Re: [RFC PATCH v3 2/3] blk-mq: Freeze and quiesce all queues for tagset in elevator_exit()

[John Garry]

On 11/03/2021 00:58, Ming Lei wrote:
> > Indeed, blk_mq_queue_tag_busy_iter() already does take a reference to its
> > queue usage counter when called, and the queue cannot be frozen to switch
> > IO scheduler until all refs are dropped. This ensures no stale references
> > to IO scheduler requests will be seen by blk_mq_queue_tag_busy_iter().
> >
> > However, there is nothing to stop blk_mq_queue_tag_busy_iter() being
> > run for another queue associated with the same tagset, and it seeing
> > a stale IO scheduler request from the other queue after they are freed.
> >
> > To stop this happening, freeze and quiesce all queues associated with the
> > tagset as the elevator is exited.
> I think this way can't be accepted since switching one queue's scheduler
> is nothing to do with other request queues attached to same HBA.
>
> This patch will cause performance regression because userspace may
> switch scheduler according to medium or workloads, at that time other
> LUNs will be affected by this patch.

Hmmm..that was my concern also. Do you think that it may cause a big 
impact? Depends totally on the workload, I suppose.

FWIW, it is useful though for solving both iterator problems.

Thanks,
John