[RFC PATCH 0/1] Add AMDGPU_INFO_GUILTY_APP ioctl

Thu May 4 01:13:26 UTC 2023

On Wed, May 3, 2023, 14:53 André Almeida <andrealmeid at igalia.com> wrote:

> Em 03/05/2023 14:08, Marek Olšák escreveu:
> > GPU hangs are pretty common post-bringup. They are not common per user,
> > but if we gather all hangs from all users, we can have lots and lots of
> > them.
> >
> > GPU hangs are indeed not very debuggable. There are however some things
> > we can do:
> > - Identify the hanging IB by its VA (the kernel should know it)
>
> How can the kernel tell which VA range is being executed? I only found
> that information at mmCP_IB1_BASE_ regs, but as stated in this thread by
> Christian this is not reliable to be read.
>

The kernel receives the VA and the size via the CS ioctl. When user queues
are enabled, the kernel will no longer receive them.

> > - Read and parse the IB to detect memory corruption.
> > - Print active waves with shader disassembly if SQ isn't hung (often
> > it's not).
> >
> > Determining which packet the CP is stuck on is tricky. The CP has 2
> > engines (one frontend and one backend) that work on the same command
> > buffer. The frontend engine runs ahead, executes some packets and
> > forwards others to the backend engine. Only the frontend engine has the
> > command buffer VA somewhere. The backend engine only receives packets
> > from the frontend engine via a FIFO, so it might not be possible to tell
> > where it's stuck if it's stuck.
>
> Do they run at the same asynchronously or does the front end waits the
> back end to execute?
>

They run asynchronously and should run asynchronously for performance, but
they can be synchronized using a special packet (PFP_SYNC_ME).

Marek

> >
> > When the gfx pipeline hangs outside of shaders, making a scandump seems
> > to be the only way to have a chance at finding out what's going wrong,
> > and only AMD-internal versions of hw can be scanned.
> >
> > Marek
> >
> > On Wed, May 3, 2023 at 11:23 AM Christian König
> > <ckoenig.leichtzumerken at gmail.com
> > <mailto:ckoenig.leichtzumerken at gmail.com>> wrote:
> >
> >     Am 03.05.23 um 17:08 schrieb Felix Kuehling:
> >      > Am 2023-05-03 um 03:59 schrieb 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 at gmail.com <mailto:timur.kristof at gmail.com>>
> 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.
> >      >
> >      > That's the worst-case scenario where you're debugging HW or FW
> >     issues.
> >      > Those should be pretty rare post-bringup. But are there hangs
> caused
> >      > by user mode driver or application bugs that are easier to debug
> and
> >      > probably don't even require a GPU reset? For example most VM
> faults
> >      > can be handled without hanging the GPU. Similarly, a shader in an
> >      > endless loop should not require a full GPU reset. In the KFD
> compute
> >      > case, that's still preemptible and the offending process can be
> >     killed
> >      > with Ctrl-C or debugged with rocm-gdb.
> >
> >     We also have infinite loop in shader abort for gfx and page faults
> are
> >     pretty rare with OpenGL (a bit more often with Vulkan) and can be
> >     handled gracefully on modern hw (they just spam the logs).
> >
> >     The majority of the problems is unfortunately that we really get hard
> >     hangs because of some hw issues. That can be caused by unlucky
> timing,
> >     power management or doing things in an order the hw doesn't expected.
> >
> >     Regards,
> >     Christian.
> >
> >      >
> >      > It's more complicated for graphics because of the more complex
> >      > pipeline and the lack of CWSR. But it should still be possible to
> do
> >      > some debugging without JTAG if the problem is in SW and not HW or
> >     FW.
> >      > It's probably worth improving that debugability without getting
> >      > hung-up on the worst case.
> >      >
> >      > Maybe user mode graphics queues will offer a better way of
> >     recovering
> >      > from these kinds of bugs, if the graphics pipeline can be unstuck
> >      > without a GPU reset, just by killing the offending user mode
> queue.
> >      >
> >      > Regards,
> >      >   Felix
> >      >
> >      >
> >      >>
> >      >> And yes that absolutely doesn't scale.
> >      >>
> >      >> Christian.
> >      >>
> >      >>>
> >      >>> Alex
> >      >>
> >
>
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