[PATCH v5 1/2] blk-mq: add tagset quiesce interface

[Christoph Hellwig]

On Tue, Jul 28, 2020 at 02:37:15AM -0700, Sagi Grimberg wrote:
>
>>>>> I like the tagset based interface.  But the idea of doing a per-hctx
>>>>> allocation and wait doesn't seem very scalable.
>>>>>
>>>>> Paul, do you have any good idea for an interface that waits on
>>>>> multiple srcu heads?  As far as I can tell we could just have a single
>>>>> global completion and counter, and each call_srcu would just just
>>>>> decrement it and then the final one would do the wakeup.  It would just
>>>>> be great to figure out a way to keep the struct rcu_synchronize and
>>>>> counter on stack to avoid an allocation.
>>>>>
>>>>> But if we can't do with an on-stack object I'd much rather just embedd
>>>>> the rcu_head in the hw_ctx.
>>>>
>>>> I think we can do that, please see the following patch which is against Sagi's V5:
>>>
>>> I don't think you can send a single rcu_head to multiple call_srcu calls.
>>
>> OK, then one variant is to put the rcu_head into blk_mq_hw_ctx, and put
>> rcu_synchronize into blk_mq_tag_set.
>
> I can cook up a spin, but I still hate the fact that I have a queue that
> ends up quiesced which I didn't want it to...

Why do we care so much about the connect_q?  Especially if we generalize
it into a passthru queue that will absolutely need the quiesce hopefully
soon.