linux-kernel - Re: [PATCH 2/3] drm/msm/gpu: Park scheduler threads for system suspend

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Date:   Thu, 17 Mar 2022 17:44:57 +0100
From:   Christian König <christian.koenig@....com>
To:     Rob Clark <robdclark@...il.com>
Cc:     Andrey Grodzovsky <andrey.grodzovsky@....com>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        freedreno <freedreno@...ts.freedesktop.org>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        Rob Clark <robdclark@...omium.org>,
        Sean Paul <sean@...rly.run>,
        Abhinav Kumar <quic_abhinavk@...cinc.com>,
        David Airlie <airlied@...ux.ie>,
        Akhil P Oommen <quic_akhilpo@...cinc.com>,
        Jonathan Marek <jonathan@...ek.ca>,
        AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@...labora.com>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Vladimir Lypak <vladimir.lypak@...il.com>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/3] drm/msm/gpu: Park scheduler threads for system
 suspend

Am 17.03.22 um 17:18 schrieb Rob Clark:
> On Thu, Mar 17, 2022 at 9:04 AM Christian König
> <christian.koenig@....com> wrote:
>> Am 17.03.22 um 16:10 schrieb Rob Clark:
>>> [SNIP]
>>> userspace frozen != kthread frozen .. that is what this patch is
>>> trying to address, so we aren't racing between shutting down the hw
>>> and the scheduler shoveling more jobs at us.
>> Well exactly that's the problem. The scheduler is supposed to shoveling
>> more jobs at us until it is empty.
>>
>> Thinking more about it we will then keep some dma_fence instance
>> unsignaled and that is and extremely bad idea since it can lead to
>> deadlocks during suspend.
> Hmm, perhaps that is true if you need to migrate things out of vram?
> It is at least not a problem when vram is not involved.

No, it's much wider than that.

See what can happen is that the memory management shrinkers want to wait 
for a dma_fence during suspend.

And if you stop the scheduler they will just wait forever.

What you need to do instead is to drain the scheduler, e.g. call 
drm_sched_entity_flush() with a proper timeout for each entity you have 
created.

Regards,
Christian.

>
>> So this patch here is an absolute clear NAK from my side. If amdgpu is
>> doing something similar that is a severe bug and needs to be addressed
>> somehow.
> I think amdgpu's use of kthread_park is not related to suspend, but
> didn't look too closely.
>
> And perhaps the solution for this problem is more complex in the case
> of amdgpu, I'm not super familiar with the constraints there.  But I
> think it is a fine solution for integrated GPUs.
>
> BR,
> -R
>
>> Regards,
>> Christian.
>>
>>> BR,
>>> -R
>>>