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 <[email protected]> --- 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 49b36ee79160..a7ff525c1267 100644 --- a/tools/testing/selftests/cgroup/test_zswap.c +++ b/tools/testing/selftests/cgroup/test_zswap.c @@ -407,6 +407,8 @@ 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; -- 2.55.0.766.g2966f0265a-goog
