----- On Sep 24, 2020, at 2:19 PM, Axel Rasmussen axelrasmussen@xxxxxxxxxx wrote: > 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). If it's not too much trouble, it would be useful to know what headers included from tracepoint.h are problematic. When I originally wrote tracepoint.h, I made sure to include the minimum set needed, but I suspect some feature-creep may have ended up including additional headers which are not strictly needed for the core instrumentation. Once we identify those, we can then make sure we split tracepoint.h into a "core instrumentation" header (tracepoint.h) and non-core stuff (e.g. tracepoint-utils.h or better name). Another possible approach is to make sure that whatever is included by the tracepoint.h "core" is split into *.h and *-{defs,types}.h, for cases where all we need from tracepoint.h is type declarations. Thanks, Mathieu > > > >> >> > + >> > +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 -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com
