Re: Using tracing_off() in __schedule_bug()

[Chase Douglas <chase.douglas@canonical.com>]

On Fri, Mar 12, 2010 at 6:34 PM, Steven Rostedt <rostedt@goodmis.org> wrote:
> On Fri, 2010-03-12 at 10:32 -0500, Chase Douglas wrote:
>> We're considering using tracing_off() in __schedule_bug() in the
>> Ubuntu kernel to help us solve "scheduling while atomic" bugs without
>> needing to compile and install a special kernel to do so. Using the
>> tracing_off() call enables us to generate smaller function traces when
>> debugging, and find the bug easier since it should be at the end of
>> any traces. The patch we are thinking of using can be found at [1].
>> You can find an overview of how bugs are solved using this approach
>> there as well.
>>
>> Are there any thoughts as to the appropriateness of this patch in
>> Ubuntu kernels? My guess is that this isn't done in the upstream
>> vanilla kernels because it would be annoying to have a stock kernel
>> littered with tracing_off() calls if you hit one when you weren't
>> meaning to. However, we're interested in hearing if there's some
>> negative consequence we haven't thought of.
>
> Actually, it has been on my TODO list to litter tracing_off() all over
> the kernel ;-)
>
> No, by all means, I would definitely take a patch that turn off tracing
> at critical areas. One of which I would like to do is with the BUG()
> call.

Sounds good to me. The only place I currently see tracing_off called
is in oops_enter.

> Hmm, thinking about it more, I would rather have a separate function,
> that would call tracing_off() if some variable is set. By default it
> would be set, but in case you want to keep tracing after a bug is hit,
> you can have a way to disable it.
>
> I need to write up a patch soon. Thanks for bring this up.

I'd be happy to help out in this endeavor if you'd like. I'm wondering
if there shouldn't be multiple levels of tracing_off support specified
at boot time (disabled on WARNING, BUG, __schedule_bug, OOPS) in an
ordered priority way. I.e. tracing_off_bug would leave tracing on for
WARNING's, but turn it off for BUG's, schedule bugs, and oopses. The
default would be tracing_off_warn, which would call tracing_off on all
of the above.

Your thoughts?

-- Chase