Add tracepoint_enabled() macro

Allows application to check if given probe is enabled.
It's useful when data it passes to tracepoint is expensive to compute.

Signed-off-by: Dmitri Shubin <dmitri.shubin@orc-group.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>

diff --git a/doc/man/lttng-ust.3 b/doc/man/lttng-ust.3
index 6f8763b9d2dcaa2f9b19cb12358de256f1d49dcd..7dae8b538d8d4161e57f54bee9e5a872815e6f02 100644 (file)
--- a/doc/man/lttng-ust.3
+++ b/doc/man/lttng-ust.3
@@ -290,6 +290,33 @@ Even though LTTng-UST supports tracepoint() call site duplicates having
 the same provider and event name, it is recommended to use a
 provider event name pair only once within the source code to help
 map events back to their call sites when analyzing the trace.
 the same provider and event name, it is recommended to use a
 provider event name pair only once within the source code to help
 map events back to their call sites when analyzing the trace.

+
+Sometimes arguments to the probe are expensive to compute (e.g.
+take call stack). To avoid the computation when the tracepoint is
+disabled one can use more 'low level' tracepoint_enabled() and
+do_tracepoint() macros as following:
+
+       if (tracepoint_enabled(ust_tests_hello, tptest)) {
+               /* prepare arguments */
+               do_tracepoint(ust_tests_hello, tptest, i, netint, values,
+                       text, strlen(text), dbl, flt);
+       }
+
+Here do_tracepoint() doesn't contain check if the tracepoint is enabled.
+Using tracepoint() in such scenario is dangerous since it also contains
+enabled check and thus race condition is possible in the following code
+if the tracepoint has been enabled after check in tracepoint_enabled()
+but before tracepoint():
+
+       if (tracepoint_enabled(provider, name)) { /* tracepoint is disabled */
+               prepare(args);
+       }
+       /* tracepoint is enabled by 'lttng' tool */
+       tracepoint(provider, name, args); /* args wasn't prepared properly */
+
+Note also that neither tracepoint_enabled() nor do_tracepoint() have
+STAP_PROBEV() call so if you need it you should emit this call yourself.
+

 .fi
 
 .SH "BUILDING/LINKING THE TRACEPOINT PROVIDER"
 .fi
 
 .SH "BUILDING/LINKING THE TRACEPOINT PROVIDER"

diff --git a/include/lttng/tracepoint.h b/include/lttng/tracepoint.h
index 63759a20ee3da5f7a7e0beb6f08219c8567abe55..1734c1b7ac8d6c0a5aaedcc9c5e9c4f3db8effc4 100644 (file)
--- a/include/lttng/tracepoint.h
+++ b/include/lttng/tracepoint.h
@@ -44,11 +44,17 @@
 extern "C" {
 #endif
 
 extern "C" {
 #endif
 

+#define tracepoint_enabled(provider, name) \
+       caa_unlikely(__tracepoint_##provider##___##name.state)
+
+#define do_tracepoint(provider, name, ...) \
+       __tracepoint_cb_##provider##___##name(__VA_ARGS__)
+

 #define tracepoint(provider, name, ...)                                            \
        do {                                                                \
                STAP_PROBEV(provider, name, ## __VA_ARGS__);                \
 #define tracepoint(provider, name, ...)                                            \
        do {                                                                \
                STAP_PROBEV(provider, name, ## __VA_ARGS__);                \

-               if (caa_unlikely(__tracepoint_##provider##___##name.state)) \
-                       __tracepoint_cb_##provider##___##name(__VA_ARGS__); \
+               if (tracepoint_enabled(provider, name))                     \
+                       do_tracepoint(provider, name, __VA_ARGS__);         \

        } while (0)
 
 #define TP_ARGS(...)       __VA_ARGS__
        } while (0)
 
 #define TP_ARGS(...)       __VA_ARGS__