linux-kernel - Re: [PATCH 0/3 v2] tracing/mm: Add tracepoint

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Date:   Fri, 25 Sep 2020 17:26:40 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     linux-kernel@...r.kernel.org
Cc:     Yafang Shao <laoar.shao@...il.com>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Michel Lespinasse <walken@...gle.com>,
        Daniel Jordan <daniel.m.jordan@...cle.com>,
        Davidlohr Bueso <dbueso@...e.de>,
        Linux MM <linux-mm@...ck.org>, Ingo Molnar <mingo@...nel.org>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Subject: Re: [PATCH 0/3 v2] tracing/mm: Add tracepoint_enabled() helper
 function for headers


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@...dmis.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(-)