Re: [REGRESSION] (>= v4.12) IO w/dmcrypt causing audio underruns

[Enric Balletbo Serra]

Hi Vito,

cc: dm-devel, Alasdair and Mike Snitzer

2018-05-28 5:32 GMT+02:00 Vito Caputo <vcaputo@xxxxxxxxxxx>:
> On Thu, Jan 25, 2018 at 12:33:21AM -0800, vcaputo@xxxxxxxxxxx wrote:
>> On Fri, Jan 19, 2018 at 11:57:32AM +0100, Enric Balletbo Serra wrote:
>> > Hi Vito,
>> >
>> > 2018-01-17 23:48 GMT+01:00  <vcaputo@xxxxxxxxxxx>:
>> > > On Mon, Dec 18, 2017 at 10:25:33AM +0100, Enric Balletbo Serra wrote:
>> > >> Hi Vito,
>> > >>
>> > >> 2017-12-01 22:33 GMT+01:00  <vcaputo@xxxxxxxxxxx>:
>> > >> > On Wed, Nov 29, 2017 at 10:39:19AM -0800, vcaputo@xxxxxxxxxxx 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@xxxxxxxxxx>
>> > >> >> 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@xxxxxxxxxx>
>> > >> >>     Signed-off-by: Enric Balletbo i Serra <enric.balletbo@xxxxxxxxxxxxx>
>> > >> >>     Signed-off-by: Mike Snitzer <snitzer@xxxxxxxxxx>
>> > >> >>
>> > >> >> ---
>> > >> >>
>> > >> >> 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.
>> > >>
>> > >
>> > > Any update on this?
>> > >
>> >
>> > I did not reproduce the issue for now. Can you try what happens if you
>> > remove the WQ_CPU_INTENSIVE in the kcryptd_io workqueue?
>> >
>> > - cc->io_queue = alloc_workqueue("kcryptd_io", WQ_HIGHPRI |
>> > WQ_CPU_INTENSIVE | WQ_MEM_RECLAIM, 1);
>> > cc->io_queue = alloc_workqueue("kcryptd_io", WQ_HIGHPRI | WQ_MEM_RECLAIM, 1);
>> >
>>
>> FWIW if I change both "kcryptd" and "kcryptd_io" workqueues to just
>> WQ_CPU_INTENSIVE, removing WQ_HIGHPRIO, the problem goes away.
>>
>> Doing this to "kcryptd_io" alone, as mentioned in my previous email, was
>> ineffective.
>>
>> Perhaps revert just the WQ_HIGHPRIO bit from the dmcrypt workqueues?
>>
>
>
> Guys... this is still a problem in 4.17-rc6.
>
> I don't understand why this is being ignored.  It's pathetic, my laptop
> can't even do a git checkout of the linux tree while playing mp3s
> without the music skipping.
>

Sorry, but it's easy to lost something on lkml, so adding the dm-devel
ML and the maintainers.

> Reverting a1b8913 completely eliminates the problem.  What gives?
>

IIRC the patch is there since 4.12 and I tried to reproduce the issue
on at least two devices, my laptop and a Chromebook Pixel 2 without
luck. Also, I am a bit surprised that nobody else has complained,
maybe I missed it, and *of course*, this doesn't mean the issue is not
there.

So, did anyone experience the same issue?

Regards,
 Enric

> Regards,
> Vito Caputo