Re: [LTP] getrusage03: account for percpu RSS counter batching

Re: [LTP] getrusage03: account for percpu RSS counter batching

[linuxtestproject.agent]

Hi Nirmoy,

On Aug 25, 2026, Nirmoy Das wrote:
> getrusage03: account for percpu RSS counter batching

> +	long batch = MAX(32L, tst_ncpus() * 2);
> +	long delta = MAXRSS_BATCHES * batch * SAFE_SYSCONF(_SC_PAGESIZE) / 1024;

Could this allowance also scale by the number of online CPUs? An mm's
anonymous RSS counter can retain up to batch - 1 pages on every CPU visited
by the child, and task migration does not flush the previous CPU's local
counter. Linux's own error bound in __percpu_counter_compare() is
batch * num_online_cpus().

For example, with 64 online CPUs and 4 KiB pages, the batch is 128 pages.
This code computes 1 MiB and therefore retains the old 20 MiB floor, while
partial batches on 41 CPUs can already omit more than 20 MiB. The failure
this patch intends to address therefore remains possible.

Verdict - Needs revision

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Re: [LTP] getrusage03: account for percpu RSS counter batching

[linuxtestproject.agent]

Hi Nirmoy,

On Sep 2, 2026, Nirmoy Das wrote:
> getrusage03: account for percpu RSS counter batching

Verdict - Reviewed

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findings are genuine. If you find issues with the review, please
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[LTP] [PATCH v3] getrusage03: account for percpu RSS counter batching

[Nirmoy Das via ltp]

After f1a7941243c1 ("mm: convert mm's rss stats into percpu_counter"),
the fixed 20 MiB tolerance can be too small on large systems. Pinning
limits pending RSS updates to one CPU, but that CPU can still hold one
batch outside the fast RSS read.

Pin the test to its current CPU and allow one batch of lower-side slack
for the 100, 300 and 400 MiB allocation checks. Keep the existing upper
and inheritance tolerances, and skip if one batch is at least 100 MiB.

On 352 CPUs with 64 KiB pages, one batch is 44 MiB. Pinning alone
reported 270336 KiB for the 300 MiB check, outside the 20 MiB tolerance.
The updated test passed 10/10 runs on the 352-CPU Vera system.

Suggested-by: Cyril Hrubis <chrubis@suse.cz>
Co-developed-by: Jan Stancek <jstancek@redhat.com>
Signed-off-by: Jan Stancek <jstancek@redhat.com>
Assisted-by: ClaudeCode:claude-opus-5-5
Signed-off-by: Nirmoy Das <nirmoyd@nvidia.com>
---
Changes in v3:
- Use tst_ncpus() and pin to the current CPU (Cyril).
- Allocate the CPU mask dynamically and log the allowance.

v2: https://lore.kernel.org/ltp/20260902160113.1207205-1-nirmoyd@nvidia.com/

 .../kernel/syscalls/getrusage/getrusage03.c   | 87 +++++++++++++++----
 1 file changed, 71 insertions(+), 16 deletions(-)

diff --git a/testcases/kernel/syscalls/getrusage/getrusage03.c b/testcases/kernel/syscalls/getrusage/getrusage03.c
index a2cdd6158..6beae130d 100644
--- a/testcases/kernel/syscalls/getrusage/getrusage03.c
+++ b/testcases/kernel/syscalls/getrusage/getrusage03.c
@@ -13,22 +13,88 @@
  * this program.
  */
 
-#include <stdlib.h>
+#define _GNU_SOURCE
 #include <stdio.h>
+#include <stdlib.h>
 
 #include "tst_test.h"
+#include "lapi/cpuset.h"
+#include "lapi/sched.h"
 #include "getrusage03.h"
 
 #define TESTBIN "getrusage03_child"
 
 static struct rusage ru;
 static long maxrss_init;
+static long lower_allowance;
 
 static const char *const resource[] = {
 	TESTBIN,
 	NULL,
 };
 
+/*
+ * Pin the test to the CPU it runs on. Every forked or exec'd child inherits
+ * the affinity, so only that CPU can hold RSS counter updates outside the
+ * fast percpu_counter read.
+ */
+static void pin_to_current_cpu(void)
+{
+	unsigned int cpu;
+	cpu_set_t *set;
+	size_t size;
+
+	if (getcpu(&cpu, NULL))
+		tst_brk(TBROK | TERRNO, "getcpu() failed");
+
+	set = CPU_ALLOC(cpu + 1);
+	if (!set)
+		tst_brk(TBROK | TERRNO, "CPU_ALLOC() failed");
+
+	size = CPU_ALLOC_SIZE(cpu + 1);
+	CPU_ZERO_S(size, set);
+	CPU_SET_S(cpu, size, set);
+	if (sched_setaffinity(0, size, set))
+		tst_brk(TBROK | TERRNO, "sched_setaffinity() failed");
+
+	CPU_FREE(set);
+	tst_res(TINFO, "Pinned to CPU %u", cpu);
+}
+
+/*
+ * The pinned CPU can still hold up to one percpu_counter batch,
+ * max(32, 2 * online CPUs) pages, outside the fast RSS read. Count CPUs
+ * before pinning because some libc implementations count only the affinity
+ * mask.
+ */
+static void setup(void)
+{
+	long online_cpus = tst_ncpus();
+	long batch = MAX(32L, online_cpus * 2);
+	long page_size = SAFE_SYSCONF(_SC_PAGESIZE);
+	long batch_kib = batch * page_size / 1024;
+
+	lower_allowance = MAX(20 * 1024L, batch_kib);
+	if (lower_allowance >= 102400L)
+		tst_brk(TCONF, "Per-CPU RSS allowance is too large: %li KiB",
+			lower_allowance);
+
+	tst_res(TINFO, "%li online CPUs, lower RSS allowance %li KiB",
+		online_cpus, lower_allowance);
+
+	pin_to_current_cpu();
+}
+
+static void check_maxrss(long actual, long expected, const char *name,
+			 const char *size)
+{
+	if (actual >= expected - lower_allowance &&
+	    actual <= expected + DELTA_MAX)
+		tst_res(TPASS, "%s ~= %s", name, size);
+	else
+		tst_res(TFAIL, "%s = %li, expected %li", name, actual, expected);
+}
+
 static void inherit_fork1(void)
 {
 	SAFE_GETRUSAGE(RUSAGE_SELF, &ru);
@@ -51,11 +117,7 @@ static void inherit_fork2(void)
 {
 	SAFE_GETRUSAGE(RUSAGE_CHILDREN, &ru);
 
-	if (is_in_delta(ru.ru_maxrss - 102400))
-		tst_res(TPASS, "initial.children ~= 100MB");
-	else
-		tst_res(TFAIL, "initial.children = %li, expected %i",
-			ru.ru_maxrss, 102400);
+	check_maxrss(ru.ru_maxrss, 102400, "initial.children", "100MB");
 
 	if (!SAFE_FORK()) {
 		SAFE_GETRUSAGE(RUSAGE_CHILDREN, &ru);
@@ -78,11 +140,7 @@ static void grandchild_maxrss(void)
 	tst_reap_children();
 	SAFE_GETRUSAGE(RUSAGE_CHILDREN, &ru);
 
-	if (is_in_delta(ru.ru_maxrss - 307200))
-		tst_res(TPASS, "child.children ~= 300MB");
-	else
-		tst_res(TFAIL, "child.children = %li, expected %i",
-			ru.ru_maxrss, 307200);
+	check_maxrss(ru.ru_maxrss, 307200, "child.children", "300MB");
 }
 
 static void zombie(void)
@@ -106,11 +164,7 @@ static void zombie(void)
 
 	tst_reap_children();
 	SAFE_GETRUSAGE(RUSAGE_CHILDREN, &ru);
-	if (is_in_delta(ru.ru_maxrss - 409600))
-		tst_res(TPASS, "post_wait.children ~= 400MB");
-	else
-		tst_res(TFAIL, "post_wait.children = %li, expected %i",
-			ru.ru_maxrss, 409600);
+	check_maxrss(ru.ru_maxrss, 409600, "post_wait.children", "400MB");
 }
 
 static void sig_ign(void)
@@ -174,6 +228,7 @@ static void run(unsigned int i)
 static struct tst_test test = {
 	.forks_child = 1,
 	.child_needs_reinit = 1,
+	.setup = setup,
 	.resource_files = resource,
 	.min_mem_avail = 512,
 	.tags = (const struct tst_tag[]) {

base-commit: dca9f0d272b2923256852fc37abd222b533d91e0
-- 
2.43.0

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Re: [LTP] getrusage03: account for percpu RSS counter batching

[linuxtestproject.agent]

Hi Nirmoy,

On 2026-10-02, Nirmoy Das wrote:
> getrusage03: account for percpu RSS counter batching

> 	long online_cpus = tst_ncpus();
> 	long batch = MAX(32L, online_cpus * 2);

Medium: On musl, tst_ncpus() counts the calling thread's affinity mask,
so an inherited taskset/cpuset restriction still underestimates the kernel's
global batch even before pinning. With 352 online CPUs, 64 KiB pages and
single-CPU inherited affinity, this chooses 20 MiB rather than 44 MiB and
rejects a valid 270336 KiB result for the 300 MiB allocation. Obtain the
system-wide online count independently of affinity, for example by counting
the CPU ranges in /sys/devices/system/cpu/online.

Verdict - Needs revision

---
Note:

The agent can sometimes produce false positives although often its
findings are genuine. If you find issues with the review, please
comment this email or ignore the suggestions.

Regards,
LTP AI Reviewer

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Mailing list info: https://lists.linux.it/listinfo/ltp