[PATCH v2 1/2] drm: Add GPU reset sysfs event

Wed Mar 16 08:48:15 UTC 2022

On Tue, 15 Mar 2022 10:54:38 -0400
Alex Deucher <alexdeucher at gmail.com> wrote:

> On Mon, Mar 14, 2022 at 11:26 AM Pekka Paalanen <ppaalanen at gmail.com> wrote:
> >
> > On Mon, 14 Mar 2022 10:23:27 -0400
> > Alex Deucher <alexdeucher at gmail.com> wrote:
> >  
> > > On Fri, Mar 11, 2022 at 3:30 AM Pekka Paalanen <ppaalanen at gmail.com> wrote:  
> > > >
> > > > On Thu, 10 Mar 2022 11:56:41 -0800
> > > > Rob Clark <robdclark at gmail.com> wrote:
> > > >  
> > > > > For something like just notifying a compositor that a gpu crash
> > > > > happened, perhaps drm_event is more suitable.  See
> > > > > virtio_gpu_fence_event_create() for an example of adding new event
> > > > > types.  Although maybe you want it to be an event which is not device
> > > > > specific.  This isn't so much of a debugging use-case as simply
> > > > > notification.  
> > > >
> > > > Hi,
> > > >
> > > > for this particular use case, are we now talking about the display
> > > > device (KMS) crashing or the rendering device (OpenGL/Vulkan) crashing?
> > > >
> > > > If the former, I wasn't aware that display device crashes are a thing.
> > > > How should a userspace display server react to those?
> > > >
> > > > If the latter, don't we have EGL extensions or Vulkan API already to
> > > > deliver that?
> > > >
> > > > The above would be about device crashes that directly affect the
> > > > display server. Is that the use case in mind here, or is it instead
> > > > about notifying the display server that some application has caused a
> > > > driver/hardware crash? If the latter, how should a display server react
> > > > to that? Disconnect the application?
> > > >
> > > > Shashank, what is the actual use case you are developing this for?
> > > >
> > > > I've read all the emails here so far, and I don't recall seeing it
> > > > explained.
> > > >  
> > >
> > > The idea is that a support daemon or compositor would listen for GPU
> > > reset notifications and do something useful with them (kill the guilty
> > > app, restart the desktop environment, etc.).  Today when the GPU
> > > resets, most applications just continue assuming nothing is wrong,
> > > meanwhile the GPU has stopped accepting work until the apps re-init
> > > their context so all of their command submissions just get rejected.
> > >  
> > > > Btw. somewhat relatedly, there has been work aiming to allow
> > > > graceful hot-unplug of DRM devices. There is a kernel doc outlining how
> > > > the various APIs should react towards userspace when a DRM device
> > > > suddenly disappears. That seems to have some overlap here IMO.
> > > >
> > > > See https://www.kernel.org/doc/html/latest/gpu/drm-uapi.html#device-hot-unplug
> > > > which also has a couple pointers to EGL and Vulkan APIs.  
> > >
> > > The problem is most applications don't use the GL or VK robustness
> > > APIs.  
> >
> > Hi,
> >
> > how would this new event help with that?  
> 
> This event would provide notification that a GPU reset occurred.
> 
> >
> > I mean, yeah, there could be a daemon that kills those GPU users, but
> > then what? You still lose any unsaved work, and may need to manually
> > restart them.
> >
> > Is the idea that it is better to have the app crash and disappear than
> > to look like it froze while it otherwise still runs?  
> 
> Yes.

Ok. That was just a wild guess.

>  The daemon could also send the user some sort of notification
> that a GPU reset occurred.
> 
> >
> > If some daemon or compositor goes killing apps that trigger GPU resets,
> > then how do we stop that for an app that actually does use the
> > appropriate EGL or Vulkan APIs to detect and remedy that situation
> > itself?  
> 
> I guess the daemon could keep some sort of whitelist.  OTOH, very few
> if any applications, especially games actually support these
> extensions.

I would think that a white-list is a non-starter due to the maintenance
nightmare and the "wrong by default" design for well behaving programs.

I am not a fan of optimising for broken software. I understand that
with games this is routine, but we're talking about everything here,
not just games. Games cannot be fixed, but the rest could if the
problem was not sweeped under the rug. It's like the inverse of the
platform problem.

> > >  You could use something like that in the compositor, but those
> > > APIs tend to be focused more on the application itself rather than the
> > > GPU in general.  E.g., Is my context lost.  Which is fine for
> > > restarting your context, but doesn't really help if you want to try
> > > and do something with another application (i.e., the likely guilty
> > > app).  Also, on dGPU at least, when you reset the GPU, vram is usually
> > > lost (either due to the memory controller being reset, or vram being
> > > zero'd on init due to ECC support), so even if you are not the guilty
> > > process, in that case you'd need to re-init your context anyway.  
> >
> > Why should something like a compositor listen for this and kill apps
> > that triggered GPU resets, instead of e.g. Mesa noticing that in the app
> > and killing itself? Mesa in the app would know if robustness API is
> > being used.  
> 
> That's another possibility, but it doesn't handle the case where the
> compositor doesn't support any sort of robustness extension so if the
> GPU was reset, you'd lose your desktop anyway even if the app kept
> running.

Why does that matter?

A GPU reset happens when it happens. If a compositor does not use
robustness extensions, it's as good as dead anyway, right?

Killing a compositor from inside in Mesa if it doesn't use robustness
might be better than leaving the compositor running blind - assuming
the compositor does not quit itself after seeing crucial EGL/Vulkan
calls failing.

> >
> > Would be really nice to have the answers to all these questions to be
> > collected and reiterated in the next version of this proposal.  
> 
> The idea is to provide the notification of a GPU reset.  What the
> various desktop environments or daemons do with it is up to them.  I
> still think there is value in a notification even if you don't kill
> apps or anything like that.  E.g., you can have a daemon running that
> gets notified and logs the error, collects debug info, sends an email,
> etc.

With new UAPI comes the demand of userspace proof, not hand-waving. You
would not be proposing this new interface if you didn't have use cases
in mind, even just one. You have to document what you imagine the new
thing to be used for, so that the appropriateness can be evaluated. If
the use case is deemed inappropriate for the proposed UAPI, you need to
find another use case to justify adding the new UAPI. If there is no
use for the UAPI, it shouldn't be added, right? Adding UAPI and hoping
someone finds use for it seems backwards to me.

Thanks,
pq
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