Re: [PATCH 1/2] tracepoints: Add helper to test if tracepoint is enabled in a header

[Axel Rasmussen <axelrasmussen@google.com>]

On Thu, Sep 24, 2020 at 10:42 AM Mathieu Desnoyers
<mathieu.desnoyers@efficios.com> wrote:
>
> ----- On Sep 24, 2020, at 1:09 PM, rostedt rostedt@goodmis.org wrote:
>
> > From: "Steven Rostedt (VMware)" <rostedt@goodmis.org>
> >
> > As tracepoints are discouraged from being added in a header because it can
> > cause side effects if other tracepoints are in headers, the common
> > workaround is to add a function call that calls a wrapper function in a
> > C file that then calls the tracepoint. But as function calls add overhead,
> > this function should only be called when the tracepoint in question is
> > enabled. To get around the overhead, a static_branch can be used that only
> > gets set when the tracepoint is enabled, and then inside the block of the
> > static branch can contain the call to the tracepoint wrapper.
> >
> > Add a tracepoint_enabled(tp) macro that gets passed the name of the
> > tracepoint, and this becomes a static_branch that is enabled when the
> > tracepoint is enabled and is a nop when the tracepoint is disabled.
> >
> > Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
> > ---
> > Documentation/trace/tracepoints.rst | 25 ++++++++++++++++++++++
> > include/linux/tracepoint-defs.h     | 33 +++++++++++++++++++++++++++++
> > 2 files changed, 58 insertions(+)
> >
> > diff --git a/Documentation/trace/tracepoints.rst
> > b/Documentation/trace/tracepoints.rst
> > index 6e3ce3bf3593..833d39ee1c44 100644
> > --- a/Documentation/trace/tracepoints.rst
> > +++ b/Documentation/trace/tracepoints.rst
> > @@ -146,3 +146,28 @@ with jump labels and avoid conditional branches.
> >       define tracepoints. Check http://lwn.net/Articles/379903,
> >       http://lwn.net/Articles/381064 and http://lwn.net/Articles/383362
> >       for a series of articles with more details.
> > +
> > +If you require calling a tracepoint from a header file, it is not
> > +recommended to call one directly or to use the trace_<tracepoint>_enabled()
> > +function call, as tracepoints in header files can have side effects if a
> > +header is included from a file that has CREATE_TRACE_POINTS set. Instead,
> > +include tracepoint-defs.h and use trace_enabled().
>
> Tracepoints per-se have no issues being used from header files. The TRACE_EVENT
> infrastructure seems to be the cause of this problem. We should fix trace events
> rather than require all users to use weird work-arounds thorough the kernel code
> base.
>
> I am not against the idea of a tracepoint_enabled(tp), but I am against the
> motivation behind this patch and the new tracepoint user requirements it documents.

Perhaps anecdotally, I've found that the situation Steven described
occurs not just because of the TRACE_EVENT infrastructure. We also run
into this problem when adding tracepoints under any "very core" APIs,
i.e. anything that is transiently included from linux/tracepoint.h.
For example, I ran into this issue while adding tracepoints under the
linux/mmap_lock.h API, because that header is somehow transiently
included by linux/tracepoint.h (sorry, I don't have the exact
transient include path on hand; I can dig it up if it would be
useful).



>
> > +
> > +In a C file::
> > +
> > +     void do_trace_foo_bar_wrapper(args)
> > +     {
> > +             trace_foo_bar(args);
> > +     }
> > +
> > +In the header file::
> > +
> > +     DECLEARE_TRACEPOINT(foo_bar);
> > +
> > +     static inline void some_inline_function()
> > +     {
> > +             [..]
> > +             if (trace_enabled(foo_bar))
>
> Is it trace_enabled() or tracepoint_enabled() ? There is a mismatch
> between the commit message/code and the documentation.
>
> Thanks,
>
> Mathieu
>
> > +                     do_trace_foo_bar_wrapper(args);
> > +             [..]
> > +     }
> > diff --git a/include/linux/tracepoint-defs.h b/include/linux/tracepoint-defs.h
> > index b29950a19205..ca2f1f77f6f8 100644
> > --- a/include/linux/tracepoint-defs.h
> > +++ b/include/linux/tracepoint-defs.h
> > @@ -48,4 +48,37 @@ struct bpf_raw_event_map {
> >       u32                     writable_size;
> > } __aligned(32);
> >
> > +/*
> > + * If a tracepoint needs to be called from a header file, it is not
> > + * recommended to call it directly, as tracepoints in header files
> > + * may cause side-effects. Instead, use trace_enabled() to test
> > + * if the tracepoint is enabled, then if it is, call a wrapper
> > + * function defined in a C file that will then call the tracepoint.
> > + *
> > + * For "trace_foo()", you would need to create a wrapper function
> > + * in a C file to call trace_foo():
> > + *   void trace_bar(args) { trace_foo(args); }
> > + * Then in the header file, declare the tracepoint:
> > + *   DECLARE_TRACEPOINT(foo);
> > + * And call your wrapper:
> > + *   static inline void some_inlined_function() {
> > + *            [..]
> > + *            if (tracepoint_enabled(foo))
> > + *                    trace_bar(args);
> > + *            [..]
> > + *   }
> > + *
> > + * Note: tracepoint_enabled(foo) is equivalent to trace_foo_enabled()
> > + *   but is safe to have in headers, where trace_foo_enabled() is not.
> > + */
> > +#define DECLARE_TRACEPOINT(tp) \
> > +     extern struct tracepoint __tracepoint_##tp
> > +
> > +#ifdef CONFIG_TRACEPOINTS
> > +# define tracepoint_enabled(tp) \
> > +     static_key_false(&(__tracepoint_##tp).key)
> > +#else
> > +# define tracepoint_enabled(tracepoint) false
> > +#endif
> > +
> > #endif
> > --
> > 2.28.0
>
> --
> Mathieu Desnoyers
> EfficiOS Inc.
> http://www.efficios.com