Re: mainline build failure for x86_64 allmodconfig with clang

[Nathan Chancellor <nathan@kernel.org>]

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@linux-foundation.org> wrote:
> >
> > On Thu, Aug 4, 2022 at 11:37 AM Sudip Mukherjee (Codethink)
> > <sudipm.mukherjee@gmail.com> 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
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.

Cheers,
Nathan