[v7,1/2] selftests/resctrl: Adjust effective L3 cache size with SNC enabled

Commit Message

Maciej Wieczor-Retman Dec. 9, 2024, 11:09 a.m. UTC

Sub-NUMA Cluster divides CPUs sharing an L3 cache into separate NUMA
nodes. Systems may support splitting into either two, three, four or six
nodes. When SNC mode is enabled the effective amount of L3 cache
available for allocation is divided by the number of nodes per L3.

It's possible to detect which SNC mode is active by comparing the number
of CPUs that share a cache with CPU0, with the number of CPUs on node0.

Detect SNC mode once and let other tests inherit that information.

Change top_srcdir in the Makefile so fallthrough macro can be used in
the switch statement.

To check if SNC detection is reliable one can check the
/sys/devices/system/cpu/offline file. If it's empty, it means all cores
are operational and the ratio should be calculated correctly. If it has
any contents, it means the detected SNC mode can't be trusted and should
be disabled.

Check if detection was not reliable due to offline cpus. If it was skip
running tests since the results couldn't be trusted.

Co-developed-by: Tony Luck <[e-mail]>
Signed-off-by: Tony Luck <[e-mail]>
Signed-off-by: Maciej Wieczor-Retman <[e-mail]>
---
Changelog v7:
- Check return value when opening empty cpus file.
- Add protection against overflow when reading string into buffer from
  empty cpus file.
- Include fallthrough macro into resctrl selftests and use it in the bit
  counting function.
- Remove function usage suggestion from comment.

Changelog v6:
- Redo snc_nodes_per_l3_cache() by removing old logic and taking into
  account existance of cpus_offline_empty().
- Remove MAX_SNC since now we just divide cache cpus by node cpus
  instead of trying to estimate the ratio.
- Skip tests if snc_unreliable was set and get_vendor() returns
  ARCH_INTEL.
- Move extern int snc_unreliable; from the next patch to this one since
  it's needed here already.
- Set snc_unreliable if node or cache cpus cannot be counted for some
  reason.
- Redo patch message after code changes.

Changelog v5:
- Set snc_mode to 1 at the start of if(!snc_mode) block.
- Move all resctrlfs.c code from patch 2/2 to this one. (Reinette)
- Fix Co-developed-by tag.
- Update SNC discovery comments for the case where it gets disabled by
  being unreliable.
- Remove exclamation mark from ksft_perror() and add full file path
  instead of "offline CPUs file".
- bit map -> bitmap.
- Remove unnecessary empty line.

Changelog v4:
- Make returned value a static local variable so the function only runs
  the logic once. (Reinette)

Changelog v3:
- Add comparison between present and online cpus to test if the
  calculated SNC mode is credible. (Reinette)
- Added comment to cache size modification to better explain why it is
  needed there. (Reinette)
- Fix facts in patch message. (Reinette)
- Change snc_ways() to snc_nodes_per_l3_cache(). (Reinette)

 tools/testing/selftests/resctrl/Makefile      |   3 +-
 tools/testing/selftests/resctrl/resctrl.h     |   5 +
 .../testing/selftests/resctrl/resctrl_tests.c |   9 +-
 tools/testing/selftests/resctrl/resctrlfs.c   | 105 ++++++++++++++++++
 4 files changed, 120 insertions(+), 2 deletions(-)

Comments

Reinette Chatre Dec. 9, 2024, 7:18 p.m. UTC | #1

Hi Maciej,

On 12/9/24 3:09 AM, Maciej Wieczor-Retman wrote:
> Sub-NUMA Cluster divides CPUs sharing an L3 cache into separate NUMA
> nodes. Systems may support splitting into either two, three, four or six
> nodes. When SNC mode is enabled the effective amount of L3 cache
> available for allocation is divided by the number of nodes per L3.
> 
> It's possible to detect which SNC mode is active by comparing the number
> of CPUs that share a cache with CPU0, with the number of CPUs on node0.
> 
> Detect SNC mode once and let other tests inherit that information.
> 
> Change top_srcdir in the Makefile so fallthrough macro can be used in
> the switch statement.
> 
> To check if SNC detection is reliable one can check the
> /sys/devices/system/cpu/offline file. If it's empty, it means all cores
> are operational and the ratio should be calculated correctly. If it has
> any contents, it means the detected SNC mode can't be trusted and should
> be disabled.
> 
> Check if detection was not reliable due to offline cpus. If it was skip
> running tests since the results couldn't be trusted.
> 
> Co-developed-by: Tony Luck <[e-mail]>
> Signed-off-by: Tony Luck <[e-mail]>
> Signed-off-by: Maciej Wieczor-Retman <[e-mail]>
> ---

...

> diff --git a/tools/testing/selftests/resctrl/Makefile b/tools/testing/selftests/resctrl/Makefile
> index f408bd6bfc3d..6b03bab6fc20 100644
> --- a/tools/testing/selftests/resctrl/Makefile
> +++ b/tools/testing/selftests/resctrl/Makefile
> @@ -1,6 +1,7 @@
>  # SPDX-License-Identifier: GPL-2.0
> +top_srcdir = ../../../..

top_srcdir is already defined in lib.mk, it should not be necessary to
redefine it.

>  
> -CFLAGS = -g -Wall -O2 -D_FORTIFY_SOURCE=2
> +CFLAGS = -g -Wall -O2 -D_FORTIFY_SOURCE=2 -I$(top_srcdir)/tools/include

It is not clear to me why my suggestion to set this after including lib.mk
was ignored. I am thinking about a change like below but looking at the other
selftest Makefiles I do see very few actually modify CFLAGS after including
lib.mk. Closest one I could find to this problem was
tools/testing/selftests/vDSO/Makefile. Do you see an issue with modifying CFLAGS
after including lib.mk?

diff --git a/tools/testing/selftests/resctrl/Makefile b/tools/testing/selftests/resctrl/Makefile
index 6b03bab6fc20..984534cfbf1b 100644
--- a/tools/testing/selftests/resctrl/Makefile
+++ b/tools/testing/selftests/resctrl/Makefile
@@ -1,7 +1,6 @@
 # SPDX-License-Identifier: GPL-2.0
-top_srcdir = ../../../..
 
-CFLAGS = -g -Wall -O2 -D_FORTIFY_SOURCE=2 -I$(top_srcdir)/tools/include
+CFLAGS = -g -Wall -O2 -D_FORTIFY_SOURCE=2
 CFLAGS += $(KHDR_INCLUDES)
 
 TEST_GEN_PROGS := resctrl_tests
@@ -9,5 +8,6 @@ TEST_GEN_PROGS := resctrl_tests
 LOCAL_HDRS += $(wildcard *.h)
 
 include ../lib.mk
+CFLAGS += -I$(top_srcdir)/tools/include
 
 $(OUTPUT)/resctrl_tests: $(wildcard *.c)

>  CFLAGS += $(KHDR_INCLUDES)
>  
>  TEST_GEN_PROGS := resctrl_tests

Rest of the patch looks good to me.

Reinette

Maciej Wieczor-Retman Dec. 16, 2024, 2:23 p.m. UTC | #2

Hello!

On 2024-12-09 at 11:18:37 -0800, Reinette Chatre wrote:
>Hi Maciej,
>
>On 12/9/24 3:09 AM, Maciej Wieczor-Retman wrote:
>> Sub-NUMA Cluster divides CPUs sharing an L3 cache into separate NUMA
>> nodes. Systems may support splitting into either two, three, four or six
>> nodes. When SNC mode is enabled the effective amount of L3 cache
>> available for allocation is divided by the number of nodes per L3.
>> 
>> It's possible to detect which SNC mode is active by comparing the number
>> of CPUs that share a cache with CPU0, with the number of CPUs on node0.
>> 
>> Detect SNC mode once and let other tests inherit that information.
>> 
>> Change top_srcdir in the Makefile so fallthrough macro can be used in
>> the switch statement.
>> 
>> To check if SNC detection is reliable one can check the
>> /sys/devices/system/cpu/offline file. If it's empty, it means all cores
>> are operational and the ratio should be calculated correctly. If it has
>> any contents, it means the detected SNC mode can't be trusted and should
>> be disabled.
>> 
>> Check if detection was not reliable due to offline cpus. If it was skip
>> running tests since the results couldn't be trusted.
>> 
>> Co-developed-by: Tony Luck <[e-mail]>
>> Signed-off-by: Tony Luck <[e-mail]>
>> Signed-off-by: Maciej Wieczor-Retman <[e-mail]>
>> ---
>
>...
>
>> diff --git a/tools/testing/selftests/resctrl/Makefile b/tools/testing/selftests/resctrl/Makefile
>> index f408bd6bfc3d..6b03bab6fc20 100644
>> --- a/tools/testing/selftests/resctrl/Makefile
>> +++ b/tools/testing/selftests/resctrl/Makefile
>> @@ -1,6 +1,7 @@
>>  # SPDX-License-Identifier: GPL-2.0
>> +top_srcdir = ../../../..
>
>top_srcdir is already defined in lib.mk, it should not be necessary to
>redefine it.
>
>>  
>> -CFLAGS = -g -Wall -O2 -D_FORTIFY_SOURCE=2
>> +CFLAGS = -g -Wall -O2 -D_FORTIFY_SOURCE=2 -I$(top_srcdir)/tools/include
>
>It is not clear to me why my suggestion to set this after including lib.mk
>was ignored.

I'm sorry it looked like it, I read your suggestion and then I looked at
tools/testing/selftests/rseq/Makefile which does exactly what I did (I looked
for selftests that use the fallthrough attribute and it was the one with the
simplest Makefile).

>I am thinking about a change like below but looking at the other
>selftest Makefiles I do see very few actually modify CFLAGS after including
>lib.mk. Closest one I could find to this problem was
>tools/testing/selftests/vDSO/Makefile. Do you see an issue with modifying CFLAGS
>after including lib.mk?

It looks good, it compiles fine and neither I nor my neovim's LSP can find any
issues with it.

>
>diff --git a/tools/testing/selftests/resctrl/Makefile b/tools/testing/selftests/resctrl/Makefile
>index 6b03bab6fc20..984534cfbf1b 100644
>--- a/tools/testing/selftests/resctrl/Makefile
>+++ b/tools/testing/selftests/resctrl/Makefile
>@@ -1,7 +1,6 @@
> # SPDX-License-Identifier: GPL-2.0
>-top_srcdir = ../../../..
> 
>-CFLAGS = -g -Wall -O2 -D_FORTIFY_SOURCE=2 -I$(top_srcdir)/tools/include
>+CFLAGS = -g -Wall -O2 -D_FORTIFY_SOURCE=2
> CFLAGS += $(KHDR_INCLUDES)
> 
> TEST_GEN_PROGS := resctrl_tests
>@@ -9,5 +8,6 @@ TEST_GEN_PROGS := resctrl_tests
> LOCAL_HDRS += $(wildcard *.h)
> 
> include ../lib.mk
>+CFLAGS += -I$(top_srcdir)/tools/include
> 
> $(OUTPUT)/resctrl_tests: $(wildcard *.c)
>
>>  CFLAGS += $(KHDR_INCLUDES)
>>  
>>  TEST_GEN_PROGS := resctrl_tests
>
>Rest of the patch looks good to me.

Thanks, I'll resend the corrected version today :)

>
>Reinette
>

Patch

diff --git a/tools/testing/selftests/resctrl/Makefile b/tools/testing/selftests/resctrl/Makefile
index f408bd6bfc3d..6b03bab6fc20 100644
--- a/tools/testing/selftests/resctrl/Makefile
+++ b/tools/testing/selftests/resctrl/Makefile
@@ -1,6 +1,7 @@ 
 # SPDX-License-Identifier: GPL-2.0
+top_srcdir = ../../../..
 
-CFLAGS = -g -Wall -O2 -D_FORTIFY_SOURCE=2
+CFLAGS = -g -Wall -O2 -D_FORTIFY_SOURCE=2 -I$(top_srcdir)/tools/include
 CFLAGS += $(KHDR_INCLUDES)
 
 TEST_GEN_PROGS := resctrl_tests
diff --git a/tools/testing/selftests/resctrl/resctrl.h b/tools/testing/selftests/resctrl/resctrl.h
index dab1953fc7a0..35fa3afee9c3 100644
--- a/tools/testing/selftests/resctrl/resctrl.h
+++ b/tools/testing/selftests/resctrl/resctrl.h
@@ -11,6 +11,7 @@ 
 #include <signal.h>
 #include <dirent.h>
 #include <stdbool.h>
+#include <ctype.h>
 #include <sys/stat.h>
 #include <sys/ioctl.h>
 #include <sys/mount.h>
@@ -21,6 +22,7 @@ 
 #include <sys/eventfd.h>
 #include <asm/unistd.h>
 #include <linux/perf_event.h>
+#include <linux/compiler.h>
 #include "../kselftest.h"
 
 #define MB			(1024 * 1024)
@@ -156,8 +158,11 @@  struct perf_event_read {
  */
 extern volatile int *value_sink;
 
+extern int snc_unreliable;
+
 extern char llc_occup_path[1024];
 
+int snc_nodes_per_l3_cache(void);
 int get_vendor(void);
 bool check_resctrlfs_support(void);
 int filter_dmesg(void);
diff --git a/tools/testing/selftests/resctrl/resctrl_tests.c b/tools/testing/selftests/resctrl/resctrl_tests.c
index 3335af815b21..5154ffd821c4 100644
--- a/tools/testing/selftests/resctrl/resctrl_tests.c
+++ b/tools/testing/selftests/resctrl/resctrl_tests.c
@@ -118,7 +118,7 @@  static bool test_vendor_specific_check(const struct resctrl_test *test)
 
 static void run_single_test(const struct resctrl_test *test, const struct user_params *uparams)
 {
-	int ret;
+	int ret, snc_mode;
 
 	if (test->disabled)
 		return;
@@ -128,8 +128,15 @@  static void run_single_test(const struct resctrl_test *test, const struct user_p
 		return;
 	}
 
+	snc_mode = snc_nodes_per_l3_cache();
+
 	ksft_print_msg("Starting %s test ...\n", test->name);
 
+	if (snc_mode == 1 && snc_unreliable && get_vendor() == ARCH_INTEL) {
+		ksft_test_result_skip("SNC detection unreliable due to offline CPUs. Test results may not be accurate if SNC enabled.\n");
+		return;
+	}
+
 	if (test_prepare(test)) {
 		ksft_exit_fail_msg("Abnormal failure when preparing for the test\n");
 		return;
diff --git a/tools/testing/selftests/resctrl/resctrlfs.c b/tools/testing/selftests/resctrl/resctrlfs.c
index d38d6dd90be4..dc7ce3cbdb27 100644
--- a/tools/testing/selftests/resctrl/resctrlfs.c
+++ b/tools/testing/selftests/resctrl/resctrlfs.c
@@ -13,6 +13,8 @@ 
 
 #include "resctrl.h"
 
+int snc_unreliable;
+
 static int find_resctrl_mount(char *buffer)
 {
 	FILE *mounts;
@@ -156,6 +158,98 @@  int get_domain_id(const char *resource, int cpu_no, int *domain_id)
 	return 0;
 }
 
+/*
+ * Count number of CPUs in a /sys bitmap
+ */
+static unsigned int count_sys_bitmap_bits(char *name)
+{
+	FILE *fp = fopen(name, "r");
+	int count = 0, c;
+
+	if (!fp)
+		return 0;
+
+	while ((c = fgetc(fp)) != EOF) {
+		if (!isxdigit(c))
+			continue;
+		switch (c) {
+		case 'f':
+			count++;
+			fallthrough;
+		case '7': case 'b': case 'd': case 'e':
+			count++;
+			fallthrough;
+		case '3': case '5': case '6': case '9': case 'a': case 'c':
+			count++;
+			fallthrough;
+		case '1': case '2': case '4': case '8':
+			count++;
+			break;
+		}
+	}
+	fclose(fp);
+
+	return count;
+}
+
+static bool cpus_offline_empty(void)
+{
+	char offline_cpus_str[64];
+	FILE *fp;
+
+	fp = fopen("/sys/devices/system/cpu/offline", "r");
+	if (!fp) {
+		ksft_perror("Could not open /sys/devices/system/cpu/offline");
+		return 0;
+	}
+
+	if (fscanf(fp, "%63s", offline_cpus_str) < 0) {
+		if (!errno) {
+			fclose(fp);
+			return 1;
+		}
+		ksft_perror("Could not read /sys/devices/system/cpu/offline");
+	}
+
+	fclose(fp);
+
+	return 0;
+}
+
+/*
+ * Detect SNC by comparing #CPUs in node0 with #CPUs sharing LLC with CPU0.
+ * If any CPUs are offline declare the detection as unreliable.
+ */
+int snc_nodes_per_l3_cache(void)
+{
+	int node_cpus, cache_cpus;
+	static int snc_mode;
+
+	if (!snc_mode) {
+		snc_mode = 1;
+		if (!cpus_offline_empty()) {
+			ksft_print_msg("Runtime SNC detection unreliable due to offline CPUs.\n");
+			ksft_print_msg("Setting SNC mode to disabled.\n");
+			snc_unreliable = 1;
+			return snc_mode;
+		}
+		node_cpus = count_sys_bitmap_bits("/sys/devices/system/node/node0/cpumap");
+		cache_cpus = count_sys_bitmap_bits("/sys/devices/system/cpu/cpu0/cache/index3/shared_cpu_map");
+
+		if (!node_cpus || !cache_cpus) {
+			ksft_print_msg("Could not determine Sub-NUMA Cluster mode.\n");
+			snc_unreliable = 1;
+			return snc_mode;
+		}
+		snc_mode = cache_cpus / node_cpus;
+
+		if (snc_mode > 1)
+			ksft_print_msg("SNC-%d mode discovered.\n", snc_mode);
+	}
+
+	return snc_mode;
+}
+
 /*
  * get_cache_size - Get cache size for a specified CPU
  * @cpu_no:	CPU number
@@ -211,6 +305,17 @@  int get_cache_size(int cpu_no, const char *cache_type, unsigned long *cache_size
 			break;
 	}
 
+	/*
+	 * The amount of cache represented by each bit in the masks
+	 * in the schemata file is reduced by a factor equal to SNC
+	 * nodes per L3 cache.
+	 * E.g. on a SNC-2 system with a 100MB L3 cache a test that
+	 * allocates memory from its local SNC node (default behavior
+	 * without using libnuma) will only see 50 MB llc_occupancy
+	 * with a fully populated L3 mask in the schemata file.
+	 */
+	if (cache_num == 3)
+		*cache_size /= snc_nodes_per_l3_cache();
 	return 0;
 }