Re: [PATCH] selftests: cgroup: make test_memcg_sock robust against delayed sock stats

[Leon Huang Fu]

On Thu, Nov 20, 2025 at 10:12 AM Guopeng Zhang <zhangguopeng@xxxxxxxxxx> wrote:
>
>
>On 11/19/25 20:27, Lance Yang wrote:
>> From: Lance Yang <lance.yang@xxxxxxxxx>
>>
>>
>> On Wed, 19 Nov 2025 18:52:16 +0800, 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. 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.
>Hi Lance,
>
>Thanks a lot for your review and helpful comments!
>>
>>Good catch! Thanks!
>>
>> With more CPU cores, updates may be distributed across cores, making it
>> slower to reach the per-CPU flush threshold, IIUC :)
>>
>Yes, that matches what I’ve seen as well — on larger systems it indeed
>takes longer for the stats to converge due to per-CPU distribution and
>the flush threshold.

Me too.

I previously proposed a potential solution to explicitly flush stats via
a new interface, "memory.stat_refresh" [1]. However, improving the
existing flush mechanism would likely be a better long-term direction.

Links:
[1] https://lore.kernel.org/linux-mm/20251110101948.19277-1-leon.huangfu@xxxxxxxxxx/

Thanks,
Leon

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