lkml.org 
[lkml]   [2018]   [May]   [27]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    SubjectRe: [REGRESSION] (>= v4.12) IO w/dmcrypt causing audio underruns
    On Thu, Jan 25, 2018 at 12:33:21AM -0800, vcaputo@pengaru.com 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@pengaru.com>:
    > > > 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@pengaru.com>:
    > > >> > On Wed, Nov 29, 2017 at 10:39:19AM -0800, vcaputo@pengaru.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@google.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@google.com>
    > > >> >> Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
    > > >> >> Signed-off-by: Mike Snitzer <snitzer@redhat.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.
    > > >>
    > > >
    > > > 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.

    Reverting a1b8913 completely eliminates the problem. What gives?

    Regards,
    Vito Caputo

    \
     
     \ /
      Last update: 2018-05-28 05:32    [W:4.406 / U:2.020 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site