count);
+ inode_dio_inc(inode);

retval = dax_io(rw, inode, iter, pos, end, get_block, &bh);

@@ -220,7 +220,7 @@ ssize_t dax_do_io(int rw, struct kiocb *iocb, struct inode *inode,
if ((retval > 0) && end_io)
end_io(iocb, pos, retval, bh.b_private);

- inode_dio_done(inode);
+ inode_dio_dec(inode);
out:
return retval;
}
diff --git a/fs/direct-io.c b/fs/direct-io.c
index e181b6b2e297..cba348e3135f 100644
--- a/fs/direct-io.c
+++ b/fs/direct-io.c
@@ -254,7 +254,9 @@ static ssize_t dio_complete(struct dio *dio, loff_t offset, ssize_t ret,
if (dio->end_io && dio->result)
dio->end_io(dio->iocb, offset, transferred, dio->private);

- inode_dio_done(dio->inode);
+ if (!(dio->flags & DIO_SKIP_DIO_COUNT))
+ inode_dio_dec(dio->inode);
+
if (is_async) {
if (dio->rw & WRITE) {
int err;
@@ -1199,7 +1201,8 @@ do_blockdev_direct_IO(int rw, struct kiocb *iocb, struct inode *inode,
/*
* Will be decremented at I/O completion time.
*/
- atomic_inc(&inode->i_dio_count);
+ if (!(dio->flags & DIO_SKIP_DIO_COUNT))
+ inode_dio_inc(inode);

retval = 0;
sdio.blkbits = blkbits;
diff --git a/fs/ext4/indirect.c b/fs/ext4/indirect.c
index 45fe924f82bc..bf77e5b73ae2 100644
--- a/fs/ext4/indirect.c
+++ b/fs/ext4/indirect.c
@@ -682,11 +682,11 @@ retry:
* via ext4_inode_block_unlocked_dio(). Check inode's state
* while holding extra i_dio_count ref.
*/
- atomic_inc(&inode->i_dio_count);
+ inode_dio_inc(inode);
smp_mb();
if (unlikely(ext4_test_inode_state(inode,
EXT4_STATE_DIOREAD_LOCK))) {
- inode_dio_done(inode);
+ inode_dio_dec(inode);
goto locked;
}
if (IS_DAX(inode))
@@ -696,7 +696,7 @@ retry:
ret = __blockdev_direct_IO(rw, iocb, inode,
inode->i_sb->s_bdev, iter, offset,
ext4_get_block, NULL, NULL, 0);
- inode_dio_done(inode);
+ inode_dio_dec(inode);
} else {
locked:
if (IS_DAX(inode))
diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 5cb9a212b86f..62d4615aa9b0 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -2978,7 +2978,7 @@ static ssize_t ext4_ext_direct_IO(int rw, struct kiocb *iocb,
* overwrite DIO as i_dio_count needs to be incremented under i_mutex.
*/
if (rw == WRITE)
- atomic_inc(&inode->i_dio_count);
+ inode_dio_inc(inode);

/* If we do a overwrite dio, i_mutex locking can be released */
overwrite = *((int *)iocb->private);
@@ -3080,7 +3080,7 @@ static ssize_t ext4_ext_direct_IO(int rw, struct kiocb *iocb,

retake_lock:
if (rw == WRITE)
- inode_dio_done(inode);
+ inode_dio_dec(inode);
/* take i_mutex locking again if we do a ovewrite dio */
if (overwrite) {
up_read(&EXT4_I(inode)->i_data_sem);
diff --git a/fs/inode.c b/fs/inode.c
index f00b16f45507..c4901c40ad65 100644
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -1946,18 +1946,31 @@ void inode_dio_wait(struct inode *inode)
EXPORT_SYMBOL(inode_dio_wait);

/*
- * inode_dio_done - signal finish of a direct I/O requests
+ * inode_dio_begin - signal start of a direct I/O requests
* @inode: inode the direct I/O happens on
*
* This is called once we've finished processing a direct I/O request,
* and is used to wake up callers waiting for direct I/O to be quiesced.
*/
-void inode_dio_done(struct inode *inode)
+void inode_dio_inc(struct inode *inode)
+{
+ atomic_inc(&inode->i_dio_count);
+}
+EXPORT_SYMBOL(inode_dio_inc);
+
+/*
+ * inode_dio_dec - signal finish of a direct I/O requests
+ * @inode: inode the direct I/O happens on
+ *
+ * This is called once we've finished processing a direct I/O request,
+ * and is used to wake up callers waiting for direct I/O to be quiesced.
+ */
+void inode_dio_dec(struct inode *inode)
{
if (atomic_dec_and_test(&inode->i_dio_count))
wake_up_bit(&inode->i_state, __I_DIO_WAKEUP);
}
-EXPORT_SYMBOL(inode_dio_done);
+EXPORT_SYMBOL(inode_dio_dec);

/*
* inode_set_flags - atomically set some inode flags
diff --git a/fs/nfs/direct.c b/fs/nfs/direct.c
index e907c8cf732e..2890317173eb 100644
--- a/fs/nfs/direct.c
+++ b/fs/nfs/direct.c
@@ -387,7 +387,7 @@ static void nfs_direct_complete(struct nfs_direct_req *dreq, bool write)
if (write)
nfs_zap_mapping(inode, inode->i_mapping);

- inode_dio_done(inode);
+ inode_dio_dec(inode);

if (dreq->iocb) {
long res = (long) dreq->error;
@@ -487,7 +487,7 @@ static ssize_t nfs_direct_read_schedule_iovec(struct nfs_direct_req *dreq,
&nfs_direct_read_completion_ops);
get_dreq(dreq);
desc.pg_dreq = dreq;
- atomic_inc(&inode->i_dio_count);
+ inode_dio_inc(inode);

while (iov_iter_count(iter)) {
struct page **pagevec;
@@ -539,7 +539,7 @@ static ssize_t nfs_direct_read_schedule_iovec(struct nfs_direct_req *dreq,
* generic layer handle the completion.
*/
if (requested_bytes == 0) {
- inode_dio_done(inode);
+ inode_dio_dec(inode);
nfs_direct_req_release(dreq);
return result < 0 ? result : -EIO;
}
@@ -873,7 +873,7 @@ static ssize_t nfs_direct_write_schedule_iovec(struct nfs_direct_req *dreq,
&nfs_direct_write_completion_ops);
desc.pg_dreq = dreq;
get_dreq(dreq);
- atomic_inc(&inode->i_dio_count);
+ inode_dio_inc(inode);

NFS_I(inode)->write_io += iov_iter_count(iter);
while (iov_iter_count(iter)) {
@@ -929,7 +929,7 @@ static ssize_t nfs_direct_write_schedule_iovec(struct nfs_direct_req *dreq,
* generic layer handle the completion.
*/
if (requested_bytes == 0) {
- inode_dio_done(inode);
+ inode_dio_dec(inode);
nfs_direct_req_release(dreq);
return result < 0 ? result : -EIO;
}
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 52cc4492cb3a..1292ce1d9be5 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -2613,6 +2613,9 @@ enum {

/* filesystem can handle aio writes beyond i_size */
DIO_ASYNC_EXTEND = 0x04,
+
+ /* inode/fs/bdev does not need truncate protection */
+ DIO_SKIP_DIO_COUNT = 0x08,
};

void dio_end_io(struct bio *bio, int error);
@@ -2633,7 +2636,8 @@ static inline ssize_t blockdev_direct_IO(int rw, struct kiocb *iocb,
#endif

void inode_dio_wait(struct inode *inode);
-void inode_dio_done(struct inode *inode);
+void inode_dio_inc(struct inode *inode);
+void inode_dio_dec(struct inode *inode);

extern void inode_set_flags(struct inode *inode, unsigned int flags,
unsigned int mask);
--
1.9.1


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