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

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Message-ID: <5ad2b75f-748a-4e93-8d11-63295bda0cbf@linux.dev>
Date: Thu, 20 Nov 2025 13:40:34 +0800
From: Lance Yang <lance.yang@...ux.dev>
To: Guopeng Zhang <zhangguopeng@...inos.cn>
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 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