Try to prevent users from enable something which will really thrash their performance

doc/doxygen/chapters/510_configure_options.doxy

@@ -505,7 +505,8 @@ Enable performance model debugging.
 <dd>
 \anchor enable-fxt-lock
 \addindex __configure__--enable-fxt-lock
-Enable additional trace events which describes locks behaviour.
+Enable additional trace events which describes locks behaviour. This is however
+extremely heavy and should only be enabled when debugging insides of StarPU.
 </dd>
 
 <dt>--enable-maxbuffers</dt>

src/core/workers.c

@@ -1282,6 +1282,9 @@ int starpu_initialize(struct starpu_conf *user_conf, int *argc, char ***argv)
 #ifdef STARPU_USE_FXT
 	_STARPU_DISP("Warning: StarPU was configured with --with-fxt, which slows down a bit, limits scalability and makes worker initialization sequential\n");
 #endif
+#ifdef STARPU_USE_FXT_LOCK_TRACSE
+	_STARPU_DISP("Warning: StarPU was configured with --enable-fxt-lock, which slows down things a huge lot, and is really only meant for StarPU insides debugging. Did you really want to enable that?\n");
+#endif
 #ifdef STARPU_PERF_DEBUG
 	_STARPU_DISP("Warning: StarPU was configured with --enable-perf-debug, which slows down a bit\n");
 #endif