Bugfix: avoid link error in Feather-Trace on x86

If no events are defined but Feater-Trace support is enabled, then the current
implementation generates a link error because the __event_table sections is
absent.

> arch/x86/built-in.o: In function `ft_disable_all_events':
> (.text+0x242af): undefined reference to `__start___event_table'

As a simple work around, we force zero-element array to always be "allocated"
in the __event_table section. This ensures that we end up with a zero-byte
section if no events are enabled, and does not affect the layout of the section
if events are present.

> bbb@ludwig:~/dev/litmus2010$ nm vmlinux | grep event_table
> ffffffff81950cdc D __event_table_dummy
> ffffffff81950cdc A __start___event_table
> ffffffff81950cdc A __stop___event_table

2 files changed, 11 insertions, 0 deletions

diff --git a/arch/x86/kernel/ft_event.c b/arch/x86/kernel/ft_event.c
index 6552e2741ecf..37cc33252713 100644
--- a/arch/x86/kernel/ft_event.c
+++ b/arch/x86/kernel/ft_event.c
@@ -22,6 +22,13 @@ struct trace_event {
 extern struct trace_event  __start___event_table[];
 extern struct trace_event  __stop___event_table[];
 
+/* Workaround: if no events are defined, then the event_table section does not
+ * exist and the above references cause linker errors. This could probably be
+ * fixed by adjusting the linker script, but it is easier to maintain for us if
+ * we simply create a dummy symbol in the event table section.
+ */
+int __event_table_dummy[0] __attribute__ ((section("__event_table")));
+
 int ft_enable_event(unsigned long id)
 {
         struct trace_event* te = __start___event_table;
diff --git a/litmus/Kconfig b/litmus/Kconfig
index f3aa7478faf7..9888589ef126 100644
--- a/litmus/Kconfig
+++ b/litmus/Kconfig
@@ -90,6 +90,10 @@ config FEATHER_TRACE
 
           Bottom line: to avoid increased overheads, choose DEBUG_RODATA=n.
 
+          Note that this option only enables the basic Feather-Trace infrastructure;
+          you still need to enable SCHED_TASK_TRACE and/or SCHED_OVERHEAD_TRACE to
+          actually enable any events.
+
 config SCHED_TASK_TRACE
         bool "Trace real-time tasks"
         depends on FEATHER_TRACE