linux-kernel - Re: [PATCH V8 00/14] mmc: Add Command Queue support

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Date:   Thu, 21 Sep 2017 12:44:12 +0300
From:   Adrian Hunter <adrian.hunter@...el.com>
To:     Ulf Hansson <ulf.hansson@...aro.org>
Cc:     linux-mmc <linux-mmc@...r.kernel.org>,
        linux-block <linux-block@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Bough Chen <haibo.chen@....com>,
        Alex Lemberg <alex.lemberg@...disk.com>,
        Mateusz Nowak <mateusz.nowak@...el.com>,
        Yuliy Izrailov <Yuliy.Izrailov@...disk.com>,
        Jaehoon Chung <jh80.chung@...sung.com>,
        Dong Aisheng <dongas86@...il.com>,
        Das Asutosh <asutoshd@...eaurora.org>,
        Zhangfei Gao <zhangfei.gao@...il.com>,
        Sahitya Tummala <stummala@...eaurora.org>,
        Harjani Ritesh <riteshh@...eaurora.org>,
        Venu Byravarasu <vbyravarasu@...dia.com>,
        Linus Walleij <linus.walleij@...aro.org>,
        Shawn Lin <shawn.lin@...k-chips.com>,
        Christoph Hellwig <hch@....de>
Subject: Re: [PATCH V8 00/14] mmc: Add Command Queue support

On 21/09/17 12:01, Ulf Hansson wrote:
> On 13 September 2017 at 13:40, Adrian Hunter <adrian.hunter@...el.com> wrote:
>> Hi
>>
>> Here is V8 of the hardware command queue patches without the software
>> command queue patches, now using blk-mq and now with blk-mq support for
>> non-CQE I/O.
>>
>> After the unacceptable debacle of the last release cycle, I expect an
>> immediate response to these patches.
>>
>> HW CMDQ offers 25% - 50% better random multi-threaded I/O.  I see a slight
>> 2% drop in sequential read speed but no change to sequential write.
>>
>> Non-CQE blk-mq showed a 3% decrease in sequential read performance.  This
>> seemed to be coming from the inferior latency of running work items compared
>> with a dedicated thread.  Hacking blk-mq workqueue to be unbound reduced the
>> performance degradation from 3% to 1%.
>>
>> While we should look at changing blk-mq to give better workqueue performance,
>> a bigger gain is likely to be made by adding a new host API to enable the
>> next already-prepared request to be issued directly from within ->done()
>> callback of the current request.
> 
> Adrian, I am reviewing this series, however let me comment on each
> change individually.
> 
> I have also run some test on my ux500 board and enabling the blkmq
> path via the new MMC Kconfig option. My idea was to run some iozone
> comparisons between the legacy path and the new blkmq path, but I just
> couldn't get to that point because of the following errors.
> 
> I am using a Kingston 4GB SDHC card, which is detected and mounted
> nicely. However, when I decide to do some writes to the card I get the
> following errors.
> 
> root@ME:/mnt/sdcard dd if=/dev/zero of=testfile bs=8192 count=5000 conv=fsync
> [  463.714294] mmci-pl18x 80126000.sdi0_per1: error during DMA transfer!
> [  464.722656] mmci-pl18x 80126000.sdi0_per1: error during DMA transfer!
> [  466.081481] mmci-pl18x 80126000.sdi0_per1: error during DMA transfer!
> [  467.111236] mmci-pl18x 80126000.sdi0_per1: error during DMA transfer!
> [  468.669647] mmci-pl18x 80126000.sdi0_per1: error during DMA transfer!
> [  469.685699] mmci-pl18x 80126000.sdi0_per1: error during DMA transfer!
> [  471.043334] mmci-pl18x 80126000.sdi0_per1: error during DMA transfer!
> [  472.052337] mmci-pl18x 80126000.sdi0_per1: error during DMA transfer!
> [  473.342651] mmci-pl18x 80126000.sdi0_per1: error during DMA transfer!
> [  474.323760] mmci-pl18x 80126000.sdi0_per1: error during DMA transfer!
> [  475.544769] mmci-pl18x 80126000.sdi0_per1: error during DMA transfer!
> [  476.539031] mmci-pl18x 80126000.sdi0_per1: error during DMA transfer!
> [  477.748474] mmci-pl18x 80126000.sdi0_per1: error during DMA transfer!
> [  478.724182] mmci-pl18x 80126000.sdi0_per1: error during DMA transfer!
> 
> I haven't yet got the point of investigating this any further, and
> unfortunate I have a busy schedule with traveling next week. I will do
> my best to look into this as soon as I can.
> 
> Perhaps you have some ideas?

The behaviour depends on whether you have MMC_CAP_WAIT_WHILE_BUSY. Try
changing that and see if it makes a difference.