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

[Adrian Hunter <adrian.hunter@intel.com>]

On 13/03/17 16:19, Jens Axboe wrote:
> 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.

I meant it gets put to the workqueue rather than issued in the context of
the submitter.