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
>
