[PATCH v7 8/8] selftests/cgroup: test_zswap: wait for asynchronous writeback

Li Wang
2026-04-24T04:02:59Z

zswap writeback is asynchronous, but test_zswap.c checks writeback
counters immediately after reclaim/trigger paths. On some platforms
(e.g. ppc64le), this can race with background writeback and cause
spurious failures even when behavior is correct.

Add wait_for_writeback() to poll get_cg_wb_count() with a bounded
timeout, and use it in:

test_zswap_writeback_one() when writeback is expected
test_no_invasive_cgroup_shrink() for the wb_group check

This keeps the original before/after assertion style while making the
tests robust against writeback completion latency.

No test behavior change, selftest stability improvement only.

Signed-off-by: Li Wang <li.wang@linux.dev>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Michal Koutný <mkoutny@suse.com>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Nhat Pham <nphamcs@gmail.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Yosry Ahmed <yosryahmed@google.com>
Acked-by: Nhat Pham <nphamcs@gmail.com>
---
tools/testing/selftests/cgroup/test_zswap.c | 28 +++++++++++++++++++--
1 file changed, 26 insertions(+), 2 deletions(-)

diff --git a/tools/testing/selftests/cgroup/test_zswap.c b/tools/testing/selftests/cgroup/test_zswap.c
index 5fe0cffb557..49b36ee7916 100644
--- a/tools/testing/selftests/cgroup/test_zswap.c
+++ b/tools/testing/selftests/cgroup/test_zswap.c
@@ -120,6 +120,27 @@ static char *setup_test_group_1M(const char *root, const char *name)
return NULL;
}

+/*
+ * Writeback is asynchronous; poll until at least one writeback has
+ * been recorded for @cg, or until @timeout_ms has elapsed.
+ */
+static long wait_for_writeback(const char *cg, int timeout_ms)
+{
+ long elapsed, count;
+ for (elapsed = 0; elapsed < timeout_ms; elapsed += 100) {
+ count = get_cg_wb_count(cg);
+
+ if (count < 0)
+ return -1;
+ if (count > 0)
+ return count;
+
+ usleep(100000);
+ }
+
+ return 0;
+}
+
/*
* Sanity test to check that pages are written into zswap.
*/
@@ -345,7 +366,10 @@ static int test_zswap_writeback_one(const char *cgroup, bool wb)
return -1;

/* Verify that zswap writeback occurred only if writeback was enabled */
- zswpwb_after = get_cg_wb_count(cgroup);
+ if (wb)
+ zswpwb_after = wait_for_writeback(cgroup, 5000);
+ else
+ zswpwb_after = get_cg_wb_count(cgroup);
if (zswpwb_after < 0)
return -1;

@@ -476,7 +500,7 @@ static int test_no_invasive_cgroup_shrink(const char *root)
}

/* Verify that only zswapped memory from gwb_group has been written back */
- if (get_cg_wb_count(wb_group) > 0 && get_cg_wb_count(zw_group) == 0)
+ if (wait_for_writeback(wb_group, 5000) > 0 && get_cg_wb_count(zw_group) == 0)
ret = KSFT_PASS;
out:
cg_enter_current(root);
--
2.53.0