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");