Re: [PATCH v2 1/2] blk-mq: add tagset quiesce interface

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

On Tue, Oct 18, 2022 at 07:19:56AM +0200, Christoph Hellwig wrote:
> On Mon, Oct 17, 2022 at 03:41:15PM -0700, Paul E. McKenney wrote:
> > Then the big question is "how long do the SRCU readers run?"
> > 
> > If all of the readers ran for exactly the same duration, there would be
> > little point in having more than one srcu_struct.
> 
> The SRCU readers are the I/O dispatch, which will have quite similar
> runtimes for the different queues.
> 
> > If the kernel knew up front how long the SRCU readers for a given entry
> > would run, it could provide an srcu_struct structure for each duration.
> > For a (fanciful) example, you could have one srcu_struct structure for
> > SSDs, another for rotating rust, a third for LAN-attached storage, and
> > a fourth for WAN-attached storage.  Maybe a fifth for lunar-based storage.
> 
> All the different request_queues in a tag_set are for the same device.
> There might be some corner cases like storare arrays where they have
> different latencies.  But we're not even waiting for the I/O completion
> here, this just protects the submission.
> 
> > Does that help, or am I off in the weeds here?
> 
> I think this was very helpful, and at least to be moving the srcu_struct
> to the tag_set sounds like a good idea to explore.
> 
> Ming, anything I might have missed?

I think it is fine to move it to tag_set, this way could simplify a
lot.

The effect could be that blk_mq_quiesce_queue() becomes a little
slow, but it is always in slow path, and synchronize_srcu() won't
wait new read-side critical-section.

Just one corner case, in case of BLK_MQ_F_BLOCKING, is there any such driver
which may block too long in ->queue_rq() sometime? such as wait for dozens
of seconds.


thanks,
Ming