Re: [PATCH v2 1/2] selftests/cgroup: test_zswap: wait for cgroup to unpopulate in test_zswap_writeback

[Michal Koutný]

On Mon, Aug 24, 2026 at 03:33:57AM +0000, Wilson Felipe Pereira <wfelipe@xxxxxxxxxx> wrote:
> 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 <wfelipe@xxxxxxxxxx>
> ---
>  tools/testing/selftests/cgroup/test_zswap.c | 2 ++
>  1 file changed, 2 insertions(+)

Acked-by: Michal Koutný <mkoutny@xxxxxxxx>

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