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

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Message-ID: <20251119122758.85610-1-ioworker0@gmail.com>
Date: Wed, 19 Nov 2025 20:27:58 +0800
From: Lance Yang <ioworker0@...il.com>
To: zhangguopeng@...inos.cn
Cc: hannes@...xchg.org,
	linux-kernel@...r.kernel.org,
	linux-kselftest@...r.kernel.org,
	linux-mm@...ck.org,
	mhocko@...nel.org,
	mkoutny@...e.com,
	muchun.song@...ux.dev,
	roman.gushchin@...ux.dev,
	shakeel.butt@...ux.dev,
	shuah@...nel.org,
	tj@...nel.org,
	leon.huangfu@...pee.com,
	Lance Yang <lance.yang@...ux.dev>
Subject: Re: [PATCH] selftests: cgroup: make test_memcg_sock robust against delayed sock stats

From: Lance Yang <lance.yang@...ux.dev>


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.

Good catch! Thanks!

With more CPU cores, updates may be distributed across cores, making it
slower to reach the per-CPU flush threshold, IIUC :)

> 
> Signed-off-by: Guopeng Zhang <zhangguopeng@...inos.cn>
> ---
>  .../selftests/cgroup/test_memcontrol.c        | 24 ++++++++++++++++++-
>  1 file changed, 23 insertions(+), 1 deletion(-)
> 
> diff --git a/tools/testing/selftests/cgroup/test_memcontrol.c b/tools/testing/selftests/cgroup/test_memcontrol.c
> index 4e1647568c5b..86d9981cddd8 100644
> --- a/tools/testing/selftests/cgroup/test_memcontrol.c
> +++ b/tools/testing/selftests/cgroup/test_memcontrol.c
> @@ -1384,6 +1384,8 @@ static int test_memcg_sock(const char *root)
>  	int bind_retries = 5, ret = KSFT_FAIL, pid, err;
>  	unsigned short port;
>  	char *memcg;
> +	long sock_post = -1;
> +	int i, retries = 30;
>  
>  	memcg = cg_name(root, "memcg_test");
>  	if (!memcg)
> @@ -1432,7 +1434,27 @@ static int test_memcg_sock(const char *root)
>  	if (cg_read_long(memcg, "memory.current") < 0)
>  		goto cleanup;
>  
> -	if (cg_read_key_long(memcg, "memory.stat", "sock "))
> +	/*
> +	 * memory.stat is updated asynchronously via the memcg rstat
> +	 * flushing worker, so the "sock " counter may stay non-zero
> +	 * for a short period of time after the TCP connection is
> +	 * closed and all socket memory has been uncharged.
> +	 *
> +	 * Poll memory.stat for up to 3 seconds and require that the
> +	 * "sock " counter eventually drops to zero.

It might be worth mentioning that the current periodic rstat flush happens
every 2 seconds (#define FLUSH_TIME (2UL*HZ)). Adding this context to the
comment would explain why the 3-second timeout was chosen ;)

> +	 */
> +	for (i = 0; i < retries; i++) {
> +		sock_post = cg_read_key_long(memcg, "memory.stat", "sock ");
> +		if (sock_post < 0)
> +			goto cleanup;
> +
> +		if (!sock_post)
> +			break;
> +
> +		usleep(100 * 1000); /* 100ms */

Nit: It would be better to define the retry count and interval as macros
(e.g., MAX_RETRIES, WAIT_INTERVAL) to avoid magic numbers and make the 3s
timeout calculation explicit.

> +	}
> +
> +	if (sock_post)
>  		goto cleanup;
>  
>  	ret = KSFT_PASS;

Thanks,
Lance