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

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Message-ID: <me7djjzkgufieiyvrsbezi4i4t6v5zsbonirjlacce6hgilb2c@sxnifdeh2gnv>
Date: Tue, 2 Dec 2025 15:12:00 -0800
From: Shakeel Butt <shakeel.butt@...ux.dev>
To: Guopeng Zhang <zhangguopeng@...inos.cn>
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 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>