[LTP] [PATCH v1] memcontrol03: Bind to single CPU on systems with 64KB pagesize

[Wei Gao]

On multi-CPU systems with 64KB page size (e.g. ppc64le), memcontrol03
intermittently fails with over-reclaim of protected pagecache under
trunk_B:

  memcontrol03.c:104: TINFO: Child 142113 in trunk_G: Allocating anon: 150994944
  memcontrol03.c:121: TPASS: Child 142113 exited
  memcontrol03.c:214: TPASS: Expect: (A/B memory.current=54394880) ~= 52428800
  ...
  memcontrol03.c:104: TINFO: Child 142114 in trunk_G: Allocating anon: 174063616
  memcontrol03.c:116: TPASS: Child 142114 killed by OOM
  memcontrol03.c:230: TFAIL: Expect: (A/B memory.current=43319296) ~= 52428800

In the kernel, per-CPU charge batching (MEMCG_CHARGE_BATCH = 64 pages)
caches up to 64 * 64KB = 4MB per CPU in memcg_stock. On a multi-CPU
system, allocating memory across CPUs allows cached stock charges to
accumulate in ancestor trunk_A (up to 4MB * nr_cpus, e.g. 16MB on a
4-CPU machine). Because trunk_A has memory.max = 200M, this extra stock
pressure forces the reclaimer to evict pagecache in trunk_B down to
~35-43MB instead of the expected ~50MB. Simply enlarging the tolerance
rate further is not good idea.

Bind the test process to a single CPU in setup() when page size is
>= 64KB. This confines stock batching to a single CPU, preventing
multi-CPU stock inflation and ensuring reproducible reclaim behavior.

Signed-off-by: Wei Gao <wegao@suse.com>
---
 .../kernel/controllers/memcg/memcontrol03.c    | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/testcases/kernel/controllers/memcg/memcontrol03.c b/testcases/kernel/controllers/memcg/memcontrol03.c
index 96224c4ee..6007d5588 100644
--- a/testcases/kernel/controllers/memcg/memcontrol03.c
+++ b/testcases/kernel/controllers/memcg/memcontrol03.c
@@ -235,6 +235,23 @@ static void test_memcg_min(void)
 	SAFE_UNLINK(TMPDIR"/tmpfile");
 }
 
+static void setup(void)
+{
+	if (getpagesize() >= 64 * 1024) {
+		cpu_set_t mask;
+		int cpu = sched_getcpu();
+
+		if (cpu < 0)
+			tst_brk(TBROK | TERRNO, "sched_getcpu() failed");
+
+		tst_res(TINFO, "Page size >= 64KB, binding to CPU %d", cpu);
+		CPU_ZERO(&mask);
+		CPU_SET(cpu, &mask);
+		if (sched_setaffinity(0, sizeof(mask), &mask))
+			tst_brk(TBROK | TERRNO, "sched_setaffinity() failed");
+	}
+}
+
 static void cleanup(void)
 {
 	cleanup_sub_groups();
@@ -243,6 +260,7 @@ static void cleanup(void)
 }
 
 static struct tst_test test = {
+	.setup = setup,
 	.cleanup = cleanup,
 	.test_all = test_memcg_min,
 	.mount_device = 1,
-- 
2.55.0