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

[Usama Arif]

On 01/05/2024 16:44, Nhat Pham wrote:
> 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?

Thanks for the review! I have added doc in v2 explaining why random 
memory is used.


> > +
> > +       /* 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 :)

I think with the method in v2, this might not be easily tracked as some 
metrics are all time (zswpout) while others are current.

> > +       }
> > +
> > +       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