Messages in this thread | | | Date | Wed, 17 Jan 2018 14:48:52 -0800 | From | vcaputo@pengaru ... | Subject | Re: [REGRESSION] (>= v4.12) IO w/dmcrypt causing audio underruns |
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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 still experience it on 4.15-rc7 when doing sustained heavyweight git checkouts without a1b8913 reverted.
Thanks, Vito Caputo
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