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Sender: Guenter Roeck <groeck7@gmail.com>
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Date: Sat, 16 Mar 2024 09:16:56 -0700
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Subject: Re: [PATCH 00/14] Add support for suppressing warning backtraces
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From: Guenter Roeck <linux@roeck-us.net>
To: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: linux-kselftest@vger.kernel.org, David Airlie <airlied@gmail.com>,
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On 3/14/24 07:37, Guenter Roeck wrote:
> On 3/14/24 06:36, Geert Uytterhoeven wrote:
>> Hi Günter,
>>
>> On Tue, Mar 12, 2024 at 6:03 PM Guenter Roeck <linux@roeck-us.net> 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.
>>
>> Thanks for your series!
>>
>> I gave it a try on m68k, just running backtrace-suppression-test,
>> and that seems to work fine.
>>
>>> Design note:
>>>    Function pointers are only added to the __bug_table section if both
>>>    CONFIG_KUNIT and CONFIG_DEBUG_BUGVERBOSE are enabled to avoid image
>>>    size increases if CONFIG_KUNIT=n. 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.
>>
>> Unfortunately this also increases kernel size in the CONFIG_KUNIT=m
>> case (ca. 80 KiB for atari_defconfig), making it less attractive to have
>> kunit and all tests enabled as modules in my standard kernel.
>>
> 
> Good point. Indeed, it does. I wanted to avoid adding a configuration option,
> but maybe I should add it after all. How about something like this ?
> 
> +config KUNIT_SUPPRESS_BACKTRACE
> +       bool "KUnit - Enable backtrace suppression"
> +       default y
> +       help
> +         Enable backtrace suppression for KUnit. If enabled, backtraces
> +         generated intentionally by KUnit tests can be suppressed. Disable
> +         to reduce kernel image size if image size is more important than
> +         suppression of backtraces generated by KUnit tests.
> +
> 

Any more comments / feedback on this ? Otherwise I'll introduce the
above configuration option in v2 of the series.

In this context, any suggestions if it should be enabled or disabled by
default ? I personally think it would be more important to be able to
suppress backtraces, but I understand that others may not be willing to
accept a ~1% image size increase with CONFIG_KUNIT=m unless
KUNIT_SUPPRESS_BACKTRACE is explicitly disabled.

Thanks,
Guenter