[PATCH v4 1/4] ublk: require unique task per io instead of unique task per hctx

From: Uday Shankar
Date: Tue Apr 15 2025 - 21:01:31 EST

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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 requiring a unique task per
I/O instead of per queue/hctx. Imbalanced load can then be balanced
across all ublk server threads by having 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 queue 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):

poller thread: T0 T1 T2 T3
0,0 0,1 0,2 0,3
1,3 1,0 1,1 1,2

Since tags appear to be allocated in sequential chunks, this setup
provides a rough approximation to distributing I/Os round-robin across
all ublk server threads, while letting I/Os stay fully thread-local.

Signed-off-by: Uday Shankar <ushankar@xxxxxxxxxxxxxxx>
Reviewed-by: Caleb Sander Mateos <csander@xxxxxxxxxxxxxxx>
---
drivers/block/ublk_drv.c | 75 ++++++++++++++++++++++--------------------------
1 file changed, 34 insertions(+), 41 deletions(-)

diff --git a/drivers/block/ublk_drv.c b/drivers/block/ublk_drv.c
index cdb1543fa4a9817aa2ca2fca66720f589cf222be..9a0d2547512fc8119460739230599d48d2c2a306 100644
--- a/drivers/block/ublk_drv.c
+++ b/drivers/block/ublk_drv.c
@@ -150,6 +150,7 @@ struct ublk_io {
int res;

struct io_uring_cmd *cmd;
+ struct task_struct *task;
};

struct ublk_queue {
@@ -157,11 +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 */
@@ -1072,11 +1071,6 @@ 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->flags & PF_EXITING;
-}
-
/* todo: handle partial completion */
static inline void __ublk_complete_rq(struct request *req)
{
@@ -1224,13 +1218,13 @@ 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;
}
@@ -1336,23 +1330,20 @@ 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];
unsigned int nr_inflight = 0;
int i;

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;
}

- if (!ubq_daemon_is_dying(ubq))
+ if (!(io->task->flags & PF_EXITING))
return BLK_EH_RESET_TIMER;

for (i = 0; i < ubq->q_depth; i++) {
- struct ublk_io *io = &ubq->ios[i];
+ io = &ubq->ios[i];

if (!(io->flags & UBLK_IO_FLAG_ACTIVE))
nr_inflight++;
@@ -1552,8 +1543,8 @@ static void ublk_commit_completion(struct ublk_device *ub,
}

/*
- * Called from ubq_daemon context via cancel fn, meantime quiesce ublk
- * blk-mq queue, so we are called exclusively with blk-mq and ubq_daemon
+ * Called from io task context via cancel fn, meantime quiesce ublk
+ * blk-mq queue, so we are called exclusively with blk-mq and io task
* context, so everything is serialized.
*/
static void ublk_abort_queue(struct ublk_device *ub, struct ublk_queue *ubq)
@@ -1669,13 +1660,13 @@ 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))
+ io = &ubq->ios[pdu->tag];
+ if (WARN_ON_ONCE(task && task != io->task))
return;

ub = ubq->dev;
need_schedule = ublk_abort_requests(ub, ubq);

- io = &ubq->ios[pdu->tag];
WARN_ON_ONCE(io->cmd != cmd);
ublk_cancel_cmd(ubq, io, issue_flags);

@@ -1836,8 +1827,6 @@ static void ublk_mark_io_ready(struct ublk_device *ub, struct ublk_queue *ubq)
mutex_lock(&ub->mutex);
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))
@@ -1952,14 +1941,14 @@ static int __ublk_ch_uring_cmd(struct io_uring_cmd *cmd,
if (!ubq || ub_cmd->q_id != ubq->q_id)
goto out;

- if (ubq->ubq_daemon && ubq->ubq_daemon != current)
- goto out;
-
if (tag >= ubq->q_depth)
goto out;

io = &ubq->ios[tag];

+ if (io->task && io->task != current)
+ goto out;
+
/* there is pending io cmd, something must be wrong */
if (io->flags & UBLK_IO_FLAG_ACTIVE) {
ret = -EBUSY;
@@ -2012,6 +2001,7 @@ static int __ublk_ch_uring_cmd(struct io_uring_cmd *cmd,

ublk_fill_io_cmd(io, cmd, ub_cmd->addr);
ublk_mark_io_ready(ub, ubq);
+ io->task = get_task_struct(current);
break;
case UBLK_IO_COMMIT_AND_FETCH_REQ:
req = blk_mq_tag_to_rq(ub->tag_set.tags[ub_cmd->q_id], tag);
@@ -2248,9 +2238,15 @@ 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));
}
@@ -2936,15 +2932,8 @@ static void ublk_queue_reinit(struct ublk_device *ub, struct ublk_queue *ubq)
{
int i;

- WARN_ON_ONCE(!(ubq->ubq_daemon && ubq_daemon_is_dying(ubq)));
-
/* All old ioucmds have to be completed */
ubq->nr_io_ready = 0;
- /* old daemon is PF_EXITING, put it now */
- 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;
ubq->canceling = false;

for (i = 0; i < ubq->q_depth; i++) {
@@ -2954,6 +2943,10 @@ static void ublk_queue_reinit(struct ublk_device *ub, struct ublk_queue *ubq)
io->flags = 0;
io->cmd = NULL;
io->addr = 0;
+
+ WARN_ON_ONCE(!(io->task && (io->task->flags & PF_EXITING)));
+ put_task_struct(io->task);
+ io->task = NULL;
}
}

@@ -2993,7 +2986,7 @@ static int ublk_ctrl_start_recovery(struct ublk_device *ub,
pr_devel("%s: start recovery for dev id %d.\n", __func__, header->dev_id);
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 the io_cmd_buf */
ub->mm = NULL;
ub->nr_queues_ready = 0;
ub->nr_privileged_daemon = 0;
@@ -3011,14 +3004,14 @@ static int ublk_ctrl_end_recovery(struct ublk_device *ub,
int ret = -EINVAL;
int i;

- 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))

--
2.34.1

Next message: Kuniyuki Iwashima: "Re: [PATCH net-next 4/8] ipv4: Use nlmsg_payload in devinet file"
Previous message: Uday Shankar: "[PATCH v4 4/4] ublk: mark ublk_queue as const for ublk_handle_need_get_data"
In reply to: Uday Shankar: "[PATCH v4 4/4] ublk: mark ublk_queue as const for ublk_handle_need_get_data"
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