linux-kernel - Re: [PATCH v1] drm/scheduler: Don't kill jobs in interrupt context

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Date:   Tue, 12 Apr 2022 22:55:52 +0300
From:   Dmitry Osipenko <dmitry.osipenko@...labora.com>
To:     Andrey Grodzovsky <andrey.grodzovsky@....com>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Tomeu Vizoso <tomeu.vizoso@...labora.com>,
        Steven Price <steven.price@....com>,
        Rob Herring <robh@...nel.org>,
        Alyssa Rosenzweig <alyssa.rosenzweig@...labora.com>,
        Rob Clark <robdclark@...il.com>
Cc:     dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        Dmitry Osipenko <digetx@...il.com>
Subject: Re: [PATCH v1] drm/scheduler: Don't kill jobs in interrupt context

On 4/12/22 22:40, Andrey Grodzovsky wrote:
> 
> On 2022-04-12 14:20, Dmitry Osipenko wrote:
>> On 4/12/22 19:51, Andrey Grodzovsky wrote:
>>> On 2022-04-11 18:15, Dmitry Osipenko wrote:
>>>> Interrupt context can't sleep. Drivers like Panfrost and MSM are taking
>>>> mutex when job is released, and thus, that code can sleep. This results
>>>> into "BUG: scheduling while atomic" if locks are contented while job is
>>>> freed. There is no good reason for releasing scheduler's jobs in IRQ
>>>> context, hence use normal context to fix the trouble.
>>>
>>> I am not sure this is the beast Idea to leave job's sw fence signalling
>>> to be
>>> executed in system_wq context which is prone to delays of executing
>>> various work items from around the system. Seems better to me to
>>> leave the
>>> fence signaling within the IRQ context and offload only the job
>>> freeing or,
>>> maybe handle rescheduling to thread context within drivers implemention
>>> of .free_job cb. Not really sure which is the better.
>> We're talking here about killing jobs when driver destroys context,
>> which doesn't feel like it needs to be a fast path. I could move the
>> signalling into drm_sched_entity_kill_jobs_cb() and use unbound wq, but
>> do we really need this for a slow path?
> 
> 
> You can't move the signaling back to drm_sched_entity_kill_jobs_cb
> since this will bring back the lockdep splat that 'drm/sched: Avoid
> lockdep spalt on killing a processes'
> was fixing.

Indeed

> I see your point and i guess we can go this way too. Another way would
> be to add to
> panfrost and msm job a  work_item and reschedule to thread context from
> within their
> .free_job callbacks but that probably to cumbersome to be justified here.

Yes, there is no clear justification for doing that.

> Andrey
> 
> 
> Reviewed-by: Andrey Grodzovsky <andrey.grodzovsky@....com>

Thank you!