Re: [RFC PATCH 0/1] Add AMDGPU_INFO_GUILTY_APP ioctl

[Christian König]

Am 02.05.23 um 20:41 schrieb Alex Deucher:
> On Tue, May 2, 2023 at 11:22 AM Timur Kristóf <timur.kristof@xxxxxxxxx> wrote:
> > [SNIP]
> > > > > > In my opinion, the correct solution to those problems would be
> > > > > > if
> > > > > > the kernel could give userspace the necessary information about
> > > > > > a
> > > > > > GPU hang before a GPU reset.
> > > > >   The fundamental problem here is that the kernel doesn't have
> > > > > that
> > > > > information either. We know which IB timed out and can
> > > > > potentially do
> > > > > a devcoredump when that happens, but that's it.
> > > >
> > > > Is it really not possible to know such a fundamental thing as what
> > > > the
> > > > GPU was doing when it hung? How are we supposed to do any kind of
> > > > debugging without knowing that?

Yes, that's indeed something at least I try to figure out for years as well.

Basically there are two major problems:
1. When the ASIC is hung you can't talk to the firmware engines any more 
and most state is not exposed directly, but just through some fw/hw 
interface.
    Just take a look at how umr reads the shader state from the SQ. 
When that block is hung you can't do that any more and basically have no 
chance at all to figure out why it's hung.

    Same for other engines, I remember once spending a week figuring 
out why the UVD block is hung during suspend. Turned out to be a 
debugging nightmare because any time you touch any register of that 
block the whole system would hang.

2. There are tons of things going on in a pipeline fashion or even 
completely in parallel. For example the CP is just the beginning of a 
rather long pipeline which at the end produces a bunch of pixels.
    In almost all cases I've seen you ran into a problem somewhere deep 
in the pipeline and only very rarely at the beginning.

> > > > I wonder what AMD's Windows driver team is doing with this problem,
> > > > surely they must have better tools to deal with GPU hangs?
> > > For better or worse, most teams internally rely on scan dumps via
> > > JTAG
> > > which sort of limits the usefulness outside of AMD, but also gives
> > > you
> > > the exact state of the hardware when it's hung so the hardware teams
> > > prefer it.
> > How does this approach scale? It's not something we can ask users to
> > do, and even if all of us in the radv team had a JTAG device, we
> > wouldn't be able to play every game that users experience random hangs
> > with.
> It doesn't scale or lend itself particularly well to external
> development, but that's the current state of affairs.

The usual approach seems to be to reproduce a problem in a lab and have 
a JTAG attached to give the hw guys a scan dump and they can then tell 
you why something didn't worked as expected.

And yes that absolutely doesn't scale.

Christian.

> Alex