On Thu, Sep 24, 2020 at 10:42 AM Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx> wrote: > > ----- On Sep 24, 2020, at 1:09 PM, 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, 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@xxxxxxxxxxx> > > --- > > 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
