Re: [RFC PATCH] blk-mq: implement queue quiesce via percpu_ref for BLK_MQ_F_BLOCKING

[Sagi Grimberg <sagi@grimberg.me>]

>>>> I think it will be a significant improvement to have a single code path.
>>>> The code will be more robust and we won't need to face issues that are
>>>> specific for blocking.
>>>>
>>>> If the cost is negligible, I think the upside is worth it.
>>>>
>>>
>>> rcu_read_lock and rcu_read_unlock has been proved as efficient enough,
>>> and I don't think percpu_refcount is better than it, so I'd suggest to
>>> not switch non-blocking into this way.
>>
>> It's not a matter of which is better, its a matter of making the code
>> more robust because it has a single code-path. If moving to percpu_ref
>> is negligible, I would suggest to move both, I don't want to have two
>> completely different mechanism for blocking vs. non-blocking.
> 
> FWIW, I proposed an hctx percpu_ref over a year ago (but for a
> completely different reason), and it was measured as too costly.
> 
>    https://lore.kernel.org/linux-block/d4a4b6c0-3ea8-f748-85b0-6b39c5023a6f@kernel.dk/

If this is the case, we shouldn't consider this as an alternative at 
all, and move forward with either the original proposal or what
ming proposed to move a counter to the tagset.