io_uring: avoid hashing O_DIRECT writes if the filesystem doesn't need it

io_uring hashes writes to a given file/inode so that it can serialize
them. This is useful if the file system needs exclusive access to the
file to perform the write, as otherwise we end up with a ton of io-wq
threads trying to lock the inode at the same time. This can cause
excessive system time.

But if the file system has flagged that it supports parallel O_DIRECT
writes, then there's no need to serialize the writes. Check for that
through FMODE_DIO_PARALLEL_WRITE and don't hash it if we don't need to.

In a basic test of 8 threads writing to a file on XFS on a gen2 Optane,
with each thread writing in 4k chunks, it improves performance from
~1350K IOPS (or ~5290MiB/sec) to ~1410K IOPS (or ~5500MiB/sec).

Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

io_uring/io_uring.c

@@ -425,7 +425,13 @@ static void io_prep_async_work(struct io_kiocb *req)
 		req->flags |= io_file_get_flags(req->file) << REQ_F_SUPPORT_NOWAIT_BIT;
 
 	if (req->flags & REQ_F_ISREG) {
-		if (def->hash_reg_file || (ctx->flags & IORING_SETUP_IOPOLL))
+		bool should_hash = def->hash_reg_file;
+
+		/* don't serialize this request if the fs doesn't need it */
+		if (should_hash && (req->file->f_flags & O_DIRECT) &&
+		    (req->file->f_mode & FMODE_DIO_PARALLEL_WRITE))
+			should_hash = false;
+		if (should_hash || (ctx->flags & IORING_SETUP_IOPOLL))
 			io_wq_hash_work(&req->work, file_inode(req->file));
 	} else if (!req->file || !S_ISBLK(file_inode(req->file)->i_mode)) {
 		if (def->unbound_nonreg_file)