| Message ID | 20231120111340.7805-18-ilpo.jarvinen@linux.intel.com |
|---|---|
| State | Superseded |
| Headers | show |
| Series | [v2,01/26] selftests/resctrl: Don't use ctrlc_handler() outside signal handling | expand |
Hi Ilpo, On 11/20/2023 3:13 AM, Ilpo Järvinen wrote: > 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 to a > variable through a volatile pointer 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. > The subject and changelog describes the need for a volatile variable. The patch introduces two volatile variables. Could you please elaborate why two volatile variables are needed? Reinette
On Tue, 28 Nov 2023, Reinette Chatre wrote: > On 11/20/2023 3:13 AM, Ilpo Järvinen wrote: > > 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 to a > > variable through a volatile pointer 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. > > > > The subject and changelog describes the need for a volatile variable. > The patch introduces two volatile variables. Could you please elaborate > why two volatile variables are needed? Well, the other "volatile variable" is a pointer to a volatile variable. I've seen gcc to kill a static volatile int so I prefer to not take change with its optimizer. Thus, I placed the sink into different compilation unit and just present a pointer to the actual "volatile" variable. I guess the sink could be marked as plain int instead but this being trickery to begin with I don't see much value either way. It's still a trick. I'll alter the changelog's wording though, "a volatile variable" is not accurate as it's "a pointer to a volatile variable".
diff --git a/tools/testing/selftests/resctrl/fill_buf.c b/tools/testing/selftests/resctrl/fill_buf.c index b9303a9d819b..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) { - 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 fe07b732cbd9..68eb1f1bac3a 100644 --- a/tools/testing/selftests/resctrl/resctrl.h +++ b/tools/testing/selftests/resctrl/resctrl.h @@ -79,6 +79,13 @@ 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");
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 to a variable through a volatile pointer 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> --- 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(-)