Re: [PATCH v5 1/4] ublk: require unique task per io instead of unique task per hctx

[Ming Lei]

On Wed, Apr 16, 2025 at 01:46:05PM -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 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>
> ---

I guess this patch need to rebase against yesterday Jens's merge.

Given this change is big from ublk serer viewpoint, it should aim at
v6.16

As I suggested on V3[1]:

- it is nice to make it as one feature, so you can make any optimization
for this feature only, such as applying BLK_MQ_F_TAG_RR for improving
IO locality

- add selftest for this usage, which is helpful for

	- verify this usage

	- avoid to break the feature with new change

	- and evaluate performance effect

[1] https://lore.kernel.org/linux-block/aABZl4Yxdf3yew4q@fedora/


thanks,
Ming