linux-kernel - [PATCH v2 21/24] selftests/resctrl: Read in less obvious order to defeat prefetch optimizations

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Date:   Tue, 18 Apr 2023 14:45:03 +0300
From:   Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
To:     linux-kselftest@...r.kernel.org,
        Reinette Chatre <reinette.chatre@...el.com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Shuah Khan <shuah@...nel.org>, linux-kernel@...r.kernel.org
Cc:     Shaopeng Tan <tan.shaopeng@...fujitsu.com>,
        Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
Subject: [PATCH v2 21/24] selftests/resctrl: Read in less obvious order to defeat prefetch optimizations

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.

Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
---
 tools/testing/selftests/resctrl/fill_buf.c | 17 ++++++++++-------
 1 file changed, 10 insertions(+), 7 deletions(-)

diff --git a/tools/testing/selftests/resctrl/fill_buf.c b/tools/testing/selftests/resctrl/fill_buf.c
index 7e0d3a1ea555..049a520498a9 100644
--- a/tools/testing/selftests/resctrl/fill_buf.c
+++ b/tools/testing/selftests/resctrl/fill_buf.c
@@ -88,14 +88,17 @@ static void *malloc_and_init_memory(size_t s)
 
 static int fill_one_span_read(unsigned char *start_ptr, unsigned char *end_ptr)
 {
-	unsigned char sum, *p;
-
+	unsigned int size = (end_ptr - start_ptr) / (CL_SIZE / 2);
+	unsigned int count = size;
+	unsigned char sum;
+
+	/*
+	 * Read the buffer in an order that is unexpected by HW prefetching
+	 * optimizations to prevent them interfering with the caching pattern.
+	 */
 	sum = 0;
-	p = start_ptr;
-	while (p < end_ptr) {
-		sum += *p;
-		p += (CL_SIZE / 2);
-	}
+	while (count--)
+		sum += start_ptr[((count * 59) % size) * CL_SIZE / 2];
 
 	return sum;
 }
-- 
2.30.2