On Wed, May 07, 2025 at 03:49:35PM -0600, Uday Shankar wrote:
> Currently, ublk_drv associates to each hardware queue (hctx) a unique
> task (called the queue's ubq_daemon) which is allowed to issue
> COMMIT_AND_FETCH commands against the hctx. If any other task attempts
> to do so, the command fails immediately with EINVAL. When considered
> together with the block layer architecture, the result is that for each
> CPU C on the system, there is a unique ublk server thread which is
> allowed to handle I/O submitted on CPU C. This can lead to suboptimal
> performance under imbalanced load generation. For an extreme example,
> suppose all the load is generated on CPUs mapping to a single ublk
> server thread. Then that thread may be fully utilized and become the
> bottleneck in the system, while other ublk server threads are totally
> idle.
>
> This issue can also be addressed directly in the ublk server without
> kernel support by having threads dequeue I/Os and pass them around to
> ensure even load. But this solution requires inter-thread communication
> at least twice for each I/O (submission and completion), which is
> generally a bad pattern for performance. The problem gets even worse
> with zero copy, as more inter-thread communication would be required to
> have the buffer register/unregister calls to come from the correct
> thread.
>
> Therefore, address this issue in ublk_drv by allowing each I/O to have
> its own daemon task. Two I/Os in the same queue are now allowed to be
> serviced by different daemon tasks - this was not possible before.
> Imbalanced load can then be balanced across all ublk server threads as
> follows:
>
> - specifying the new UBLK_F_RR_TAGS flag when creating the ublk device,
> to ensure round-robin tag allocation
> - having the ublk server threads issue FETCH_REQs in a round-robin
> manner. As a small toy example, consider a system with a single ublk
> device having 2 queues, each of depth 4. A ublk server having 4
> threads could issue its FETCH_REQs against this device as follows
> (where each entry is the qid,tag pair that the FETCH_REQ targets):
>
> ublk server thread: T0 T1 T2 T3
> 0,0 0,1 0,2 0,3
> 1,3 1,0 1,1 1,2
>
> The combination of these two changes allow a ublk server to balance
> load. For example, suppose a program quickly issues 4 I/Os concurrently.
> Before this change, all 4 I/Os would have had to be serviced by the same
> thread, and that thread may become a bottleneck. With this change, those
> 4 I/Os would get tags 0-3 for the same queue, and would thus spread out
> evenly over all the ublk server threads. Peak bandwidth in CPU-limited
> workloads under imbalanced load thus will increase.
>
> The UBLK_F_RR_TAGS flag is also added as a feature, so that ublk servers
> can essentially test for the presence of this patch and tailor their
> behavior accordingly.
>
> Signed-off-by: Uday Shankar <ushan...@purestorage.com>
> Reviewed-by: Caleb Sander Mateos <csan...@purestorage.com>
> ---
> drivers/block/ublk_drv.c | 82
> ++++++++++++++++++++++---------------------
> include/uapi/linux/ublk_cmd.h | 8 +++++
> 2 files changed, 50 insertions(+), 40 deletions(-)
>
> diff --git a/drivers/block/ublk_drv.c b/drivers/block/ublk_drv.c
> index
> cb612151e9a1de7630c49a2b78d278e005d52856..ff5beab32220ea96a1e15ab26c02f4883a6c3d6e
> 100644
> --- a/drivers/block/ublk_drv.c
> +++ b/drivers/block/ublk_drv.c
> @@ -66,7 +66,8 @@
> | UBLK_F_USER_COPY \
> | UBLK_F_ZONED \
> | UBLK_F_USER_RECOVERY_FAIL_IO \
> - | UBLK_F_UPDATE_SIZE)
> + | UBLK_F_UPDATE_SIZE \
> + | UBLK_F_RR_TAGS)
>
> #define UBLK_F_ALL_RECOVERY_FLAGS (UBLK_F_USER_RECOVERY \
> | UBLK_F_USER_RECOVERY_REISSUE \
> @@ -148,6 +149,8 @@ struct ublk_io {
> /* valid if UBLK_IO_FLAG_OWNED_BY_SRV is set */
> struct request *req;
> };
> +
> + struct task_struct *task;
> };
>
> struct ublk_queue {
> @@ -155,11 +158,9 @@ struct ublk_queue {
> int q_depth;
>
> unsigned long flags;
> - struct task_struct *ubq_daemon;
> struct ublksrv_io_desc *io_cmd_buf;
>
> bool force_abort;
> - bool timeout;
> bool canceling;
> bool fail_io; /* copy of dev->state == UBLK_S_DEV_FAIL_IO */
> unsigned short nr_io_ready; /* how many ios setup */
> @@ -1069,11 +1070,6 @@ static inline struct ublk_uring_cmd_pdu
> *ublk_get_uring_cmd_pdu(
> return io_uring_cmd_to_pdu(ioucmd, struct ublk_uring_cmd_pdu);
> }
>
> -static inline bool ubq_daemon_is_dying(struct ublk_queue *ubq)
> -{
> - return !ubq->ubq_daemon || ubq->ubq_daemon->flags & PF_EXITING;
> -}
> -
> /* todo: handle partial completion */
> static inline void __ublk_complete_rq(struct request *req)
> {
> @@ -1207,13 +1203,13 @@ static void ublk_dispatch_req(struct ublk_queue *ubq,
> /*
> * Task is exiting if either:
> *
> - * (1) current != ubq_daemon.
> + * (1) current != io->task.
> * io_uring_cmd_complete_in_task() tries to run task_work
> - * in a workqueue if ubq_daemon(cmd's task) is PF_EXITING.
> + * in a workqueue if cmd's task is PF_EXITING.
> *
> * (2) current->flags & PF_EXITING.
> */
> - if (unlikely(current != ubq->ubq_daemon || current->flags &
> PF_EXITING)) {
> + if (unlikely(current != io->task || current->flags & PF_EXITING)) {
> __ublk_abort_rq(ubq, req);
> return;
> }
> @@ -1286,13 +1282,10 @@ static void ublk_queue_cmd_list(struct ublk_queue
> *ubq, struct rq_list *l)
> static enum blk_eh_timer_return ublk_timeout(struct request *rq)
> {
> struct ublk_queue *ubq = rq->mq_hctx->driver_data;
> + struct ublk_io *io = &ubq->ios[rq->tag];
>
> if (ubq->flags & UBLK_F_UNPRIVILEGED_DEV) {
> - if (!ubq->timeout) {
> - send_sig(SIGKILL, ubq->ubq_daemon, 0);
> - ubq->timeout = true;
> - }
> -
> + send_sig(SIGKILL, io->task, 0);
> return BLK_EH_DONE;
> }
>
> @@ -1405,17 +1398,6 @@ static void ublk_queue_reinit(struct ublk_device *ub,
> struct ublk_queue *ubq)
> /* All old ioucmds have to be completed */
> ubq->nr_io_ready = 0;
>
> - /*
> - * old daemon is PF_EXITING, put it now
> - *
> - * It could be NULL in case of closing one quisced device.
> - */
> - if (ubq->ubq_daemon)
> - put_task_struct(ubq->ubq_daemon);
> - /* We have to reset it to NULL, otherwise ub won't accept new FETCH_REQ
> */
> - ubq->ubq_daemon = NULL;
> - ubq->timeout = false;
> -
> for (i = 0; i < ubq->q_depth; i++) {
> struct ublk_io *io = &ubq->ios[i];
>
> @@ -1426,6 +1408,17 @@ static void ublk_queue_reinit(struct ublk_device *ub,
> struct ublk_queue *ubq)
> io->flags &= UBLK_IO_FLAG_CANCELED;
> io->cmd = NULL;
> io->addr = 0;
> +
> + /*
> + * old task is PF_EXITING, put it now
> + *
> + * It could be NULL in case of closing one quiesced
> + * device.
> + */
> + if (io->task) {
> + put_task_struct(io->task);
> + io->task = NULL;
> + }
> }
> }
>
> @@ -1447,7 +1440,7 @@ static void ublk_reset_ch_dev(struct ublk_device *ub)
> for (i = 0; i < ub->dev_info.nr_hw_queues; i++)
> ublk_queue_reinit(ub, ublk_get_queue(ub, i));
>
> - /* set to NULL, otherwise new ubq_daemon cannot mmap the io_cmd_buf */
> + /* set to NULL, otherwise new tasks cannot mmap io_cmd_buf */
> ub->mm = NULL;
> ub->nr_queues_ready = 0;
> ub->nr_privileged_daemon = 0;
> @@ -1722,7 +1715,7 @@ static void ublk_uring_cmd_cancel_fn(struct
> io_uring_cmd *cmd,
> return;
>
> task = io_uring_cmd_get_task(cmd);
> - if (WARN_ON_ONCE(task && task != ubq->ubq_daemon))
> + if (WARN_ON_ONCE(task && task != ubq->ios[pdu->tag].task))
> return;
>
> if (!ubq->canceling)
> @@ -1861,8 +1854,6 @@ static void ublk_mark_io_ready(struct ublk_device *ub,
> struct ublk_queue *ubq)
> {
> ubq->nr_io_ready++;
> if (ublk_queue_ready(ubq)) {
> - ubq->ubq_daemon = current;
> - get_task_struct(ubq->ubq_daemon);
> ub->nr_queues_ready++;
>
> if (capable(CAP_SYS_ADMIN))
> @@ -1995,6 +1986,7 @@ static int ublk_fetch(struct io_uring_cmd *cmd, struct
> ublk_queue *ubq,
> }
>
> ublk_fill_io_cmd(io, cmd, buf_addr);
> + WRITE_ONCE(io->task, get_task_struct(current));
> ublk_mark_io_ready(ub, ubq);
> out:
> mutex_unlock(&ub->mutex);
> @@ -2062,6 +2054,7 @@ static int __ublk_ch_uring_cmd(struct io_uring_cmd *cmd,
> const struct ublksrv_io_cmd *ub_cmd)
> {
> struct ublk_device *ub = cmd->file->private_data;
> + struct task_struct *task;
> struct ublk_queue *ubq;
> struct ublk_io *io;
> u32 cmd_op = cmd->cmd_op;
> @@ -2076,13 +2069,14 @@ static int __ublk_ch_uring_cmd(struct io_uring_cmd
> *cmd,
> goto out;
>
> ubq = ublk_get_queue(ub, ub_cmd->q_id);
> - if (ubq->ubq_daemon && ubq->ubq_daemon != current)
> - goto out;
>
> if (tag >= ubq->q_depth)
> goto out;
>
> io = &ubq->ios[tag];
> + task = READ_ONCE(io->task);
> + if (task && task != current)
> + goto out;
>
> /* there is pending io cmd, something must be wrong */
> if (io->flags & UBLK_IO_FLAG_ACTIVE) {
> @@ -2332,9 +2326,15 @@ static void ublk_deinit_queue(struct ublk_device *ub,
> int q_id)
> {
> int size = ublk_queue_cmd_buf_size(ub, q_id);
> struct ublk_queue *ubq = ublk_get_queue(ub, q_id);
> + struct ublk_io *io;
> + int i;
> +
> + for (i = 0; i < ubq->q_depth; i++) {
> + io = &ubq->ios[i];
> + if (io->task)
> + put_task_struct(io->task);
> + }
>
> - if (ubq->ubq_daemon)
> - put_task_struct(ubq->ubq_daemon);
> if (ubq->io_cmd_buf)
> free_pages((unsigned long)ubq->io_cmd_buf, get_order(size));
> }
> @@ -2487,6 +2487,8 @@ static int ublk_add_tag_set(struct ublk_device *ub)
> ub->tag_set.numa_node = NUMA_NO_NODE;
> ub->tag_set.cmd_size = sizeof(struct ublk_rq_data);
> ub->tag_set.driver_data = ub;
> + if (ub->dev_info.flags & UBLK_F_RR_TAGS)
> + ub->tag_set.flags |= BLK_MQ_F_TAG_RR;
> return blk_mq_alloc_tag_set(&ub->tag_set);
> }
>
> @@ -3062,14 +3064,14 @@ static int ublk_ctrl_end_recovery(struct ublk_device
> *ub,
> int ublksrv_pid = (int)header->data[0];
> int ret = -EINVAL;
>
> - pr_devel("%s: Waiting for new ubq_daemons(nr: %d) are ready, dev id
> %d...\n",
> - __func__, ub->dev_info.nr_hw_queues, header->dev_id);
> - /* wait until new ubq_daemon sending all FETCH_REQ */
> + pr_devel("%s: Waiting for all FETCH_REQs, dev id %d...\n", __func__,
> + header->dev_id);
> +
> if (wait_for_completion_interruptible(&ub->completion))
> return -EINTR;
>
> - pr_devel("%s: All new ubq_daemons(nr: %d) are ready, dev id %d\n",
> - __func__, ub->dev_info.nr_hw_queues, header->dev_id);
> + pr_devel("%s: All FETCH_REQs received, dev id %d\n", __func__,
> + header->dev_id);
>
> mutex_lock(&ub->mutex);
> if (ublk_nosrv_should_stop_dev(ub))
> diff --git a/include/uapi/linux/ublk_cmd.h b/include/uapi/linux/ublk_cmd.h
> index
> be5c6c6b16e098838a2bf790e588b79656defdda..3ba8f26d60bc70ade95b4f4d5990157af2bd5b2e
> 100644
> --- a/include/uapi/linux/ublk_cmd.h
> +++ b/include/uapi/linux/ublk_cmd.h
> @@ -219,6 +219,14 @@
> */
> #define UBLK_F_UPDATE_SIZE (1ULL << 10)
>
> +/*
> + * Force tags to be allocated round-robin on each queue. If ublk server
> + * threads also issue FETCH_REQs against the queues in a round-robin
> + * manner, load can be balanced across all threads even if it is issued
> + * to the queues in an imbalanced way.
> + */
> +#define UBLK_F_RR_TAGS (1ULL << 11)
Please make UBLK_F_RR_TAGS as one standalone patch, which is also helpful
for evaluating round-robin performance, and doesn't have to be used by
per-io task.
Thanks,
Ming
