Re: [PATCH 16/24] selftests/resctrl: Rewrite Cache Allocation Technology (CAT) test

[Maciej Wieczór-Retman]

On 2023-10-24 at 12:26:26 +0300, Ilpo Järvinen wrote:
>CAT test spawns two processes into two different control groups with
>exclusive schemata. Both the processes alloc a buffer from memory
>matching their allocated LLC block size and flush the entire buffer out
>of caches. Since the processes are reading through the buffer only once
>during the measurement and initially all the buffer was flushed, the
>test isn't testing CAT.
>
>Rewrite the CAT test to allocate a buffer sized to half of LLC. Then
>perform a sequence of tests with different LLC alloc sizes starting
>from half of the CBM bits down to 1-bit CBM. Flush the buffer before
>each test and read the buffer twice. Observe the LLC misses on the
>second read through the buffer. As the allocated LLC block gets smaller
>and smaller, the LLC misses will become larger and larger giving a
>strong signal on CAT working properly.
>
>The new CAT test is using only a single process because it relies on
>measured effect against another run of itself rather than another
>process adding noise. The rest of the system is allocated the CBM bits
>not used by the CAT test to keep the test isolated.
>
>Replace count_bits() with count_contiguous_bits() to get the first bit
>position in order to be able to calculate masks based on it.
>
>This change has been tested with a number of systems from different
>generations.
>
>Suggested-by: Reinette Chatre <reinette.chatre@xxxxxxxxx>
>Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@xxxxxxxxxxxxxxx>
>---
> tools/testing/selftests/resctrl/cat_test.c  | 286 +++++++++-----------
> tools/testing/selftests/resctrl/fill_buf.c  |   6 +-
> tools/testing/selftests/resctrl/resctrl.h   |   5 +-
> tools/testing/selftests/resctrl/resctrlfs.c |  44 +--
> 4 files changed, 137 insertions(+), 204 deletions(-)
>
>diff --git a/tools/testing/selftests/resctrl/cat_test.c b/tools/testing/selftests/resctrl/cat_test.c
>index e71690a9bbb3..7518c520c5cc 100644
>--- a/tools/testing/selftests/resctrl/cat_test.c
>+++ b/tools/testing/selftests/resctrl/cat_test.c
>@@ -11,65 +11,68 @@
> #include "resctrl.h"
> #include <unistd.h>
> 
>-#define RESULT_FILE_NAME1	"result_cat1"
>-#define RESULT_FILE_NAME2	"result_cat2"
>+#define RESULT_FILE_NAME	"result_cat"
> #define NUM_OF_RUNS		5
>-#define MAX_DIFF_PERCENT	4
>-#define MAX_DIFF		1000000
> 
> /*
>- * Change schemata. Write schemata to specified
>- * con_mon grp, mon_grp in resctrl FS.
>- * Run 5 times in order to get average values.
>+ * Minimum difference in LLC misses between a test with n+1 bits CBM mask to
>+ * the test with n bits. With e.g. 5 vs 4 bits in the CBM mask, the minimum
>+ * difference must be at least MIN_DIFF_PERCENT_PER_BIT * (4 - 1) = 3 percent.
>+ *
>+ * The relationship between number of used CBM bits and difference in LLC
>+ * misses is not expected to be linear. With a small number of bits, the
>+ * margin is smaller than with larger number of bits. For selftest purposes,
>+ * however, linear approach is enough because ultimately only pass/fail
>+ * decision has to be made and distinction between strong and stronger
>+ * signal is irrelevant.
>  */
>-static int cat_setup(struct resctrl_val_param *p)
>-{
>-	char schemata[64];
>-	int ret = 0;
>-
>-	/* Run NUM_OF_RUNS times */
>-	if (p->num_of_runs >= NUM_OF_RUNS)
>-		return END_OF_TESTS;
>-
>-	if (p->num_of_runs == 0) {
>-		sprintf(schemata, "%lx", p->mask);
>-		ret = write_schemata(p->ctrlgrp, schemata, p->cpu_no,
>-				     p->resctrl_val);
>-	}
>-	p->num_of_runs++;
>-
>-	return ret;
>-}
>+#define MIN_DIFF_PERCENT_PER_BIT	1
> 
> static int show_results_info(__u64 sum_llc_val, int no_of_bits,
>-			     unsigned long cache_span, unsigned long max_diff,
>-			     unsigned long max_diff_percent, unsigned long num_of_runs,
>-			     bool platform)
>+			     unsigned long cache_span, long min_diff_percent,
>+			     unsigned long num_of_runs, bool platform,
>+			     __s64 *prev_avg_llc_val)
> {
> 	__u64 avg_llc_val = 0;
>-	float diff_percent;
>-	int ret;
>+	float avg_diff;
>+	int ret = 0;
> 
> 	avg_llc_val = sum_llc_val / num_of_runs;
>-	diff_percent = ((float)cache_span - avg_llc_val) / cache_span * 100;
>+	if (*prev_avg_llc_val) {
>+		float delta = (__s64)(avg_llc_val - *prev_avg_llc_val);
> 
>-	ret = platform && abs((int)diff_percent) > max_diff_percent;
>+		avg_diff = delta / *prev_avg_llc_val;
>+		ret = platform && (avg_diff * 100) < (float)min_diff_percent;
> 
>-	ksft_print_msg("%s Check cache miss rate within %lu%%\n",
>-		       ret ? "Fail:" : "Pass:", max_diff_percent);
>+		ksft_print_msg("%s Check cache miss rate changed more than %.1f%%\n",
>+			       ret ? "Fail:" : "Pass:", (float)min_diff_percent);

Shouldn't "Fail" and "Pass" be flipped in the ternary operator? Or the condition
sign above "<" should be ">"?

Now it looks like if (avg_diff * 100) is smaller than the min_diff_percent the
test is supposed to fail but the text suggests it's the other way around.

I also ran this selftest and that's the output:

# Pass: Check cache miss rate changed more than 3.0%
# Percent diff=45.8
# Number of bits: 4
# Average LLC val: 322489
# Cache span (lines): 294912
# Pass: Check cache miss rate changed more than 2.0%
# Percent diff=38.0
# Number of bits: 3
# Average LLC val: 445005
# Cache span (lines): 221184
# Pass: Check cache miss rate changed more than 1.0%
# Percent diff=27.2
# Number of bits: 2
# Average LLC val: 566145
# Cache span (lines): 147456
# Pass: Check cache miss rate changed more than 0.0%
# Percent diff=18.3
# Number of bits: 1
# Average LLC val: 669657
# Cache span (lines): 73728
ok 1 CAT: test

The diff percentages are much larger than the thresholds they're supposed to
be within and the test is passed.

>-	ksft_print_msg("Percent diff=%d\n", abs((int)diff_percent));
>+		ksft_print_msg("Percent diff=%.1f\n", avg_diff * 100);
>+	}
>+	*prev_avg_llc_val = avg_llc_val;
> 
> 	show_cache_info(no_of_bits, avg_llc_val, cache_span, true);
> 
> 	return ret;
> }
> 
>@@ -143,54 +168,64 @@ static int cat_test(struct resctrl_val_param *param, size_t span)
> 	if (ret)
> 		return ret;
> 
>+	buf = alloc_buffer(span, 1);
>+	if (buf == NULL)

Similiar to patch 01/24, wouldn't this:
	if (!buf)
be better?

>+		return -1;
>+

-- 
Kind regards
Maciej Wieczór-Retman