selftests/cgroup: test_zswap: wait for asynchronous writeback

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.

Link: https://lore.kernel.org/20260424040059.12940-9-li.wang@linux.dev
Signed-off-by: Li Wang <li.wang@linux.dev>
Acked-by: Nhat Pham <nphamcs@gmail.com>
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: 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>
Cc: Chengming Zhou <chengming.zhou@linux.dev>
Cc: Jiayuan Chen <jiayuan.chen@linux.dev>
Cc: Waiman Long <longman@redhat.com>
Cc: Yosry Ahmed <yosry@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/tools/testing/selftests/cgroup/test_zswap.c b/tools/testing/selftests/cgroup/test_zswap.c
index 5fe0cffb5575d710742a3b81cf8d9274970474ad..49b36ee791606fa53e3abb60f761de5554059cc6 100644 (file)
--- a/tools/testing/selftests/cgroup/test_zswap.c
+++ b/tools/testing/selftests/cgroup/test_zswap.c
@@ -120,6 +120,27 @@ fail:
        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);