linux-kernel - Re: [Intel-gfx] WQ_UNBOUND warning since recent workqueue refactoring

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Date:   Wed, 30 Aug 2023 21:56:16 +0300
From:   Imre Deak <imre.deak@...el.com>
To:     Tejun Heo <tj@...nel.org>, Heiner Kallweit <hkallweit1@...il.com>
Cc:     intel-gfx@...ts.freedesktop.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>
Subject: Re: [Intel-gfx] WQ_UNBOUND warning since recent workqueue refactoring

On Wed, Aug 30, 2023 at 07:51:13AM -1000, Tejun Heo wrote:
Hi,

> 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?

No, this shouldn't happen.

I assume it happens in
https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git master

after cfd48ad8c4a9 ("drm/i915: Fix HPD polling, reenabling the output poll work as needed")

which could result in the above problem.

Could you give a try to
https://lore.kernel.org/all/20230809104307.1218058-1-imre.deak@intel.com/

and if that doesn't help provide more information/logs, by opening a
ticket at:
https://gitlab.freedesktop.org/drm/intel/-/issues/new

Thanks,
Imre

> * If so, can we make them use an unbound workqueue instead?
> 
> Thanks.
> 
> -- 
> tejun