Commit 8a8fe42d authored by Caleb Sander Mateos's avatar Caleb Sander Mateos Committed by Jens Axboe
Browse files

ublk: optimize UBLK_IO_REGISTER_IO_BUF on daemon task 



ublk_register_io_buf() performs an expensive atomic refcount increment,
as well as a lot of pointer chasing to look up the struct request.

Create a separate ublk_daemon_register_io_buf() for the daemon task to
call. Initialize ublk_io's reference count to a large number, introduce
a field task_registered_buffers to count the buffers registered on the
daemon task, and atomically subtract the large number minus
task_registered_buffers in ublk_commit_and_fetch().

Also obtain the struct request directly from ublk_io's req field instead
of looking it up on the tagset.

Signed-off-by: default avatarCaleb Sander Mateos <csander@purestorage.com>
Reviewed-by: default avatarMing Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20250620151008.3976463-12-csander@purestorage.com


Signed-off-by: default avatarJens Axboe <axboe@kernel.dk>
parent 7ab74108

drivers/block/ublk_drv.c

+61 −9
Original line number Diff line number Diff line
@@ -148,6 +148,13 @@ struct ublk_uring_cmd_pdu { 
/* atomic RW with ubq->cancel_lock */

#define UBLK_IO_FLAG_CANCELED	0x80000000



/*

 * Initialize refcount to a large number to include any registered buffers.

 * UBLK_IO_COMMIT_AND_FETCH_REQ will release these references minus those for

 * any buffers registered on the io daemon task.

 */

#define UBLK_REFCOUNT_INIT (REFCOUNT_MAX / 2)



struct ublk_io {

	/* userspace buffer address from io cmd */

	__u64	addr;
@@ -166,14 +173,17 @@ struct ublk_io { 
	/*

	 * The number of uses of this I/O by the ublk server

	 * if user copy or zero copy are enabled:

	 * - 1 from dispatch to the server until UBLK_IO_COMMIT_AND_FETCH_REQ

	 * - UBLK_REFCOUNT_INIT from dispatch to the server

	 *   until UBLK_IO_COMMIT_AND_FETCH_REQ

	 * - 1 for each inflight ublk_ch_{read,write}_iter() call

	 * - 1 for each io_uring registered buffer

	 * - 1 for each io_uring registered buffer not registered on task

	 * The I/O can only be completed once all references are dropped.

	 * User copy and buffer registration operations are only permitted

	 * if the reference count is nonzero.

	 */

	refcount_t ref;

	/* Count of buffers registered on task and not yet unregistered */

	unsigned task_registered_buffers;



	/* auto-registered buffer, valid if UBLK_IO_FLAG_AUTO_BUF_REG is set */

	u16 buf_index;
@@ -686,7 +696,7 @@ static inline void ublk_init_req_ref(const struct ublk_queue *ubq, 
		struct ublk_io *io)

{

	if (ublk_need_req_ref(ubq))

		refcount_set(&io->ref, 1);

		refcount_set(&io->ref, UBLK_REFCOUNT_INIT);

}



static inline bool ublk_get_req_ref(const struct ublk_queue *ubq,
@@ -709,6 +719,15 @@ static inline void ublk_put_req_ref(const struct ublk_queue *ubq, 
	}

}



static inline void ublk_sub_req_ref(struct ublk_io *io, struct request *req)

{

	unsigned sub_refs = UBLK_REFCOUNT_INIT - io->task_registered_buffers;



	io->task_registered_buffers = 0;

	if (refcount_sub_and_test(sub_refs, &io->ref))

		__ublk_complete_rq(req);

}



static inline bool ublk_need_get_data(const struct ublk_queue *ubq)

{

	return ubq->flags & UBLK_F_NEED_GET_DATA;
@@ -1197,7 +1216,6 @@ ublk_auto_buf_reg_fallback(const struct ublk_queue *ubq, struct ublk_io *io) 
	struct ublksrv_io_desc *iod = ublk_get_iod(ubq, tag);



	iod->op_flags |= UBLK_IO_F_NEED_REG_BUF;

	refcount_set(&io->ref, 1);

}



static bool ublk_auto_buf_reg(const struct ublk_queue *ubq, struct request *req,
@@ -1216,9 +1234,8 @@ static bool ublk_auto_buf_reg(const struct ublk_queue *ubq, struct request *req, 
		blk_mq_end_request(req, BLK_STS_IOERR);

		return false;

	}

	/* one extra reference is dropped by ublk_io_release */

	refcount_set(&io->ref, 2);



	io->task_registered_buffers = 1;

	io->buf_ctx_handle = io_uring_cmd_ctx_handle(io->cmd);

	/* store buffer index in request payload */

	io->buf_index = pdu->buf.index;
@@ -1230,10 +1247,10 @@ static bool ublk_prep_auto_buf_reg(struct ublk_queue *ubq, 
				   struct request *req, struct ublk_io *io,

				   unsigned int issue_flags)

{

	ublk_init_req_ref(ubq, io);

	if (ublk_support_auto_buf_reg(ubq) && ublk_rq_has_data(req))

		return ublk_auto_buf_reg(ubq, req, io, issue_flags);



	ublk_init_req_ref(ubq, io);

	return true;

}
@@ -1506,6 +1523,7 @@ static void ublk_queue_reinit(struct ublk_device *ub, struct ublk_queue *ubq) 
		}



		WARN_ON_ONCE(refcount_read(&io->ref));

		WARN_ON_ONCE(io->task_registered_buffers);

	}

}
@@ -2041,6 +2059,35 @@ static int ublk_register_io_buf(struct io_uring_cmd *cmd, 
	return 0;

}



static int

ublk_daemon_register_io_buf(struct io_uring_cmd *cmd,

			    const struct ublk_queue *ubq, struct ublk_io *io,

			    unsigned index, unsigned issue_flags)

{

	unsigned new_registered_buffers;

	struct request *req = io->req;

	int ret;



	/*

	 * Ensure there are still references for ublk_sub_req_ref() to release.

	 * If not, fall back on the thread-safe buffer registration.

	 */

	new_registered_buffers = io->task_registered_buffers + 1;

	if (unlikely(new_registered_buffers >= UBLK_REFCOUNT_INIT))

		return ublk_register_io_buf(cmd, ubq, io, index, issue_flags);



	if (!ublk_support_zero_copy(ubq) || !ublk_rq_has_data(req))

		return -EINVAL;



	ret = io_buffer_register_bvec(cmd, req, ublk_io_release, index,

				      issue_flags);

	if (ret)

		return ret;



	io->task_registered_buffers = new_registered_buffers;

	return 0;

}



static int ublk_unregister_io_buf(struct io_uring_cmd *cmd,

				  const struct ublk_device *ub,

				  unsigned int index, unsigned int issue_flags)
@@ -2164,7 +2211,10 @@ static int ublk_commit_and_fetch(const struct ublk_queue *ubq, 
	if (unlikely(blk_should_fake_timeout(req->q)))

		return 0;



	ublk_put_req_ref(ubq, io, req);

	if (ublk_need_req_ref(ubq))

		ublk_sub_req_ref(io, req);

	else

		__ublk_complete_rq(req);

	return 0;

}
@@ -2262,7 +2312,8 @@ static int __ublk_ch_uring_cmd(struct io_uring_cmd *cmd, 


	switch (_IOC_NR(cmd_op)) {

	case UBLK_IO_REGISTER_IO_BUF:

		return ublk_register_io_buf(cmd, ubq, io, ub_cmd->addr, issue_flags);

		return ublk_daemon_register_io_buf(cmd, ubq, io, ub_cmd->addr,

						   issue_flags);

	case UBLK_IO_COMMIT_AND_FETCH_REQ:

		ret = ublk_commit_and_fetch(ubq, io, cmd, ub_cmd, issue_flags);

		if (ret)
@@ -2501,6 +2552,7 @@ static void ublk_deinit_queue(struct ublk_device *ub, int q_id) 
		if (io->task)

			put_task_struct(io->task);

		WARN_ON_ONCE(refcount_read(&io->ref));

		WARN_ON_ONCE(io->task_registered_buffers);

	}



	if (ubq->io_cmd_buf)