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

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

----- 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.

> +
> +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