[PATCH] selftests/cgroup: Skip test_no_invasive_cgroup_shrink if zswap shrinker is enabled

[Joshua Hahn <joshua.hahnjy@gmail.com>]

The test_no_invasive_cgroup_shrink selftest checks that when a cgroup
has zswapped out more pages than memory.zswap.max, that it does not
trigger writeback for other cgroups. To do this, it compares the
writeback count in a control cgroup and makes sure that it is 0,
and compares it to the writeback count in a aggressor cgroup who
does expect to see writeback.

When the zswap shrinker is enabled however, there is an additional path
to zswap writeback, which can happen independently of reaching the
memory.zswap.max limit.

The writeback counter cannot distinguish between invasive noisy-neighbor
driven writeback and the zswap shrinker work, so the test becomes
invalid.

Skip test_no_invasive_cgroup_shrink if the zswap shrinker is enabled.

Signed-off-by: Joshua Hahn <joshua.hahnjy@gmail.com>
---
 tools/testing/selftests/cgroup/test_zswap.c | 22 +++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/tools/testing/selftests/cgroup/test_zswap.c b/tools/testing/selftests/cgroup/test_zswap.c
index 9c5bd503c3f73..6f9b5c631d030 100644
--- a/tools/testing/selftests/cgroup/test_zswap.c
+++ b/tools/testing/selftests/cgroup/test_zswap.c
@@ -20,6 +20,8 @@ static int page_size;
 
 #define PATH_ZSWAP "/sys/module/zswap"
 #define PATH_ZSWAP_ENABLED "/sys/module/zswap/parameters/enabled"
+#define PATH_ZSWAP_SHRINKER_ENABLED \
+	"/sys/module/zswap/parameters/shrinker_enabled"
 #define PATH_ZSWAP_STORED_PAGES "/sys/kernel/debug/zswap/stored_pages"
 
 static int read_int(const char *path, size_t *value)
@@ -446,6 +448,16 @@ static int test_zswap_writeback_disabled(const char *root)
 	return test_zswap_writeback(root, false);
 }
 
+static bool zswap_shrinker_enabled(void)
+{
+	char value[2];
+
+	if (read_text(PATH_ZSWAP_SHRINKER_ENABLED, value, sizeof(value)) <= 0)
+		return false;
+
+	return value[0] == 'Y';
+}
+
 /*
  * When trying to store a memcg page in zswap, if the memcg hits its memory
  * limit in zswap, writeback should affect only the zswapped pages of that
@@ -461,6 +473,16 @@ static int test_no_invasive_cgroup_shrink(const char *root)
 	char *zw_allocation = NULL, *wb_allocation = NULL;
 	char *zw_group = NULL, *wb_group = NULL;
 
+	/*
+	 * The shrinker can write back zw_group's pages despite not hitting the
+	 * zswap limit. Skip the test if the zswap shrinker is enabled since the
+	 * test cannot distinguish invasive writeback from shrinker work.
+	 */
+	if (zswap_shrinker_enabled()) {
+		ksft_print_msg("zswap shrinker is enabled\n");
+		return KSFT_SKIP;
+	}
+
 	snprintf(zswap_max_buf, sizeof(zswap_max_buf), "%d", page_size);
 	snprintf(mem_max_buf, sizeof(mem_max_buf), "%zu", allocation_size / 2);
 
-- 
2.53.0-Meta