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

[Baolin Wang]

On Tue, Nov 12, 2019 at 12:59 AM Arnd Bergmann <arnd@xxxxxxxx> wrote:
>
> 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.

OK. Now we will complete a request in block softirq, which means the
irq thread of host driver should call blk_mq_complete_request() to
complete this request (triggering the block softirq) and call
blk_mq_run_hw_queues() to dispatch another request in this context.

>
> 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.

Right. Need to improve this as you suggested.

>
> > > - 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

Right.

> 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?

Yes, seems reasonable to me.

>
> > > - 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.

Yes, we can use queue_rq()/commit_rqs() and bd->last (now bd->last may
can not work well, see [1]), but like we talked before, for packed
request, we still need some new interfaces (for example, a interface
used to start a packed request, and a interface used to complete a
packed request), but at last we got a consensus that we should re-use
the CQE interfaces instead of new invention.

[1] https://lore.kernel.org/patchwork/patch/1102897/

>
> > > - 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.

I checked the 'struct blk_mq_ops', and I did not find a ops can be
used to do DMA management. And I also checked UFS driver, it also did
the DMA mapping in the queue_rq() (scsi_queue_rq() --->
ufshcd_queuecommand() ---> ufshcd_map_sg()). Maybe I missed something?

Moreover like I said above, for the packed request, we still need
implement something (like the software queue) based on the CQE
interfaces to help to handle packed requests.