Re: [PATCH 00/14] introduce the BFQ-v0 I/O scheduler as an extra scheduler

[Jens Axboe]

On 10/27/2016 04:27 PM, Linus Walleij wrote:
> On Thu, Oct 27, 2016 at 11:08 PM, Jens Axboe <axboe@xxxxxxxxx> wrote:
>
> > blk-mq has evolved to support a variety of devices, there's nothing
> > special about mmc that can't work well within that framework.
>
> There is. Read mmc_queue_thread() in drivers/mmc/card/queue.c
>
> This repeatedly calls req = blk_fetch_request(q);, starting one request
> and then getting the next one off the queue, including reading
> a few NULL requests off the end of the queue (to satisfy the
> semantics of its state machine.
>
> It then preprocess each request by esstially calling .pre() and .post()
> hooks all the way down to the driver, flushing its mapped
> sglist from CPU to DMA device memory (not a problem on x86 and
> other DMA-coherent archs, but a big win on the incoherent ones).
>
> In the attempt that was posted recently this is achieved by lying
> and saying the HW queue is two items deep and eating requests
> off that queue calling pre/post on them.
>
> But as there actually exist MMC cards with command queueing, this
> would become hopeless to handle, the hw queue depth has to reflect
> the real depth. What we need is for the block core to call pre/post
> hooks on each request.
>
> The "only" thing that doesn't work well after that is that CFQ is no
> longer in action, which will have interesting effects on MMC throughput
> in any fio-like stress test as it is mostly single-hw-queue.

That will cause you pain with any IO scheduler that has more complex
state, like CFQ and BFQ... I looked at the code but I don't quite get
why it is handling requests like that. Care to expand? Is it a
performance optimization? It looks fairly convoluted for some reason. I
would imagine that latency would be one of the more important aspects
for mmc, yet the driver has a context switch for each sync IO.

--
Jens Axboe