Message-ID: <4c0ed590-4237-435d-40b3-21dffa9f9f00@igalia.com>
Date:   Wed, 3 May 2023 15:52:58 -0300
From:   André Almeida <andrealmeid@...lia.com>
To:     Marek Olšák <maraeo@...il.com>
Cc:     Felix Kuehling <felix.kuehling@....com>,
        Alex Deucher <alexdeucher@...il.com>,
        Timur Kristóf <timur.kristof@...il.com>,
        Christian König <ckoenig.leichtzumerken@...il.com>,
        "Pelloux-Prayer, Pierre-Eric" <pierre-eric.pelloux-prayer@....com>,
        michel.daenzer@...lbox.org,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        linux-kernel@...r.kernel.org,
        Samuel Pitoiset <samuel.pitoiset@...il.com>,
        amd-gfx list <amd-gfx@...ts.freedesktop.org>,
        kernel-dev@...lia.com,
        "Deucher, Alexander" <alexander.deucher@....com>,
        Christian König <christian.koenig@....com>
Subject: Re: [RFC PATCH 0/1] Add AMDGPU_INFO_GUILTY_APP ioctl

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.

> - 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?

> 
> 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@...il.com 
> <mailto:ckoenig.leichtzumerken@...il.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@...il.com <mailto:timur.kristof@...il.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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