Re: [PATCH] selftests: cgroup: add tests to verify the zswap writeback path

[Nhat Pham]

On Wed, May 1, 2024 at 3:04 AM Usama Arif <usamaarif642@xxxxxxxxx> wrote:
>
> The condition for writeback can be triggered by allocating random
> memory more than memory.high to push memory into zswap, more than
> zswap.max to trigger writeback if enabled, but less than memory.max
> so that OOM is not triggered. Both values of memory.zswap.writeback
> are tested.

Thanks for adding the test, Usama :) A couple of suggestions below.

>
> Signed-off-by: Usama Arif <usamaarif642@xxxxxxxxx>
> ---
>  tools/testing/selftests/cgroup/test_zswap.c | 83 +++++++++++++++++++++
>  1 file changed, 83 insertions(+)
>
> diff --git a/tools/testing/selftests/cgroup/test_zswap.c b/tools/testing/selftests/cgroup/test_zswap.c
> index f0e488ed90d8..fe0e7221525c 100644
> --- a/tools/testing/selftests/cgroup/test_zswap.c
> +++ b/tools/testing/selftests/cgroup/test_zswap.c
> @@ -94,6 +94,19 @@ static int allocate_bytes(const char *cgroup, void *arg)
>         return 0;
>  }
>
> +static int allocate_random_bytes(const char *cgroup, void *arg)
> +{
> +       size_t size = (size_t)arg;
> +       char *mem = (char *)malloc(size);
> +
> +       if (!mem)
> +               return -1;
> +       for (int i = 0; i < size; i++)
> +               mem[i] = rand() % 128;
> +       free(mem);
> +       return 0;
> +}
> +
>  static char *setup_test_group_1M(const char *root, const char *name)
>  {
>         char *group_name = cg_name(root, name);
> @@ -248,6 +261,74 @@ static int test_zswapin(const char *root)
>         return ret;
>  }
>
> +/* Test to verify the zswap writeback path */
> +static int test_zswap_writeback(const char *root, bool wb)
> +{
> +       int ret = KSFT_FAIL;
> +       char *test_group;
> +       long zswpwb_before, zswpwb_after;
> +
> +       test_group = cg_name(root,
> +               wb ? "zswap_writeback_enabled_test" : "zswap_writeback_disabled_test");
> +       if (!test_group)
> +               goto out;
> +       if (cg_create(test_group))
> +               goto out;
> +       if (cg_write(test_group, "memory.max", "8M"))
> +               goto out;
> +       if (cg_write(test_group, "memory.high", "2M"))
> +               goto out;
> +       if (cg_write(test_group, "memory.zswap.max", "2M"))
> +               goto out;
> +       if (cg_write(test_group, "memory.zswap.writeback", wb ? "1" : "0"))
> +               goto out;
> +
> +       zswpwb_before = cg_read_key_long(test_group, "memory.stat", "zswpwb ");
> +       if (zswpwb_before < 0) {
> +               ksft_print_msg("failed to get zswpwb_before\n");
> +               goto out;
> +       }
> +
> +       /*
> +        * Allocate more than memory.high to push memory into zswap,
> +        * more than zswap.max to trigger writeback if enabled,
> +        * but less than memory.max so that OOM is not triggered
> +        */
> +       if (cg_run(test_group, allocate_random_bytes, (void *)MB(3)))
> +               goto out;

I think we should document better why we allocate random bytes (rather
than just using the existing allocation helper).

This random allocation pattern (rand() % 128) is probably still
compressible by zswap, albeit poorly. I assume this is so that zswap
would not be able to just absorb all the swapped out pages?

> +
> +       /* Verify that zswap writeback occurred only if writeback was enabled */
> +       zswpwb_after = cg_read_key_long(test_group, "memory.stat", "zswpwb ");
> +       if (wb) {
> +               if (zswpwb_after <= zswpwb_before) {
> +                       ksft_print_msg("writeback enabled and zswpwb_after <= zswpwb_before\n");
> +                       goto out;
> +               }
> +       } else {
> +               if (zswpwb_after != zswpwb_before) {
> +                       ksft_print_msg("writeback disabled and zswpwb_after != zswpwb_before\n");
> +                       goto out;
> +               }

It'd be nice if we can check that in this case, the number of pages
that are "swapped out" matches the cgroup's zswpout stats :)


> +       }
> +
> +       ret = KSFT_PASS;
> +
> +out:
> +       cg_destroy(test_group);
> +       free(test_group);
> +       return ret;
> +}
> +
> +static int test_zswap_writeback_enabled(const char *root)
> +{
> +       return test_zswap_writeback(root, true);
> +}
> +
> +static int test_zswap_writeback_disabled(const char *root)
> +{
> +       return test_zswap_writeback(root, false);
> +}
> +
>  /*
>   * When trying to store a memcg page in zswap, if the memcg hits its memory
>   * limit in zswap, writeback should affect only the zswapped pages of that
> @@ -425,6 +506,8 @@ struct zswap_test {
>         T(test_zswap_usage),
>         T(test_swapin_nozswap),
>         T(test_zswapin),
> +       T(test_zswap_writeback_enabled),
> +       T(test_zswap_writeback_disabled),
>         T(test_no_kmem_bypass),
>         T(test_no_invasive_cgroup_shrink),
>  };
> --
> 2.43.0
>