Re: [PATCH 06/16] mmc: core: replace waitqueue with worker

[Jens Axboe <axboe@kernel.dk>]

On 03/13/2017 03:25 AM, Adrian Hunter wrote:
> On 11/03/17 00:05, Jens Axboe wrote:
>> On 03/10/2017 07:21 AM, Adrian Hunter wrote:
>>>> Essentially I take out that thread and replace it with this one worker
>>>> introduced in this very patch. I agree the driver can block in many ways
>>>> and that is why I need to have it running in process context, and this
>>>> is what the worker introduced here provides.
>>>
>>> The last time I looked at the blk-mq I/O scheduler code, it pulled up to
>>> qdepth requests from the I/O scheduler and left them on a local list while
>>> running ->queue_rq().  That means blocking in ->queue_rq() leaves some
>>> number of requests in limbo (not issued but also not in the I/O scheduler)
>>> for that time.
>>
>> Look again, if we're not handling the requeued dispatches, we pull one
>> at the time from the scheduler.
>>
> 
> That's good :-)
> 
> Now the next thing ;-)
> 
> It looks like we either set BLK_MQ_F_BLOCKING and miss the possibility of
> issuing synchronous requests immediately, or we don't set BLK_MQ_F_BLOCKING
> in which case we are never allowed to sleep in ->queue_rq().  Is that true?

Only one of those statements is true - if you don't set BLK_MQ_F_BLOCKING,
then you may never block in your ->queue_rq() function. But if you do set it,
it does not preclude immediate issue of sync requests.

-- 
Jens Axboe