When reading memory in order, HW prefetching optimizations will interfere with measuring how caches and memory are being accessed. This adds noise into the results.
Change the fill_buf reading loop to not use an obvious in-order access using multiply by a prime and modulo.
Using a prime multiplier with modulo ensures the entire buffer is eventually read. 23 is small enough that the reads are spread out but wrapping does not occur very frequently (wrapping too often can trigger L2 hits more frequently which causes noise to the test because getting the data from LLC is not required).
It was discovered that not all primes work equally well and some can cause wildly unstable results (e.g., in an earlier version of this patch, the reads were done in reversed order and 59 was used as the prime resulting in unacceptably high and unstable results in MBA and MBM test on some architectures).
Link: https://lore.kernel.org/linux-kselftest/TYAPR01MB6330025B5E6537F94DA49ACB8B4... Signed-off-by: Ilpo Järvinen ilpo.jarvinen@linux.intel.com --- tools/testing/selftests/resctrl/fill_buf.c | 38 +++++++++++++++++----- 1 file changed, 30 insertions(+), 8 deletions(-)
diff --git a/tools/testing/selftests/resctrl/fill_buf.c b/tools/testing/selftests/resctrl/fill_buf.c index 9d0b0bf4b85a..326d530425d0 100644 --- a/tools/testing/selftests/resctrl/fill_buf.c +++ b/tools/testing/selftests/resctrl/fill_buf.c @@ -51,16 +51,38 @@ static void mem_flush(unsigned char *buf, size_t buf_size) sb(); }
+/* + * Buffer index step advance to workaround HW prefetching interfering with + * the measurements. + * + * Must be a prime to step through all indexes of the buffer. + * + * Some primes work better than others on some architectures (from MBA/MBM + * result stability point of view). + */ +#define FILL_IDX_MULT 23 + static int fill_one_span_read(unsigned char *buf, size_t buf_size) { - unsigned char *end_ptr = buf + buf_size; - unsigned char sum, *p; - - sum = 0; - p = buf; - while (p < end_ptr) { - sum += *p; - p += (CL_SIZE / 2); + unsigned int size = buf_size / (CL_SIZE / 2); + unsigned int i, idx = 0; + unsigned char sum = 0; + + /* + * Read the buffer in an order that is unexpected by HW prefetching + * optimizations to prevent them interfering with the caching pattern. + * + * The read order is (in terms of halves of cachelines): + * i * FILL_IDX_MULT % size + * The formula is open-coded below to avoiding modulo inside the loop + * as it improves MBA/MBM result stability on some architectures. + */ + for (i = 0; i < size; i++) { + sum += buf[idx * (CL_SIZE / 2)]; + + idx += FILL_IDX_MULT; + while (idx >= size) + idx -= size; }
return sum;