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

[Paolo Valente <paolo.valente@linaro.org>]

> Il giorno 22 nov 2019, alle ore 10:50, Arnd Bergmann <arnd@arndb.de> ha scritto:
> 
> (adding Paolo as well, maybe he has some more insights)
> 
> On Mon, Nov 18, 2019 at 11:04 AM (Exiting) Baolin Wang
> <baolin.wang@linaro.org> wrote:
>> On Tue, 12 Nov 2019 at 16:48, Baolin Wang <baolin.wang7@gmail.com> wrote:
>>> On Tue, Nov 12, 2019 at 12:59 AM Arnd Bergmann <arnd@arndb.de> wrote:
>>>> On Mon, Nov 11, 2019 at 1:58 PM Baolin Wang <baolin.wang@linaro.org> wrote:
>>>>>> - 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.
>> 
>> After some investigation and offline discussion with you, I still have
>> some concerns about your suggestion.
>> 
>> 1) Now blk-mq have not supplied some ops to prepare a request, which is
>> used to do some DMA management asynchronously. But yes, we can
>> introduce new ops for blk-mq. But there are still some remaining
>> preparation in mmc_mq_queue_rq(), like mmc part switch. For software
>> queue, we can prepare a request totally after issuing one.
> 
> I suppose to make the submission non-blocking, all operations that
> currently block in the submission path may have to be changed first.
> 
> For the case of a partition switch (same for retune), I suppose
> something like this can be done:
> 
> - in queue_rq() check whether a partition switch is needed. If not,
>  submit the current rq
> - if a partition switch is needed, submit the partition switch cmd
>  instead, and return busy status
> - when the completion arrives for the partition switch, call back into
>  blk_mq to have it call queue_rq again.
> 
> Or possibly even (this might not be possible without signifcant
> restructuring):
> 
> - when preparing a request that would require a partition switch,
>  insert another meta-request to switch the partition ahead of it.
> 
> I do realize that this is a significant departure from how it was done
> in the past, but it seems cleaner that way to me.
> 
>> 2) I wonder if it is appropriate that using the irq threaded context
>> to dispatch next request, actually we will still introduce a context
>> switch here. Now we will complete a request in the hard irq handler
>> and kick the softirq to do time-consuming operations, like DMA
>> unmapping , and will start next request in the hard irq handler
>> without context switch. Moreover if we remove the BLK_MQ_F_BLOCKING in
>> future like you suggested, then we can remove all context switch. And
>> I think we can dispatch next request in the softirq context (actually
>> the CQE already did).
> 
> I hope Hannes (or someone else) can comment here, as I don't
> know exactly what his objection to kicking off the next cmd in the
> hardirq was.
> 
> I think generally, deferring all slow operations to an irqthread
> rather than a softirq is a good idea, but I share your concern that
> this can introduce an unnecessary latency between the the
> the IRQ is signaled and the time the following cmd is sent to the
> hardware.
> 
> Doing everything in a single (irqthread) context is clearly simpler,
> so this would need to be measured carefully to avoid unnecessary
> complexity, but I think don't see anything stopping us from having
> the fast-path where the low-level driver first checks for any possible
> error conditions in hardirq context and the fires off a prepared cmd
> right away whenever it can before triggering the irqthread that does
> everything else. I think this has to be a per-driver optimization, so
> the common case would just have an irqthread.
> 
>> 3) For packed request support, I did not see an example that block
>> driver can dispatch a request from the IO scheduler in queue_rq() and
>> no APIs supported from blk-mq. And we do not know where can dispatch a
>> request in queue_rq(), from IO scheduler? from ctx? or from
>> hctx->dispatch list? and if this request can not be passed to host
>> now, how to do it? Seems lots of complicated things.
> 
> The only way I can see is the ->last flag, so if blk_mq submits multiple
> requests in a row to queue_rq() with this flag cleared and  calls
> ->commit_rqs() after the last one. This seems to be what the scsi
> disk driver and the nvme driver rely on, and we should be able to use
> it the same way for packed cmds, by checking each time in queue_rq()
> whether requests can/should be combined and reporting busy otherwise
> (after preparing a combined mmc cmd).
> blk_mq will then call commit_rqs, which should do the actual submission
> to the hardware driver.
> 
> Now as you point out, the *big* problem with this is that we never
> get multiple requests together in practice, i.e. the last flag is almost
> always set, and any optimization around it has no effect.
> 
> This is where I'm a bit lost in the code as well, but it seems that
> this is part of the current bfq design that only sends one request down
> the driver stack at a time, and this would have to change first before
> we can rely on this for packing requests.
> 
> Paolo, can you comment on why this is currently done, or if it can
> be changed? It seems to me that sending multiple requests at
> once would also have a significant benefit on the per-request overhead
> on NVMe devices with with bfq.
> 

Hi,
actually, "one request dispatched at a time" is not a peculiarity of
bfq.  Any scheduler can provide only one request at a time, with the
current blk-mq API for I/O schedulers.

Yet, when it is time to refill an hardware queue, blk-mq pulls as many
requests as it deems appropriate from the scheduler, by invoking the
latter multiple times.  See blk_mq_do_dispatch_sched() in
block/blk-mq-sched.c.

I don't know where the glitch for MMC is with respect to this scheme.

Thanks,
Paolo


>> Moreover, we still need some interfaces for the packed request
>> handling, from previous discussion, we still need something like MMC
>> software queue based on  the CQE to help to handle the packed request.
>> 
>> So I think I still need to introduce the MMC software queue, on the one
>> hand is that it can really improve the performance from fio data and
>> avoid a long latency, on the other hand we can expand it to support
>> packed request easily in future. Thanks.
>> 
>> (Anyway I will still post the V7 to address Adrian's comments and to
>> see if we can get a consensus there).
> 
> 
>     Arnd