linux-kernel - Re: [PATCH v4 2/3] selftests: cgroup: make test_memcg

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Message-ID: <240ebe22-6e1c-4481-8a5e-b929bbfad178@kylinos.cn>
Date: Wed, 3 Dec 2025 11:27:47 +0800
From: Guopeng Zhang <zhangguopeng@...inos.cn>
To: Shakeel Butt <shakeel.butt@...ux.dev>
Cc: tj@...nel.org, hannes@...xchg.org, mhocko@...nel.org,
 roman.gushchin@...ux.dev, mkoutny@...e.com, muchun.song@...ux.dev,
 lance.yang@...ux.dev, shuah@...nel.org, linux-mm@...ck.org,
 linux-kselftest@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 2/3] selftests: cgroup: make test_memcg_sock robust
 against delayed sock stats



On 12/3/25 07:12, Shakeel Butt wrote:
> On Mon, Nov 24, 2025 at 08:38:15PM +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. 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.
>>
>> Signed-off-by: Guopeng Zhang <zhangguopeng@...inos.cn>
>> Suggested-by: Lance Yang <lance.yang@...ux.dev>
>> ---
>>  .../selftests/cgroup/test_memcontrol.c        | 20 ++++++++++++++++++-
>>  1 file changed, 19 insertions(+), 1 deletion(-)
>>
>> diff --git a/tools/testing/selftests/cgroup/test_memcontrol.c b/tools/testing/selftests/cgroup/test_memcontrol.c
>> index 4e1647568c5b..dda12e5c6457 100644
>> --- a/tools/testing/selftests/cgroup/test_memcontrol.c
>> +++ b/tools/testing/selftests/cgroup/test_memcontrol.c
>> @@ -21,6 +21,8 @@
>>  #include "kselftest.h"
>>  #include "cgroup_util.h"
>>  
>> +#define MEMCG_SOCKSTAT_WAIT_RETRIES        30              /* 3s total */
> 
> No need for the comment at the end as it will be stale when someone
> change DEFAULT_WAIT_INTERVAL_US in future.
> 
> Anyways it's a nit.
> 
> Reviewed-by: Shakeel Butt <shakeel.butt@...ux.dev>
Hi Shakeel,

Thanks for the review and for the Reviewed-by.

Good point about the comment. I’ll drop the "/* 3s total */" part in the next version so it does not become stale if DEFAULT_WAIT_INTERVAL_US changes.

Thanks,
Guopeng