Re: [PATCH v2 5/7] selftests: memcg: Reduce the expected swap.peak with larger page size

[Waiman Long <longman@redhat.com>]

On 3/23/26 4:24 AM, Li Wang wrote:
> On Fri, Mar 20, 2026 at 04:42:39PM -0400, Waiman Long wrote:
>> When running the test_memcg_swap_max_peak test which sets swap.max
>> to 30M on an arm64 system with 64k page size, the test failed as the
>> swap.peak could only reach up only to 27,328,512 bytes (about 25.45
>> MB which is lower than the expected 29M) before the allocating task
>> got oom-killed.
>>
>> It is likely due to the fact that it takes longer to write out a larger
>> page to swap and hence a lower swap.peak is being reached. Setting
>> memory.high to 29M to throttle memory allocation when nearing memory.max
>> helps, but it still could only reach up to 29,032,448 bytes (about
>> 27.04M). As a result, we have to reduce the expected swap.peak with
>> larger page size. Now swap.peak is expected to reach only 27M with 64k
>> page, 29M with 4k page and 28M with 16k page.
>>
>> Signed-off-by: Waiman Long <longman@redhat.com>
>> ---
>>   .../selftests/cgroup/test_memcontrol.c        | 26 ++++++++++++++++---
>>   1 file changed, 22 insertions(+), 4 deletions(-)
>>
>> diff --git a/tools/testing/selftests/cgroup/test_memcontrol.c b/tools/testing/selftests/cgroup/test_memcontrol.c
>> index c078fc458def..3832ded1e47b 100644
>> --- a/tools/testing/selftests/cgroup/test_memcontrol.c
>> +++ b/tools/testing/selftests/cgroup/test_memcontrol.c
>> @@ -1032,6 +1032,7 @@ static int test_memcg_swap_max_peak(const char *root)
>>   	char *memcg;
>>   	long max, peak;
>>   	struct stat ss;
>> +	long swap_peak;
>>   	int swap_peak_fd = -1, mem_peak_fd = -1;
>>   
>>   	/* any non-empty string resets */
>> @@ -1119,6 +1120,23 @@ static int test_memcg_swap_max_peak(const char *root)
>>   	if (cg_write(memcg, "memory.max", "30M"))
>>   		goto cleanup;
>>   
>> +	/*
>> +	 * The swap.peak that can be reached will depend on the system page
>> +	 * size. With larger page size (e.g. 64k), it takes more time to write
>> +	 * the anonymous memory page to swap and so the peak reached will be
>> +	 * lower before the memory allocation process get oom-killed. One way
>> +	 * to allow the swap.peak to go higher is to throttle memory allocation
>> +	 * by setting memory.high to, say, 29M to give more time to swap out the
>> +	 * memory before oom-kill. This is still not enough for it to reach
>> +	 * 29M reachable with 4k page. So we still need to reduce the expected
>> +	 * swap.peak accordingly.
>> +	 */
>> +	swap_peak = (page_size == KB(4)) ? MB(29) :
>> +		   ((page_size <= KB(16)) ? MB(28) : MB(27));
> Or, go with a dynamic adjustment based on page size?
>
>      swap_peak = MB(29) - ilog2(page_size / KB(4)) * MB(1);
>
It is a good suggestion. I will adopt a dynamic base adjustment as 
suggested.

Cheers,
Longman