linux-kernel - Re: Problems caused by dm crypt: use WQ_HIGHPRI for the IO and crypt workqueues

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Date:   Tue, 14 May 2019 15:12:48 -0700
From:   Doug Anderson <dianders@...omium.org>
To:     Mike Snitzer <snitzer@...hat.com>
Cc:     Tim Murray <timmurray@...gle.com>,
        Guenter Roeck <groeck@...omium.org>,
        Enric Balletbo i Serra <enric.balletbo@...labora.com>,
        Vito Caputo <vcaputo@...garu.com>,
        LKML <linux-kernel@...r.kernel.org>, dm-devel@...hat.com,
        Tejun Heo <tj@...nel.org>
Subject: Re: Problems caused by dm crypt: use WQ_HIGHPRI for the IO and crypt workqueues

Hi,

On Tue, May 14, 2019 at 10:29 AM Mike Snitzer <snitzer@...hat.com> wrote:

> > tl;dr: High priority (even without CPU_INTENSIVE) definitely causes
> > interference with my high priority work starving it for > 8 ms, but
> > dm-crypt isn't unique here--loopback devices also have problems.
>
> Well I read it all ;)
>
> I don't have a commit 37a186225a0c, the original commit in querstion is
> a1b89132dc4 right?

commit 37a186225a0c ("platform/chrome: cros_ec_spi: Transfer messages
at high priority") is only really relevant to my particular test case
of using cros_ec to reproduce the problem.

> But I think we need a deeper understanding from workqueue maintainers on
> what the right way forward is here.  I cc'd Tejun in my previous reply
> but IIRC he no longer looks after the workqueue code.
>
> I think it'd be good for you to work with the original author of commit
> a1b89132dc4 (Tim, on cc) to see if you can reach consensus on what works
> for both of your requirements.

Basically what I decided in the end was that the workqueue code didn't
offer enough flexibility in terms of priorities.  To get realtime
priority I needed to fallback to using kthread_create_worker() to
create my worker.  Presumably if you want something nicer than the
"min_nice" priority you get with the high priority workqueue flag then
you'd have to do something similar (but moving in the opposite
direction).

> Given 7 above, if your new "cros_ec at realtime" series fixes it.. ship
> it?

Yeah, that's the plan.  Right now I have
<https://lkml.kernel.org/r/20190514183935.143463-2-dianders@chromium.org>
but Guenter pointed out some embarrassing bugs in my patch so I'll
post up a v4 tomorrow.  I pointed to a Chrome OS review that has a
preview of my v4 if you for some reason can't wait.  That can be found
at <https://crrev.com/c/1612165>.

-Doug