Re: [PATCH v4 00/14] Add support for suppressing warning backtraces

David Gow
2025-03-29T08:05:57Z

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On Thu, 13 Mar 2025 at 19:44, Alessandro Carminati <acarmina@redhat.com> wrote:
>
> Some unit tests intentionally trigger warning backtraces by passing bad
> parameters to kernel API functions. Such unit tests typically check the
> return value from such calls, not the existence of the warning backtrace.
>
> Such intentionally generated warning backtraces are neither desirable
> nor useful for a number of reasons.
> - They can result in overlooked real problems.
> - A warning that suddenly starts to show up in unit tests needs to be
> investigated and has to be marked to be ignored, for example by
> adjusting filter scripts. Such filters are ad-hoc because there is
> no real standard format for warnings. On top of that, such filter
> scripts would require constant maintenance.
>
> One option to address problem would be to add messages such as "expected
> warning backtraces start / end here" to the kernel log. However, that
> would again require filter scripts, it might result in missing real
> problematic warning backtraces triggered while the test is running, and
> the irrelevant backtrace(s) would still clog the kernel log.
>
> Solve the problem by providing a means to identify and suppress specific
> warning backtraces while executing test code. Support suppressing multiple
> backtraces while at the same time limiting changes to generic code to the
> absolute minimum. Architecture specific changes are kept at minimum by
> retaining function names only if both CONFIG_DEBUG_BUGVERBOSE and
> CONFIG_KUNIT are enabled.
>
> The first patch of the series introduces the necessary infrastructure.
> The second patch introduces support for counting suppressed backtraces.
> This capability is used in patch three to implement unit tests.
> Patch four documents the new API.
> The next two patches add support for suppressing backtraces in drm_rect
> and dev_addr_lists unit tests. These patches are intended to serve as
> examples for the use of the functionality introduced with this series.
> The remaining patches implement the necessary changes for all
> architectures with GENERIC_BUG support.
>
> With CONFIG_KUNIT enabled, image size increase with this series applied is
> approximately 1%. The image size increase (and with it the functionality
> introduced by this series) can be avoided by disabling
> CONFIG_KUNIT_SUPPRESS_BACKTRACE.
>
> This series is based on the RFC patch and subsequent discussion at
> https://patchwork.kernel.org/project/linux-kselftest/patch/02546e59-1afe-4b08-ba81-d94f3b691c9a@moroto.mountain/
> and offers a more comprehensive solution of the problem discussed there.
>
> Design note:
> Function pointers are only added to the __bug_table section if both
> CONFIG_KUNIT_SUPPRESS_BACKTRACE and CONFIG_DEBUG_BUGVERBOSE are enabled
> to avoid image size increases if CONFIG_KUNIT is disabled. There would be
> some benefits to adding those pointers all the time (reduced complexity,
> ability to display function names in BUG/WARNING messages). That change,
> if desired, can be made later.
>
> Checkpatch note:
> Remaining checkpatch errors and warnings were deliberately ignored.
> Some are triggered by matching coding style or by comments interpreted
> as code, others by assembler macros which are disliked by checkpatch.
> Suggestions for improvements are welcome.
>
> Changes since RFC:
> - Introduced CONFIG_KUNIT_SUPPRESS_BACKTRACE
> - Minor cleanups and bug fixes
> - Added support for all affected architectures
> - Added support for counting suppressed warnings
> - Added unit tests using those counters
> - Added patch to suppress warning backtraces in dev_addr_lists tests
>
> Changes since v1:
> - Rebased to v6.9-rc1
> - Added Tested-by:, Acked-by:, and Reviewed-by: tags
> [I retained those tags since there have been no functional changes]
> - Introduced KUNIT_SUPPRESS_BACKTRACE configuration option, enabled by
> default.
>
> Changes since v2:
> - Rebased to v6.9-rc2
> - Added comments to drm warning suppression explaining why it is needed.
> - Added patch to move conditional code in arch/sh/include/asm/bug.h
> to avoid kerneldoc warning
> - Added architecture maintainers to Cc: for architecture specific patches
> - No functional changes
>
> Changes since v3:
> - Rebased to v6.14-rc6
> - Dropped net: "kunit: Suppress lock warning noise at end of dev_addr_lists tests"
> since 3db3b62955cd6d73afde05a17d7e8e106695c3b9
> - Added __kunit_ and KUNIT_ prefixes.
> - Tested on interessed architectures.
>
> ----

Sorry: I also thought this had already landed.

I'm definitely in favour of us taking this, though agree that we
definitely can't afford to break the s390x build.

I've (re-)reviewed the early patches as well, and am generally acking
the series (though some of the architecture-specific patches are
definitely beyond my expertise to review fully).

Acked-by: David Gow <davidgow@google.com>

Cheers,
-- David

> Guenter Roeck (14):
> bug/kunit: Core support for suppressing warning backtraces
> kunit: bug: Count suppressed warning backtraces
> kunit: Add test cases for backtrace warning suppression
> kunit: Add documentation for warning backtrace suppression API
> drm: Suppress intentional warning backtraces in scaling unit tests
> x86: Add support for suppressing warning backtraces
> arm64: Add support for suppressing warning backtraces
> loongarch: Add support for suppressing warning backtraces
> parisc: Add support for suppressing warning backtraces
> s390: Add support for suppressing warning backtraces
> sh: Add support for suppressing warning backtraces
> sh: Move defines needed for suppressing warning backtraces
> riscv: Add support for suppressing warning backtraces
> powerpc: Add support for suppressing warning backtraces
>
> Documentation/dev-tools/kunit/usage.rst | 30 ++++++-
> arch/arm64/include/asm/asm-bug.h | 27 ++++--
> arch/arm64/include/asm/bug.h | 8 +-
> arch/loongarch/include/asm/bug.h | 42 +++++++---
> arch/parisc/include/asm/bug.h | 29 +++++--
> arch/powerpc/include/asm/bug.h | 37 +++++++--
> arch/riscv/include/asm/bug.h | 38 ++++++---
> arch/s390/include/asm/bug.h | 17 +++-
> arch/sh/include/asm/bug.h | 28 ++++++-
> arch/x86/include/asm/bug.h | 21 +++--
> drivers/gpu/drm/tests/drm_rect_test.c | 16 ++++
> include/asm-generic/bug.h | 16 +++-
> include/kunit/bug.h | 56 +++++++++++++
> include/kunit/test.h | 1 +
> include/linux/bug.h | 13 +++
> lib/bug.c | 51 +++++++++++-
> lib/kunit/Kconfig | 9 ++
> lib/kunit/Makefile | 7 +-
> lib/kunit/backtrace-suppression-test.c | 104 ++++++++++++++++++++++++
> lib/kunit/bug.c | 42 ++++++++++
> 20 files changed, 519 insertions(+), 73 deletions(-)
> create mode 100644 include/kunit/bug.h
> create mode 100644 lib/kunit/backtrace-suppression-test.c
> create mode 100644 lib/kunit/bug.c
>
> --
> 2.34.1
>

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