linux-kernel - Re: [PATCH v2] selftests: cgroup: make test_memcg

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Message-ID: <2fda06e4-19ac-497d-8f42-73972bdd761e@kylinos.cn>
Date: Thu, 20 Nov 2025 14:09:14 +0800
From: Guopeng Zhang <zhangguopeng@...inos.cn>
To: Lance Yang <lance.yang@...ux.dev>
Cc: shuah@...nel.org, mkoutny@...e.com, linux-mm@...ck.org,
 muchun.song@...ux.dev, linux-kselftest@...r.kernel.org,
 shakeel.butt@...ux.dev, linux-kernel@...r.kernel.org, tj@...nel.org,
 hannes@...xchg.org, mhocko@...nel.org, roman.gushchin@...ux.dev
Subject: Re: [PATCH v2] selftests: cgroup: make test_memcg_sock robust against
 delayed sock stats



On 11/20/25 13:40, Lance Yang wrote:
> 
> 
> On 2025/11/20 11:16, Guopeng Zhang wrote:
>> test_memcg_sock() currently requires that memory.stat's "sock " counter
>> is exactly zero immediately after the TCP server exits. On a busy system
>> this assumption is too strict:
>>
>>    - Socket memory may be freed with a small delay (e.g. RCU callbacks).
>>    - memcg statistics are updated asynchronously via the rstat flushing
>>      worker, so the "sock " value in memory.stat can stay non-zero for a
>>      short period of time even after all socket memory has been uncharged.
>>
>> As a result, test_memcg_sock() can intermittently fail even though socket
>> memory accounting is working correctly.
>>
>> Make the test more robust by polling memory.stat for the "sock "
>> counter and allowing it some time to drop to zero instead of checking
>> it only once. The timeout is set to 3 seconds to cover the periodic
>> rstat flush interval (FLUSH_TIME = 2*HZ by default) plus some
>> scheduling slack. If the counter does not become zero within the
>> timeout, the test still fails as before.
>>
>> On my test system, running test_memcontrol 50 times produced:
>>
>>    - Before this patch:  6/50 runs passed.
>>    - After this patch:  50/50 runs passed.
>>
>> Suggested-by: Lance Yang <lance.yang@...ux.dev>
>> Signed-off-by: Guopeng Zhang <zhangguopeng@...inos.cn>
>> ---
>> v2:
>>   - Mention the periodic rstat flush interval (FLUSH_TIME = 2*HZ) in
>>     the comment and clarify the rationale for the 3s timeout.
>>   - Replace the hard-coded retry count and wait interval with macros
>>     to avoid magic numbers and make the 3s timeout calculation explicit.
>> ---
>>   .../selftests/cgroup/test_memcontrol.c        | 30 ++++++++++++++++++-
>>   1 file changed, 29 insertions(+), 1 deletion(-)
>>
>> diff --git a/tools/testing/selftests/cgroup/test_memcontrol.c b/tools/testing/selftests/cgroup/test_memcontrol.c
>> index 4e1647568c5b..7bea656658a2 100644
>> --- a/tools/testing/selftests/cgroup/test_memcontrol.c
>> +++ b/tools/testing/selftests/cgroup/test_memcontrol.c
>> @@ -24,6 +24,9 @@
>>   static bool has_localevents;
>>   static bool has_recursiveprot;
>>   +#define MEMCG_SOCKSTAT_WAIT_RETRIES        30              /* 3s total */
>> +#define MEMCG_SOCKSTAT_WAIT_INTERVAL_US    (100 * 1000)    /* 100 ms */
> 
> Nit: Defines are usually placed at the top of the file (e.g., after the
> #include block). Placing them between global variables and functions
> looks a bit out of place, IMHO ...
> Otherwise, feel free to add:
> 
> Reviewed-by: Lance Yang <lance.yang@...ux.dev>
> [...]
> 
> Cheers,
> Lance
Thanks for the review and the suggestion!

Good point about the placement of the defines — I’ll move
MEMCG_SOCKSTAT_WAIT_* after the #include block in v3 as you suggested,
and I’ll add your:

    Reviewed-by: Lance Yang <lance.yang@...ux.dev>

as well.

Thanks again!

Best regards,
Guopeng