linux-kernel - Re: WQ_UNBOUND warning since recent workqueue refactoring

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Date:   Wed, 30 Aug 2023 07:51:13 -1000
From:   Tejun Heo <tj@...nel.org>
To:     Heiner Kallweit <hkallweit1@...il.com>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        Tvrtko Ursulin <tvrtko.ursulin@...ux.intel.com>,
        intel-gfx@...ts.freedesktop.org
Subject: Re: WQ_UNBOUND warning since recent workqueue refactoring

Hello,

(cc'ing i915 folks)

On Wed, Aug 30, 2023 at 04:57:42PM +0200, Heiner Kallweit wrote:
> Recently I started to see the following warning on linux-next and presumably
> this may be related to the refactoring of the workqueue core code.
> 
> [   56.900223] workqueue: output_poll_execute [drm_kms_helper] hogged CPU for >10000us 4 times, consider switching to WQ_UNBOUND
> [   56.923226] workqueue: i915_hpd_poll_init_work [i915] hogged CPU for >10000us 4 times, consider switching to WQ_UNBOUND
> [   97.860430] workqueue: output_poll_execute [drm_kms_helper] hogged CPU for >10000us 8 times, consider switching to WQ_UNBOUND
> [   97.884453] workqueue: i915_hpd_poll_init_work [i915] hogged CPU for >10000us 8 times, consider switching to WQ_UNBOUND
> 
> Adding WQ_UNBOUND to these queues didn't change the behavior.

That should have made them go away as the code path isn't active at all for
WQ_UNBOUND workqueues. Can you please double check?

> Maybe relevant: I run the affected system headless.

i915 folks, workqueue recently added debug warnings which trigger when a
per-cpu work item hogs the CPU for too long - 10ms in this case. This is
problematic because such work item can stall other per-cpu work items.

* Is it expected for the above two work functions to occupy the CPU for over
  10ms repeatedly?

* If so, can we make them use an unbound workqueue instead?

Thanks.

-- 
tejun