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ublk: optimize UBLK_IO_REGISTER_IO_BUF on daemon task

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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: Caleb Sander Mateos <csander@purestorage.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20250620151008.3976463-12-csander@purestorage.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>
This commit is contained in:
Caleb Sander Mateos 2025-06-20 09:10:05 -06:00 • committed by Jens Axboe
parent 7ab741081b
commit 8a8fe42d76
1 changed files with 61 additions and 9 deletions
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70
drivers/block/ublk_drv.c
View file

@ -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)