Re: [PATCH] mm/memcg: Skip high limit check in root memcg

[Haifeng Xu]

On 2023/2/21 20:20, Michal Hocko wrote:
> On Tue 21-02-23 18:29:39, Haifeng Xu wrote:
>>
>>
>> On 2023/2/14 23:56, Michal Hocko wrote:
>>> On Fri 10-02-23 09:45:50, Haifeng Xu wrote:
>>>> The high limit checks the memory usage from given memcg to root memcg.
>>>> However, there is no limit in root memcg. So this check makes no sense
>>>> and we can ignore it.
>>>
>>> Is this check actually addining any benefit? Have you measured aby
>>> performance gains by this change?
>>>
>>>> Signed-off-by: Haifeng Xu <haifeng.xu@xxxxxxxxxx>
>>>> ---
>>>>  mm/memcontrol.c | 4 ++++
>>>>  1 file changed, 4 insertions(+)
>>>>
>>>> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
>>>> index 73afff8062f9..a31a56598f29 100644
>>>> --- a/mm/memcontrol.c
>>>> +++ b/mm/memcontrol.c
>>>> @@ -2780,6 +2780,10 @@ static int try_charge_memcg(struct mem_cgroup *memcg, gfp_t gfp_mask,
>>>>  	do {
>>>>  		bool mem_high, swap_high;
>>>>  
>>>> +		/* There is no need for root memcg to check high limit */
>>>> +		if (mem_cgroup_is_root(memcg))
>>>> +			break;
>>>> +
>>>>  		mem_high = page_counter_read(&memcg->memory) >
>>>>  			READ_ONCE(memcg->memory.high);
>>>>  		swap_high = page_counter_read(&memcg->swap) >
>>>> -- 
>>>> 2.25.1
>>>
>>
>> test steps:
>> 1. mkdir -p /sys/fs/cgroup/memory/test
>> 2. echo $$ > /sys/fs/cgroup/memory/test/cgroup.procs
>> 3. ./mmap_test
>>
>> The test result show that with or without the patch, the time taken is almost the same.
> 
> This is in line with my expectation. So the question is whether the
> additional check is really worth it. 

This patch doesn't bring any obvious benifit or harm, but the high limit check in root memcg seems a little weird.
Maybe we can add this check？It all depends on your viewpoint.

Thanks.