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Date: Fri, 3 May 2024 16:11:11 +0100
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Subject: Re: [PATCH] selftests: cgroup: add tests to verify the zswap
 writeback path
To: Nhat Pham <nphamcs@gmail.com>
Cc: akpm@linux-foundation.org, hannes@cmpxchg.org, yosryahmed@google.com,
 chengming.zhou@linux.dev, linux-mm@kvack.org, linux-kernel@vger.kernel.org,
 kernel-team@meta.com
References: <20240501100446.1454264-1-usamaarif642@gmail.com>
 <CAKEwX=NKMOJQ2KuiBcs-62NzqAhRDOvEhWvAzjsAbUtg=65fvA@mail.gmail.com>
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From: Usama Arif <usamaarif642@gmail.com>
In-Reply-To: <CAKEwX=P3CzkrkB8iQ5-BvgaFko03hUULtz=8z-qe_d3LZ7Ge2Q@mail.gmail.com>
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On 03/05/2024 00:30, Nhat Pham wrote:
> On Thu, May 2, 2024 at 12:05 PM Usama Arif <usamaarif642@gmail.com> wrote:
>>
>> On 01/05/2024 16:44, Nhat Pham wrote:
>>> On Wed, May 1, 2024 at 3:04 AM Usama Arif <usamaarif642@gmail.com> 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@gmail.com>
>>>> ---
>>>>    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.
> Hmm would pgsteal be a good candidate for this purpose?
> Just throwing out ideas - I'll leave this up to you to decide :)

So I think what the equation would be is pgsteal = zswpout + number of 
pages that couldnt be compressed and directly moved to swap? We have 
pgsteal and zswpout, but I cant see from existing metrics if we can get 
the 3rd thing.

As a reference these are some of the metrics at the end of writeback 
enabled test from v2:

zswpin 1024
zswpout 1801
zswpwb 297

pgsteal 2057

zswap 0
zswapped 0

We need a metric that is 2057-1801 = 256, but can't find any :)