Re: [PATCH 1/3 v2] tracepoints: Add helper to test if tracepoint is enabled in a header

[Axel Rasmussen]

On Fri, Sep 25, 2020 at 2:18 PM Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
>
> From: "Steven Rostedt (VMware)" <rostedt@xxxxxxxxxxx>
>
> As tracepoints are discouraged from being added in a header because it can
> cause side effects if other tracepoints are in headers, as well as bloat the
> kernel as the trace_<tracepoint>() function is not a small inline, 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 this overhead, a static_branch can be used to only
> have the tracepoint wrapper get called when the tracepoint is enabled.
>
> 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@xxxxxxxxxxx>
> ---
>  Documentation/trace/tracepoints.rst | 27 +++++++++++++++++++++++
>  include/linux/tracepoint-defs.h     | 34 +++++++++++++++++++++++++++++
>  2 files changed, 61 insertions(+)
>
> diff --git a/Documentation/trace/tracepoints.rst b/Documentation/trace/tracepoints.rst
> index 6e3ce3bf3593..68579ebd1e4c 100644
> --- a/Documentation/trace/tracepoints.rst
> +++ b/Documentation/trace/tracepoints.rst
> @@ -146,3 +146,30 @@ 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, as
> +well as the trace_<tracepoint>() is not that small of an inline
> +and can bloat the kernel if used by other inlined functions. Instead,
> +include tracepoint-defs.h and use tracepoint_enabled().
> +
> +In a C file::
> +
> +       void do_trace_foo_bar_wrapper(args)
> +       {
> +               trace_foo_bar(args);
> +       }
> +
> +In the header file::
> +
> +       DECLEARE_TRACEPOINT(foo_bar);

Should be "DECLARE_..."

> +
> +       static inline void some_inline_function()
> +       {
> +               [..]
> +               if (tracepoint_enabled(foo_bar))
> +                       do_trace_foo_bar_wrapper(args);
> +               [..]
> +       }
> diff --git a/include/linux/tracepoint-defs.h b/include/linux/tracepoint-defs.h
> index b29950a19205..60625973faaf 100644
> --- a/include/linux/tracepoint-defs.h
> +++ b/include/linux/tracepoint-defs.h
> @@ -48,4 +48,38 @@ 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 and bloat the kernel. Instead, use
> + * tracepoint_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_bar()", you would need to create a wrapper function
> + * in a C file to call trace_foo_bar():
> + *   void do_trace_foo_bar(args) { trace_foo_bar(args); }
> + * Then in the header file, declare the tracepoint:
> + *   DECLARE_TRACEPOINT(foo_bar);
> + * And call your wrapper:
> + *   static inline void some_inlined_function() {
> + *            [..]
> + *            if (tracepoint_enabled(foo_bar))
> + *                    do_trace_foo_bar(args);
> + *            [..]
> + *   }
> + *
> + * Note: tracepoint_enabled(foo_bar) is equivalent to trace_foo_bar_enabled()
> + *   but is safe to have in headers, where trace_foo_bar_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
>
>