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

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Message-ID: <CAF6AEGsyFAOPmHqT7YX1wsukP4-gYAstCukr89r9w28V0YSCUw@mail.gmail.com>
Date:   Thu, 17 Mar 2022 10:35:24 -0700
From:   Rob Clark <robdclark@...il.com>
To:     Christian König <christian.koenig@....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

On Thu, Mar 17, 2022 at 9:45 AM Christian König
<christian.koenig@....com> wrote:
>
> 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.

we don't wait on fences in shrinker, only purging or evicting things
that are already ready.  Actually, waiting on fences in shrinker path
sounds like a pretty bad idea.

> 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.

yeah, it would work to drain the scheduler.. I guess that might be the
more portable approach as far as generic solution for suspend.

BR,
-R

> 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
> >>>
>