Re: [PATCH 0/3 v2] tracing/mm: Add tracepoint_enabled() helper function for headers

Mathieu Desnoyers
2020-09-25T22:23:38Z

No worries, I'll get it from lore.

Thanks,

Mathieu

----- Steven Rostedt <rostedt@goodmis.org> wrote:
>
> Bah, My cut-and-paste of my "quilt mail --send" chopped off Mathieu's email.
>
> Mathieu, I didn't meant to not Cc you on this. Do you need me to bounce
> the rest to you or you can get it from lore?
>
> -- Steve
>
>
> On Fri, 25 Sep 2020 17:12:06 -0400
> Steven Rostedt <rostedt@goodmis.org> wrote:
>
> > Tracepoints are not safe to be called directly from header files as they may
> > be included by C code that has CREATE_TRACE_POINTS defined, and this would
> > cause side effects and possibly break the build in hard to debug ways. Not
> > to mention it also will bloat the code being in commonly used inline
> > functions.
> >
> > Instead, it is recommended to call a tracepoint helper function that is
> > defined in a C file that calls the tracepoint. But we would only want this
> > function to be called if the tracepoint is enabled, as function calls add
> > overhead.
> >
> > The trace_<tracepoint>_enabled() function is also not safe to be called in a
> > header file as it is created by the tracepoint header, which suffers the
> > same fate if CREATE_TRACE_POINTS is defined. Instead, the tracepoint needs
> > to be declared as an extern, and the helper function can test the static key
> > to call the helper function that calls the tracepoint.
> >
> > This has been done by open coding the tracepoint extern and calling the
> > static key directly:
> >
> > commit 95813b8faa0cd ("mm/page_ref: add tracepoint to track down page reference manipulation")
> > commit 7f47d8cc039f ("x86, tracing, perf: Add trace point for MSR accesses")
> >
> > does this (back in 2015). Now we have another use case, so a helper function
> > should be created to keep the internals of the tracepoints from being spread
> > out in other subsystems.
> >
> > Link: https://lore.kernel.org/r/20200922125113.12ef1e03@gandalf.local.home
> >
> > This adds tracepoint_enabled() helper macro and DECLARE_TRACEPOINT() macro
> > to allow this to be done without exposing the internals of the tracepoints.
> >
> > The first patch adds the infrastructure, the second converts page_ref over
> > to it, and third converts over msr.h.
> >
> > Steven Rostedt (VMware) (3):
> > tracepoints: Add helper to test if tracepoint is enabled in a header
> > mm/page_ref: Convert the open coded tracepoint enabled to the new helper
> > x86: Use tracepoint_enabled() for msr tracepoints instead of open coding it
> >
> > ----
> >
> > Changes since v1 (https://lore.kernel.org/r/20200924170928.466191266@goodmis.org):
> >
> > - Fixed using "trace_enabled()" instead of "tracepoint_enabled()"
> > (Mathieu Desnoyers reported)
> >
> > - Reworded to include comments about bloating the kernel when tracepoints
> > are used in commonly used inlined functions.
> >
> > - Added the msr update as well.
> >
> >
> > Documentation/trace/tracepoints.rst | 27 ++++++++++++++++++++++++
> > arch/x86/include/asm/msr.h | 18 +++++++---------
> > include/linux/page_ref.h | 42 ++++++++++++++++++-------------------
> > include/linux/tracepoint-defs.h | 34 ++++++++++++++++++++++++++++++
> > 4 files changed, 90 insertions(+), 31 deletions(-)
>

--
Mathieu Desnoyers
EfficiOS Inc.