Date:   Mon, 18 Dec 2017 10:25:33 +0100
From:   Enric Balletbo Serra <eballetbo@...il.com>
To:     vcaputo@...garu.com
Cc:     linux-kernel <linux-kernel@...r.kernel.org>, timmurray@...gle.com,
        tj@...nel.org
Subject: Re: [REGRESSION] (>= v4.12) IO w/dmcrypt causing audio underruns

Hi Vito,

2017-12-01 22:33 GMT+01:00  <vcaputo@...garu.com>:
> On Wed, Nov 29, 2017 at 10:39:19AM -0800, vcaputo@...garu.com wrote:
>> Hello,
>>
>> Recently I noticed substantial audio dropouts when listening to MP3s in
>> `cmus` while doing big and churny `git checkout` commands in my linux git
>> tree.
>>
>> It's not something I've done much of over the last couple months so I
>> hadn't noticed until yesterday, but didn't remember this being a problem in
>> recent history.
>>
>> As there's quite an accumulation of similarly configured and built kernels
>> in my grub menu, it was trivial to determine approximately when this began:
>>
>> 4.11.0: no dropouts
>> 4.12.0-rc7: dropouts
>> 4.14.0-rc6: dropouts (seem more substantial as well, didn't investigate)
>>
>> Watching top while this is going on in the various kernel versions, it's
>> apparent that the kworker behavior changed.  Both the priority and quantity
>> of running kworker threads is elevated in kernels experiencing dropouts.
>>
>> Searching through the commit history for v4.11..v4.12 uncovered:
>>
>> commit a1b89132dc4f61071bdeaab92ea958e0953380a1
>> Author: Tim Murray <timmurray@...gle.com>
>> Date:   Fri Apr 21 11:11:36 2017 +0200
>>
>>     dm crypt: use WQ_HIGHPRI for the IO and crypt workqueues
>>
>>     Running dm-crypt with workqueues at the standard priority results in IO
>>     competing for CPU time with standard user apps, which can lead to
>>     pipeline bubbles and seriously degraded performance.  Move to using
>>     WQ_HIGHPRI workqueues to protect against that.
>>
>>     Signed-off-by: Tim Murray <timmurray@...gle.com>
>>     Signed-off-by: Enric Balletbo i Serra <enric.balletbo@...labora.com>
>>     Signed-off-by: Mike Snitzer <snitzer@...hat.com>
>>
>> ---
>>
>> Reverting a1b8913 from 4.14.0-rc6, my current kernel, eliminates the
>> problem completely.
>>
>> Looking at the diff in that commit, it looks like the commit message isn't
>> even accurate; not only is the priority of the dmcrypt workqueues being
>> changed - they're also being made "CPU intensive" workqueues as well.
>>
>> This combination appears to result in both elevated scheduling priority and
>> greater quantity of participant worker threads effectively starving any
>> normal priority user task under periods of heavy IO on dmcrypt volumes.
>>
>> I don't know what the right solution is here.  It seems to me we're lacking
>> the appropriate mechanism for charging CPU resources consumed on behalf of
>> user processes in kworker threads to the work-causing process.
>>
>> What effectively happens is my normal `git` user process is able to
>> greatly amplify what share of CPU it takes from the system by generating IO
>> on what happens to be a high-priority CPU-intensive storage volume.
>>
>> It looks potentially complicated to fix properly, but I suspect at its core
>> this may be a fairly longstanding shortcoming of the page cache and its
>> asynchronous design.  Something that has been exacerbated substantially by
>> the introduction of CPU-intensive storage subsystems like dmcrypt.
>>
>> If we imagine the whole stack simplified, where all the IO was being done
>> synchronously in-band, and the dmcrypt kernel code simply ran in the
>> IO-causing process context, it would be getting charged to the calling
>> process and scheduled accordingly.  The resource accounting and scheduling
>> problems all emerge with the page cache, buffered IO, and async background
>> writeback in a pool of unrelated worker threads, etc.  That's how it
>> appears to me anyways...
>>
>> The system used is a X61s Thinkpad 1.8Ghz with 840 EVO SSD, lvm on dmcrypt.
>> The kernel .config is attached in case it's of interest.
>>
>> Thanks,
>> Vito Caputo
>
>
>
> Ping...
>
> Could somebody please at least ACK receiving this so I'm not left wondering
> if my mails to lkml are somehow winding up flagged as spam, thanks!

Sorry I did not notice your email before you ping me directly. It's
interesting that issue, though we didn't notice this problem. It's a
bit far since I tested this patch but I'll setup the environment again
and do more tests to understand better what is happening.

Thanks,
 Enric

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