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 > >
