Re: [PATCH v3] selftests: cgroup: make test_memcg_sock robust against delayed sock stats

[Guopeng Zhang <zhangguopeng@kylinos.cn>]

On 11/20/25 23:35, Michal Koutný wrote:
> Hello Guopeng.
> 
> +Cc Leon Huang Fu <leon.huangfu@shopee.com>   
> 
> On Thu, Nov 20, 2025 at 02:04:06PM +0800, Guopeng Zhang <zhangguopeng@kylinos.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.)
> 
Hi Michal,

Thank you for the suggestion.

I tested using `echo 1 > /sys/module/rcutree/parameters/do_rcu_barrier`, but
unfortunately the effect was not very good on my setup. As you mentioned, a
reader may actually wait for the multi-stage RCU pipeline, so a plain timeout
seems more reliable here.
>>   - 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?
> 
Yes. From my observations, it usually succeeds after about 10–15 retries on
average (roughly 1–1.5 seconds), and occasionally it takes more than 20 retries
(>2 seconds). This looks broadly in line with the periodic rstat flushing
interval (~2 seconds) plus some scheduling slack.
>>
>> Suggested-by: Lance Yang <lance.yang@linux.dev>
>> Reviewed-by: Lance Yang <lance.yang@linux.dev>
>> Signed-off-by: Guopeng Zhang <zhangguopeng@kylinos.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

That’s a great idea. I agree this pattern could be useful for other
`memory.stat` checks as well, and I will implement an encapsulated helper/macro
along those lines as per your suggestion.

Thanks,
Guopeng