blk-mq queue selection and queue_rq preemption

[Sagi Grimberg <sagig@dev.mellanox.co.il>]

Hey Jens, Christoph & Co,

I raised this question at LSF but didn't get a clear answer on this matter.
So it seems to me that the hctx selection and the actual request 
dispatch (queue_rq) are preemptive:
(1) blk_mq_get_ctx(q);
(2) map_queue(q, ctx->cpu);
...
(3) blk_mq_put_ctx(ctx);
(4) blk_mq_run_hw_queue(hctx, async);

It is possible that an MQ device driver may want to implement a lockless 
scheme counting on (running) CPU <-> hctx attachment.
Generally speaking, I think that LLDs will be more comfortable knowing 
that they are not preemptive in the dispatch flow.

My question is, is this a must? if so can you please explain why?

Is it possible to put the hctx (restoring preemption) after run_hw_queue 
allowing to LLDs to be sure that the selected queue
match the running CPU?

Thanks,

Sagi.