[PATCH 1/2] selftests/cgroup: test_zswap: retry subtree_control write on EBUSY in test_zswap_writeback

When running test_zswap on a single-core VM (-smp 1) with 4GB of RAM,
test_zswap_writeback reliably 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, waitpid() reaps the zombie PID immediately,
but the removal of struct task_struct from the cgroup task list is
performed asynchronously via an RCU callback (release_task). On
single-core systems, this RCU callback is delayed behind CPU softirqs,
causing "+memory" to fail if written immediately after waitpid() returns.

Fix this by adding an EBUSY retry loop with usleep(1000) around cg_write(),
matching the existing cgroup cleanup pattern in cg_destroy().

Signed-off-by: Wilson Felipe Pereira <[email protected]>
---
 tools/testing/selftests/cgroup/test_zswap.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/tools/testing/selftests/cgroup/test_zswap.c b/tools/testing/selftests/cgroup/test_zswap.c
index 49b36ee791606..312d51887a1d8 100644
--- a/tools/testing/selftests/cgroup/test_zswap.c
+++ b/tools/testing/selftests/cgroup/test_zswap.c
@@ -407,8 +407,11 @@ 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_write(test_group, "cgroup.subtree_control", "+memory"))
-		goto out;
+	while (cg_write(test_group, "cgroup.subtree_control", "+memory")) {
+		if (errno != EBUSY)
+			goto out;
+		usleep(1000);
+	}
 
 	test_group_child = cg_name(test_group, "zswap_writeback_test_child");
 	if (!test_group_child)
-- 
2.55.0.571.g244d577d93-goog