linux-kernel - Re: [PATCH v3] selftests: cgroup: make test_memcg

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Message-ID: <p655qedqjaakrnqpytc6dltejfluxo6jrffcltfz2ivonmk6lb@bxf5xlgo4iw2>
Date: Thu, 20 Nov 2025 16:35:02 +0100
From: Michal Koutný <mkoutny@...e.com>
To: Guopeng Zhang <zhangguopeng@...inos.cn>
Cc: tj@...nel.org, hannes@...xchg.org, mhocko@...nel.org, 
	roman.gushchin@...ux.dev, shakeel.butt@...ux.dev, 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 v3] selftests: cgroup: make test_memcg_sock robust
 against delayed sock stats

Hello Guopeng.

+Cc Leon Huang Fu <leon.huangfu@...pee.com>   

On Thu, Nov 20, 2025 at 02:04:06PM +0800, Guopeng Zhang <zhangguopeng@...inos.cn> 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).

(FTR, I remember there is `echo 1 > /sys/module/rcutree/parameters/do_rcu_barrier`,
however, I'm not sure it works always as expected (a reader may actually
wait for multi-stage RCU pipeline), so plain timeout is more reliable.)

>   - 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.

I like the approach of adaptive waiting to settle in such tests.

> 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.

BTW Have you looked into the number of retries until success?
Was it in accordance with the flushing interval?

> 
> Suggested-by: Lance Yang <lance.yang@...ux.dev>
> Reviewed-by: Lance Yang <lance.yang@...ux.dev>
> Signed-off-by: Guopeng Zhang <zhangguopeng@...inos.cn>
> ---
> v3:
>  - Move MEMCG_SOCKSTAT_WAIT_* defines after the #include block as
>    suggested.
> 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..8ff7286fc80b 100644
> --- a/tools/testing/selftests/cgroup/test_memcontrol.c
> +++ b/tools/testing/selftests/cgroup/test_memcontrol.c
> @@ -21,6 +21,9 @@
>  #include "kselftest.h"
>  #include "cgroup_util.h"
>  
> +#define MEMCG_SOCKSTAT_WAIT_RETRIES        30              /* 3s total */
> +#define MEMCG_SOCKSTAT_WAIT_INTERVAL_US    (100 * 1000)    /* 100 ms */
> +
>  static bool has_localevents;
>  static bool has_recursiveprot;
>  
> @@ -1384,6 +1387,8 @@ 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;
>  
>  	memcg = cg_name(root, "memcg_test");
>  	if (!memcg)
> @@ -1432,7 +1437,30 @@ 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, which runs periodically (every 2 seconds,
> +	 * see FLUSH_TIME). On a busy system, 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 (~FLUSH_TIME plus some
> +	 * scheduling slack) and require that the "sock " counter
> +	 * eventually drops to zero.
> +	 */
> +	for (i = 0; i < MEMCG_SOCKSTAT_WAIT_RETRIES; i++) {
> +		sock_post = cg_read_key_long(memcg, "memory.stat", "sock ");
> +		if (sock_post < 0)
> +			goto cleanup;
> +
> +		if (!sock_post)
> +			break;
> +
> +		usleep(MEMCG_SOCKSTAT_WAIT_INTERVAL_US);
> +	}

I think this may be useful also for othe tests (at least other
memory.stat checks), so some encapsulated implementation like a macro
with parameters
	cg_read_assert_gt_with_retries(cg, file, field, exp, timeout, retries)
WDYT?

Michal

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