Re: [PATCH] block: Improve shared tag set performance

[Keith Busch <kbusch@kernel.org>]

On Fri, Oct 20, 2023 at 09:45:53AM -0700, Bart Van Assche wrote:
> 
> On 10/20/23 09:25, Keith Busch wrote:
> > The legacy block request layer didn't have a tag resource shared among
> > multiple request queues. Each queue had their own mempool for allocating
> > requests. The mempool, I think, would always guarantee everyone could
> > get at least one request.
> 
> I think that the above is irrelevant in this context. As an example, SCSI
> devices have always shared a pool of tags across multiple logical
> units. This behavior has not been changed by the conversion of the
> SCSI core from the legacy block layer to blk-mq.
> 
> For other (hardware) block devices it didn't matter either that there
> was no upper limit to the number of requests the legacy block layer
> could allocate. All hardware block devices I know support fixed size
> queues for queuing requests to the block device.

I am not sure I understand your point. Those lower layers always were
able to get at least one request per request_queue. They can do whatever
they want with it after that. This change removes that guarantee for
blk-mq in some cases, right? I just don't think you can readily conclude
that is "safe" by appealing to the legacy behavior, that's all.