[PATCH v3 20/29] selftests/resctrl: Replace file write with volatile variable

Ilpo Järvinen
2023-12-11T12:21:40Z

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>
---

v3:
- Altered changelog's wording to not say "a volatile pointer"
---
tools/testing/selftests/resctrl/fill_buf.c | 26 ++++---------------
tools/testing/selftests/resctrl/resctrl.h | 7 +++++
.../testing/selftests/resctrl/resctrl_tests.c | 4 +++
3 files changed, 16 insertions(+), 21 deletions(-)

diff --git a/tools/testing/selftests/resctrl/fill_buf.c b/tools/testing/selftests/resctrl/fill_buf.c
index 635f938b11f0..8fe9574db9d8 100644
--- a/tools/testing/selftests/resctrl/fill_buf.c
+++ b/tools/testing/selftests/resctrl/fill_buf.c
@@ -78,10 +78,9 @@ static void fill_one_span_write(unsigned char *buf, size_t buf_size)
}
}

-static int fill_cache_read(unsigned char *buf, size_t buf_size, bool once)
+static void fill_cache_read(unsigned char *buf, size_t buf_size, bool once)
{
int ret = 0;
- FILE *fp;

while (1) {
ret = fill_one_span_read(buf, buf_size);
@@ -90,26 +89,16 @@ static int fill_cache_read(unsigned char *buf, size_t buf_size, bool once)
}

/* Consume read result so that reading memory is not optimized out. */
- fp = fopen("/dev/null", "w");
- if (!fp) {
- ksft_perror("Unable to write to /dev/null");
- return -1;
- }
- fprintf(fp, "Sum: %d ", ret);
- fclose(fp);
-
- return 0;
+ *value_sink = ret;
}

-static int fill_cache_write(unsigned char *buf, size_t buf_size, bool once)
+static void fill_cache_write(unsigned char *buf, size_t buf_size, bool once)
{
while (1) {
fill_one_span_write(buf, buf_size);
if (once)
break;
}
-
- return 0;
}

static unsigned char *alloc_buffer(size_t buf_size, int memflush)
@@ -143,21 +132,16 @@ static unsigned char *alloc_buffer(size_t buf_size, int memflush)
int run_fill_buf(size_t buf_size, int memflush, int op, bool once)
{
unsigned char *buf;
- int ret;

buf = alloc_buffer(buf_size, memflush);
if (!buf)
return -1;

if (op == 0)
- ret = fill_cache_read(buf, buf_size, once);
+ fill_cache_read(buf, buf_size, once);
else
- ret = fill_cache_write(buf, buf_size, once);
+ fill_cache_write(buf, buf_size, once);
free(buf);
- if (ret) {
- printf("\n Error in fill cache\n");
- return -1;
- }

return 0;
}
diff --git a/tools/testing/selftests/resctrl/resctrl.h b/tools/testing/selftests/resctrl/resctrl.h
index 419669502959..4e1f7a4bbaf5 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];
diff --git a/tools/testing/selftests/resctrl/resctrl_tests.c b/tools/testing/selftests/resctrl/resctrl_tests.c
index 2bbe3045a018..f0d573aa363b 100644
--- a/tools/testing/selftests/resctrl/resctrl_tests.c
+++ b/tools/testing/selftests/resctrl/resctrl_tests.c
@@ -10,6 +10,10 @@
*/
#include "resctrl.h"

+/* Volatile memory sink to prevent compiler optimizations */
+static volatile int sink_target;
+volatile int *value_sink = &sink_target;
+
static int detect_vendor(void)
{
FILE *inf = fopen("/proc/cpuinfo", "r");
--
2.30.2