Re: [PATCH v6 0/4] Add MMC software queue support

[Arnd Bergmann]

On Mon, Nov 11, 2019 at 1:58 PM Baolin Wang <baolin.wang@xxxxxxxxxx> wrote:
> On Mon, 11 Nov 2019 at 17:28, Arnd Bergmann <arnd@xxxxxxxx> wrote:
> > On Mon, Nov 11, 2019 at 8:35 AM Baolin Wang <baolin.wang@xxxxxxxxxx> wrote:
> > - Removing all the context switches and workqueues from the data submission
> >   path is also the right idea. As you found, there is still a workqueue inside
> >   of blk_mq that is used because it may get called from atomic context but
> >   the submission may get blocked in __mmc_claim_host(). This really
> >   needs to be changed as well, but not in the way I originally suggested:
> >   As Hannes suggested, the host interrrupt handler should always use
> >   request_threaded_irq() to have its own process context, and then pass a
> >   flag to blk_mq to say that we never need another workqueue there.
>
> So you mean we should complete the request in the host driver irq
> thread context, then issue another request in this context by calling
> blk_mq_run_hw_queues()?

Yes. I assumed there was already code that would always run
blk_mq_run_hw_queue() at I/O completion, but I can't find where
that happens today.

As I understand, the main difference to today is that
__blk_mq_delay_run_hw_queue() can call into __blk_mq_run_hw_queue
directly rather than using the delayed work queue once we
can skip the BLK_MQ_F_BLOCKING check.

> > - With that change in place calling a blocking __mmc_claim_host() is
> >   still a problem, so there should still be a nonblocking mmc_try_claim_host()
> >   for the submission path, leading to a BLK_STS_DEV_RESOURCE (?)
> >   return code from mmc_mq_queue_rq(). Basically mmc_mq_queue_rq()
> >   should always return right away, either after having queued the next I/O
> >   or with an error, but not waiting for the device in any way.
>
> Actually not only the mmc_claim_host() will block the MMC request
> processing, in this routine, the mmc_blk_part_switch() and
> mmc_retune() can also block the request processing. Moreover the part
> switching and tuning should be sync operations, and we can not move
> them to a work or a thread.

Ok, I see.

Those would also cause requests to be sent to the device or the host
controller, right? Maybe we can treat them as "a non-IO request
has successfully been queued to the device" events, returning
busy from the mmc_mq_queue_rq() function and then running
the queue again when they complete?

> > - For the packed requests, there is apparently a very simple way to implement
> >   that without a software queue: mmc_mq_queue_rq() is allowed to look at
> >   and dequeue all requests that are currently part of the request_queue,
> >   so it should take out as many as it wants to submit at once and send
> >   them all down to the driver together, avoiding the need for any further
> >   round-trips to blk_mq or maintaining a queue in mmc.
>
> You mean we can dispatch a request directly from
> elevator->type->ops.dispatch_request()?  but we still need some helper
> functions to check if these requests can be packed (the package
> condition), and need to invent new APIs to start a packed request (or
> using cqe interfaces, which means we still need to implement some cqe
> callbacks).

I don't know how the dispatch_request() function fits in there,
what Hannes told me is that in ->queue_rq() you can always
look at the following requests that are already queued up
and take the next ones off the list. Looking at bd->last
tells you if there are additional requests. If there are, you can
look at the next one from blk_mq_hw_ctx (not sure how, but
should not be hard to find)

I also see that there is a commit_rqs() callback that may
go along with queue_rq(), implementing that one could make
this easier as well.

> > - The DMA management (bounce buffer, map, unmap) that is currently
> >   done in mmc_blk_mq_issue_rq() should ideally be done in the
> >   init_request()/exit_request()  (?) callbacks from mmc_mq_ops so this
> >   can be done asynchronously, out of the critical timing path for the
> >   submission. With this, there won't be any need for a software queue.
>
> This is not true, now the blk-mq will allocate some static request
> objects (usually the static requests number should be the same with
> the hardware queue depth) saved in struct blk_mq_tags. So the
> init_request() is used to initialize the static requests when
> allocating them, and call exit_request to free the static requests
> when freeing the 'struct blk_mq_tags', such as the queue is dead. So
> we can not move the DMA management into the init_request/exit_request.

Ok, I must have misremembered which callback that is then, but I guess
there is some other place to do it.

       Arnd