On 2022-08-04 16:43, Nathan Chancellor wrote:
> On Thu, Aug 04, 2022 at 09:24:41PM +0200, Arnd Bergmann wrote:
>> On Thu, Aug 4, 2022 at 8:52 PM Linus Torvalds
>> <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>>>
>>> On Thu, Aug 4, 2022 at 11:37 AM Sudip Mukherjee (Codethink)
>>> <sudipm.mukherjee@xxxxxxxxx> wrote:cov_trace_cmp
>>>>
>>>> git bisect points to 3876a8b5e241 ("drm/amd/display: Enable building new display engine with KCOV enabled").
>>>
>>> Ahh. So that was presumably why it was disabled before - because it
>>> presumably does disgusting things that make KCOV generate even bigger
>>> stack frames than it already has.
>>>
>>> Those functions do seem to have fairly big stack footprints already (I
>>> didn't try to look into why, I assume it's partly due to aggressive
>>> inlining, and probably some automatic structures on stack). But gcc
>>> doesn't seem to make it all that much worse with KCOV (and my clang
>>> build doesn't enable KCOV).
>>>
>>> So it's presumably some KCOV-vs-clang thing. Nathan?
>
> Looks like Arnd beat me to it :)
>
>> The dependency was originally added to avoid a link failure in 9d1d02ff3678
>> ("drm/amd/display: Don't build DCN1 when kcov is enabled") after I reported the
>> problem in https://lists.freedesktop.org/archives/dri-devel/2018-August/186131.html>>>
>> The commit from the bisection just turns off KCOV for the entire directory
>> to avoid the link failure, so it's not actually a problem with KCOV vs clang,
>> but I think a problem with clang vs badly written code that was obscured
>> in allmodconfig builds prior to this.
>
> Right, I do think the sanitizers make things worse here too, as those get
> enabled with allmodconfig. I ran some really quick tests with allmodconfig and
> a few instrumentation options flipped on/off:
>
> allmodconfig (CONFIG_KASAN=y, CONFIG_KCSAN=n, CONFIG_KCOV=y, and CONFIG_UBSAN=y):
>
> warning: stack frame size (2216) exceeds limit (2048) in 'dml30_ModeSupportAndSystemConfigurationFull' [-Wframe-larger-than]
> warning: stack frame size (2184) exceeds limit (2048) in 'dml31_ModeSupportAndSystemConfigurationFull' [-Wframe-larger-than]
> warning: stack frame size (2176) exceeds limit (2048) in 'dml32_ModeSupportAndSystemConfigurationFull' [-Wframe-larger-than]
>
> allmodconfig + CONFIG_KASAN=n:
>
> warning: stack frame size (2112) exceeds limit (2048) in 'dml32_ModeSupportAndSystemConfigurationFull' [-Wframe-larger-than]
>
> allmodconfig + CONFIG_KCOV=n:
>
> warning: stack frame size (2216) exceeds limit (2048) in 'dml30_ModeSupportAndSystemConfigurationFull' [-Wframe-larger-than]
> warning: stack frame size (2184) exceeds limit (2048) in 'dml31_ModeSupportAndSystemConfigurationFull' [-Wframe-larger-than]
> warning: stack frame size (2176) exceeds limit (2048) in 'dml32_ModeSupportAndSystemConfigurationFull' [-Wframe-larger-than]
>
> allmodconfig + CONFIG_UBSAN=n:
>
> warning: stack frame size (2584) exceeds limit (2048) in 'dml30_ModeSupportAndSystemConfigurationFull' [-Wframe-larger-than]
> warning: stack frame size (2680) exceeds limit (2048) in 'dml31_ModeSupportAndSystemConfigurationFull' [-Wframe-larger-than]
> warning: stack frame size (2352) exceeds limit (2048) in 'dml32_ModeSupportAndSystemConfigurationFull' [-Wframe-larger-than]
>
> allmodconfig + CONFIG_KASAN=n + CONFIG_KCSAN=y + CONFIG_UBSAN=n:
>
> warning: stack frame size (2504) exceeds limit (2048) in 'dml30_ModeSupportAndSystemConfigurationFull' [-Wframe-larger-than]
> warning: stack frame size (2600) exceeds limit (2048) in 'dml31_ModeSupportAndSystemConfigurationFull' [-Wframe-larger-than]
> warning: stack frame size (2264) exceeds limit (2048) in 'dml32_ModeSupportAndSystemConfigurationFull' [-Wframe-larger-than]
>
> allmodconfig + CONFIG_KASAN=n + CONFIG_KCSAN=n + CONFIG_UBSAN=n:
>
> warning: stack frame size (2072) exceeds limit (2048) in 'dml31_ModeSupportAndSystemConfigurationFull' [-Wframe-larger-than]
>
> There might be other debugging configurations that make this worse too,
> as I don't see those warnings on my distribution configuration.
>
>> The dml30_ModeSupportAndSystemConfigurationFull() function exercises
>> a few paths in the compiler that are otherwise rare. On thing it does is to
>> pass up to 60 arguments to other functions, and it heavily uses float and
>> double variables. Both of these make it rather fragile when it comes to
>> unusual compiler options, so the files keep coming up whenever a new
>> instrumentation feature gets added. There is probably some other flag
>> in allmodconfig that we can disable to improve this again, but I have not
>> checked this time.
>
> I do notice that these files build with a non-configurable
> -Wframe-large-than value:
>
> $ rg frame_warn_flag drivers/gpu/drm/amd/display/dc/dml/Makefile
> 54:frame_warn_flag := -Wframe-larger-than=2048
Tbh, I was looking at the history and I can't find a good reason this
was added. It should be safe to drop this. I would much rather use
the CONFIG_FRAME_WARN value than override it.
AFAIK most builds use 2048 by default anyways.
> 70:CFLAGS_$(AMDDALPATH)/dc/dml/dcn30/display_mode_vba_30.o := $(dml_ccflags) $(frame_warn_flag)
> 72:CFLAGS_$(AMDDALPATH)/dc/dml/dcn31/display_mode_vba_31.o := $(dml_ccflags) $(frame_warn_flag)
> 76:CFLAGS_$(AMDDALPATH)/dc/dml/dcn32/display_mode_vba_32.o := $(dml_ccflags) $(frame_warn_flag)
>
> I suppose that could just be bumped as a quick workaround? Two of those
> files have a comment that implies modifying them in non-trivial ways is
> not recommended.
>
> /*
> * NOTE:
> * This file is gcc-parsable HW gospel, coming straight from HW engineers.
> *
> * It doesn't adhere to Linux kernel style and sometimes will do things in odd
> * ways. Unless there is something clearly wrong with it the code should
> * remain as-is as it provides us with a guarantee from HW that it is correct.
> */
>
> I do note that commit 1b54a0121dba ("drm/amd/display: Reduce stack size
> in the mode support function") did have a workaround for GCC. It appears
> clang will still inline mode_support_configuration(). If I mark it as
> 'noinline', the warning disappears in that file.
>
That'd be the best quick fix. I guess if we split out functions to fix
stack usage we should mark them as 'noinline' in the future to avoid
agressive compiler optimizations.
Harry
> Cheers,
> Nathan
