[PATCH v6 1/3] selftests/cgroup: test_zswap: wait for cgroup to unpopulate in test_zswap_writeback

When running test_zswap on a single-core VM (-smp 1) with 4GB of RAM,
test_zswap_writeback intermittently fails on the initial run after boot.

In test_zswap_writeback(), after waitpid() reaps the child process created
by test_zswap_writeback_one(), writing "+memory" to cgroup.subtree_control
can fail with -EBUSY. Under cgroup v2, enabling domain subtree controllers
is forbidden while any tasks remain in cgroup.procs.

When a child process exits, exit_notify() wakes the parent process,
allowing waitpid() to return immediately. However, the cgroup populated
task count (nr_populated_csets) is only decremented when the exiting
task is switched away via finish_task_switch() -> cgroup_task_dead(). On
single-core systems, the parent runs before the dead child has been
switched out, causing "+memory" to fail with -EBUSY if written immediately
after waitpid() returns.

Fix this by waiting for cgroup.events to report "populated 0\n" via
cg_read_strcmp_wait() before enabling subtree control.

Signed-off-by: Wilson Felipe Pereira <redacted>
Acked-by: Michal Koutný <mkoutny@suse.com>
---
 tools/testing/selftests/cgroup/test_zswap.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/tools/testing/selftests/cgroup/test_zswap.c b/tools/testing/selftests/cgroup/test_zswap.c
index 609c48f38524..8f2c9aa4776c 100644
--- a/tools/testing/selftests/cgroup/test_zswap.c
+++ b/tools/testing/selftests/cgroup/test_zswap.c

@@ -408,6 +408,8 @@ static int test_zswap_writeback(const char *root, bool wb)
 	 * Thus, the parent's setting shall be what's in effect. */
 	if (cg_write(test_group, "memory.zswap.max", "max"))
 		goto out;
+	if (cg_read_strcmp_wait(test_group, "cgroup.events", "populated 0\n"))
+		goto out;
 	if (cg_write(test_group, "cgroup.subtree_control", "+memory"))
 		goto out;
 

-- 
2.55.0.979.g7e5102b832-goog