Re: [PATCH v2 1/2] ublk: speed up ublk server exit handling

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

On Thu, Jul 03, 2025 at 11:41:07PM -0600, Uday Shankar wrote:
> Recently, we've observed a few cases where a ublk server is able to
> complete restart more quickly than the driver can process the exit of
> the previous ublk server. The new ublk server comes up, attempts
> recovery of the preexisting ublk devices, and observes them still in
> state UBLK_S_DEV_LIVE. While this is possible due to the asynchronous
> nature of io_uring cleanup and should therefore be handled properly in
> the ublk server, it is still preferable to make ublk server exit
> handling faster if possible, as we should strive for it to not be a
> limiting factor in how fast a ublk server can restart and provide
> service again.
> 
> Analysis of the issue showed that the vast majority of the time spent in
> handling the ublk server exit was in calls to blk_mq_quiesce_queue,
> which is essentially just a (relatively expensive) call to
> synchronize_rcu. The ublk server exit path currently issues an
> unnecessarily large number of calls to blk_mq_quiesce_queue, for two
> reasons:
> 
> 1. It tries to call blk_mq_quiesce_queue once per ublk_queue. However,
>    blk_mq_quiesce_queue targets the request_queue of the underlying ublk
>    device, of which there is only one. So the number of calls is larger
>    than necessary by a factor of nr_hw_queues.
> 2. In practice, it calls blk_mq_quiesce_queue _more_ than once per
>    ublk_queue. This is because of a data race where we read
>    ubq->canceling without any locking when deciding if we should call
>    ublk_start_cancel. It is thus possible for two calls to
>    ublk_uring_cmd_cancel_fn against the same ublk_queue to both call
>    ublk_start_cancel against the same ublk_queue.
> 
> Fix this by making the "canceling" flag a per-device state. This
> actually matches the existing code better, as there are several places
> where the flag is set or cleared for all queues simultaneously, and
> there is the general expectation that cancellation corresponds with ublk
> server exit. This per-device canceling flag is then checked under a
> (new) lock (addressing the data race (2) above), and the queue is only
> quiesced if it is cleared (addressing (1) above). The result is just one
> call to blk_mq_quiesce_queue per ublk device.
> 
> To minimize the number of cache lines that are accessed in the hot path,
> the per-queue canceling flag is kept. The values of the per-device
> canceling flag and all per-queue canceling flags should always match.
> 
> In our setup, where one ublk server handles I/O for 128 ublk devices,
> each having 24 hardware queues of depth 4096, here are the results
> before and after this patch, where teardown time is measured from the
> first call to io_ring_ctx_wait_and_kill to the return from the last
> ublk_ch_release:
> 
> 						before		after
> number of calls to blk_mq_quiesce_queue:	6469		256
> teardown time:					11.14s		2.44s
> 
> There are still some potential optimizations here, but this takes care
> of a big chunk of the ublk server exit handling delay.
> 
> Signed-off-by: Uday Shankar <ushankar@purestorage.com>

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


thanks,
Ming