Re: [PATCH v8 1/9] ublk: have a per-io daemon instead of a per-queue daemon

[Ming Lei <ming.lei@redhat.com>]

On Thu, May 29, 2025 at 05:47:10PM -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 by
> 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
> 
> This setup allows for load that is concentrated on one hctx/ublk_queue
> to be spread out across all ublk server threads, alleviating the issue
> described above.
> 
> Add the new UBLK_F_PER_IO_DAEMON feature to ublk_drv, which ublk servers
> can use to essentially test for the presence of this change and tailor
> their behavior accordingly.
> 
> Signed-off-by: Uday Shankar <ushankar@purestorage.com>
> Reviewed-by: Caleb Sander Mateos <csander@purestorage.com>

Reviewed-by: Ming Lei <ming.lei@redhat.com>



Thanks,
Ming