Add tracelog documentation to lttng-ust(3)

[lttng-ust.git] / doc / man / lttng-ust.3

diff --git a/doc/man/lttng-ust.3 b/doc/man/lttng-ust.3
index 6f8763b9d2dcaa2f9b19cb12358de256f1d49dcd..f57d7f880926fe6481248a16b731e5ad0d5cc28d 100644 (file)
--- a/doc/man/lttng-ust.3
+++ b/doc/man/lttng-ust.3
@@ -48,6 +48,40 @@ whereas tracepoint.h is meant for thorough instrumentation of a code
 base to be integrated with an upstream project.
 .PP
 
 base to be integrated with an upstream project.
 .PP
 

+.SH "USAGE WITH TRACELOG"
+.PP
+If you want to migrate existing logging (info, errors, ...)
+to LTTng UST, you can use the tracelog() interface.
+To do it, in a nutshell:
+
+1) #include <lttng/tracelog.h>
+
+2) /* in your code, use like a printf, with extra loglevel info. */
+   tracelog(info, "Message with integer %d", 1234);
+
+3) Link your program against liblttng-ust.so.
+
+4) Enable UST events when tracing with the following sequence of commands
+   from lttng-tools:
+
+   lttng create
+   lttng enable-event -u "lttng_ust_tracelog:*"
+   lttng start
+   [... run your program ...]
+   lttng stop
+   lttng view
+
+That's it!
+
+You can replace the enable-event line above with a selection of
+loglevels, e.g.:
+
+   lttng enable-event -u -a --loglevel INFO
+
+Which will gather all events from INFO and more important loglevels.
+
+.PP
+

 .SH "USAGE WITH TRACEPOINT"
 .PP
 The simple way to generate the lttng-ust tracepoint probes is to use the
 .SH "USAGE WITH TRACEPOINT"
 .PP
 The simple way to generate the lttng-ust tracepoint probes is to use the


@@ -290,6 +324,33 @@ 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"