selftests/resctrl: Replace file write with volatile variable

The fill_buf code prevents compiler optimizating the entire read loop
away by writing the final value of the variable into a file. While it
achieves the goal, writing into a file requires significant amount of
work within the innermost test loop and also error handling.

A simpler approach is to take advantage of volatile. Writing through
a pointer to a volatile variable is enough to prevent compiler from
optimizing the write away, and therefore compiler cannot remove the
read loop either.

Add a volatile 'value_sink' into resctrl_tests.c and make fill_buf to
write into it. As a result, the error handling in fill_buf.c can be
simplified.

Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Reviewed-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

1 files changed, 7 insertions, 0 deletions

diff --git a/tools/testing/selftests/resctrl/resctrl.h b/tools/testing/selftests/resctrl/resctrl.h
index f22b7897251e..5b9bea505120 100644
--- a/tools/testing/selftests/resctrl/resctrl.h
+++ b/tools/testing/selftests/resctrl/resctrl.h
@@ -78,6 +78,13 @@ struct perf_event_read {
 #define CMT_STR			"cmt"
 #define CAT_STR			"cat"
 
+/*
+ * Memory location that consumes values compiler must not optimize away.
+ * Volatile ensures writes to this location cannot be optimized away by
+ * compiler.
+ */
+extern volatile int *value_sink;
+
 extern pid_t bm_pid, ppid;
 
 extern char llc_occup_path[1024];