Re: [RFC PATCH 3/3] mmc: core: Allow mmc block device to re-claim the host

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

On 15/05/17 17:05, Ulf Hansson wrote:
> On 12 May 2017 at 10:36, Adrian Hunter <adrian.hunter@intel.com> wrote:
>> On 11/05/17 15:39, Ulf Hansson wrote:
>>> The current mmc block device implementation is tricky when it comes to
>>> claim and release of the host, while processing I/O requests. In principle
>>> we need to claim the host at the first request entering the queue and then
>>> we need to release the host, as soon as the queue becomes empty. This
>>> complexity relates to the asynchronous request mechanism that the mmc block
>>> device driver implements.
>>>
>>> For the legacy block interface that we currently implements, the above
>>> issue can be addressed, as we can find out when the queue really becomes
>>> empty.
>>>
>>> However, to find out whether the queue is empty, isn't really an applicable
>>> method when using the new blk-mq interface, as requests are instead pushed
>>> to us via the struct struct blk_mq_ops and its function pointers.
>>
>> That is not entirely true.  We can pull requests by running the queue i.e.
>> blk_mq_run_hw_queues(q, false), returning BLK_MQ_RQ_QUEUE_BUSY and stopping
>> / starting the queue as needed.
> 
> I am not sure how that would work. It doesn't sound very effective to
> me, but I may be wrong.

The queue depth is not the arbiter of whether we can issue a request.  That
means there will certainly be times when we have to return
BLK_MQ_RQ_QUEUE_BUSY from ->queue_rq() and perhaps stop the queue as well.

We could start with ->queue_rq() feeding every request to the existing
thread and work towards having it submit requests immediately when possible.
 Currently mmc core cannot submit mmc_requests without waiting, but the
command queue implementation can for read/write requests when the host
controller and card are runtime resumed and the card is switched to the
correct internal partition (and we are not currently discarding flushing or
recovering), so it might be simpler to start with cmdq ;-)

> 
>>
>> But, as I have written before, we could start with the most trivial
>> implementation.  ->queue_rq() puts the requests in a list and then the
>> thread removes them from the list.
> 
> That would work...
> 
>>
>> That would be a good start because it would avoid having to deal with other
>> issues at the same time.
> 
> ...however this doesn't seem like a step in the direction we want to
> take when porting to blkmq.
> 
> There will be an extra context switch for each an every request, won't there?

No, for synchronous requests, it would be the same as now. ->queue_rq()
would be called in the context of the submitter and would wake the thread
just like ->request_fn() does now.

> 
> My point is, to be able to convert to blkmq, we must also avoid
> performance regression - at leas as long as possible.

It would still be better to start simple, and measure the performance, than
to guess where the bottlenecks are.

> 
>>
>>>
>>> Being able to support the asynchronous request method using the blk-mq
>>> interface, means we have to allow the mmc block device driver to re-claim
>>> the host from different tasks/contexts, as we may have > 1 request to
>>> operate upon.
>>>
>>> Therefore, let's extend the mmc_claim_host() API to support reference
>>> counting for the mmc block device.
>>
>> Aren't you overlooking the possibility that there are many block devices per
>> host. i.e. one per eMMC internal partition.
> 
> Right now, yes you are right. I hope soon not. :-)
> 
> These internal eMMC partitions are today suffering from the similar
> problems as we have for mmc ioctls. That means, requests are being I/O
> scheduled separately for each internal partition. Meaning requests for
> one partition could starve requests for another.
> 
> I really hope we can fix this in some way or the other. Probably
> building upon Linus Walleij's series for fixing the problems for mmc
> ioctls [1] is the way to go.
> 
> Then, when we have managed to fix these issues, I think my approach
> for extending the mmc_claim_host() API could be a possible
> intermediate step when trying to complete the blkmq port. Then we can
> continue to try to remove/re-work the claim host lock altogether as an
> optimization task.
> 
> Kind regards
> Uffe
> 
> [1]
> https://www.spinics.net/lists/linux-block/msg12677.html
>