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

[Christian König]

Am 17.03.22 um 17:18 schrieb Rob Clark:
> On Thu, Mar 17, 2022 at 9:04 AM Christian König
> <christian.koenig@xxxxxxx> 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