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

[Christoph Hellwig <hch@lst.de>]

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?