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

[vcaputo]

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 still experience it on 4.15-rc7 when doing sustained heavyweight git
checkouts without a1b8913 reverted.
 
Thanks,
Vito Caputo