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

Steven Rostedt
2020-09-25T21:26:45Z

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(-)