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

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Date:   Fri, 28 Oct 2016 00:27:28 +0200
From:   Linus Walleij <linus.walleij@...aro.org>
To:     Jens Axboe <axboe@...nel.dk>
Cc:     Ulf Hansson <ulf.hansson@...aro.org>,
        Paolo Valente <paolo.valente@...aro.org>,
        Christoph Hellwig <hch@...radead.org>,
        Arnd Bergmann <arnd@...db.de>,
        Bart Van Assche <bart.vanassche@...disk.com>,
        Jan Kara <jack@...e.cz>, Tejun Heo <tj@...nel.org>,
        linux-block@...r.kernel.org,
        Linux-Kernal <linux-kernel@...r.kernel.org>,
        Mark Brown <broonie@...nel.org>,
        Hannes Reinecke <hare@...e.de>,
        Grant Likely <grant.likely@...retlab.ca>,
        James Bottomley <James.Bottomley@...senpartnership.com>
Subject: Re: [PATCH 00/14] introduce the BFQ-v0 I/O scheduler as an extra scheduler

On Thu, Oct 27, 2016 at 11:08 PM, Jens Axboe <axboe@...nel.dk> 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.

Yours,
Linus Walleij