linux-kernel - Re: [PATCH V12 0/5] mmc: Add Command Queue support

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Date:   Thu, 26 Oct 2017 15:32:43 +0200
From:   Linus Walleij <linus.walleij@...aro.org>
To:     Adrian Hunter <adrian.hunter@...el.com>
Cc:     Ulf Hansson <ulf.hansson@...aro.org>,
        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>,
        Shawn Lin <shawn.lin@...k-chips.com>,
        Christoph Hellwig <hch@....de>
Subject: Re: [PATCH V12 0/5] mmc: Add Command Queue support

On Tue, Oct 24, 2017 at 10:40 AM, Adrian Hunter <adrian.hunter@...el.com> wrote:

> Here is V12 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.

Since I had my test setup going I gave this a spin with the same set
of tests that I used before/after my MQ patches.

It is using the same setup and same eMMC, but I hade to rebase onto
Ulf's very latest next branch to apply your patches.

I default-enabled multiqueue.

Results:

sync
echo 3 > /proc/sys/vm/drop_caches
sync
time dd if=/dev/mmcblk3 of=/dev/null bs=1M count=1024
1024+0 records in
1024+0 records out
1073741824 bytes (1.0GB) copied, 24.251922 seconds, 42.2MB/s
real    0m 24.25s
user    0m 0.03s
sys     0m 3.80s

mount /dev/mmcblk3p1 /mnt/
cd /mnt/
sync
echo 3 > /proc/sys/vm/drop_caches
sync
time find . > /dev/null
real    0m 3.24s
user    0m 0.22s
sys     0m 1.23s

sync
echo 3 > /proc/sys/vm/drop_caches
sync
iozone -az -i0 -i1 -i2 -s 20m -I -f /mnt/foo.test

                                                   random    random
   kB  reclen    write  rewrite    read    reread    read     write
20480       4     1615     1571     6612     6714     6494      531
20480       8     2143     2295    11559    11563    11499     1164
20480      16     3894     4202    17826    17823    17755     1369
20480      32     5816     7489    23741    23759    23709     3016
20480      64     7393     9167    27532    27526    27502     3591
20480     128     7328     8097    29184    29161    29159     5592
20480     256     7194     8752    29424    29434    29424     6700
20480     512     8984     9930    29903    29911    29909     7420
20480    1024     7072     7446    27684    27685    27681     7444
20480    2048     6840     8199    27398    27420    27418     6766
20480    4096     8137     6805    28091    28089    28093     8209
20480    8192     7255     7485    28386    28384    28383     7479
20480   16384     7078     7448    28584    28585    28585     7447

In short: no performance regressions.

Performance-wise this is on par with my own patch set for MQ.

As you know my pet peeve is "enable MQ by default" and I see no
reason from a performance perspective not to enable MQ by default
on this patch set or mine for that matter.

> 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.

My patch series switches the stack around to make it possible
to do this. But it doesn't go the whole way to complete the requests
from interrupt context.

Since we have to send commands for retune etc request finalization
cannot easily be done from interrupt context.

But I am thinking about testing to hack it
using some ugly approaches ... like assuming we don't need any
retune etc and just say all is fine and optimistically complete the
request directly in the interrupt handler if all was OK and wait
for errors to happen before retuning.

Yours,
Linus Walleij