Re: [PATCH] mm: remove redundant check in handle_mm_fault

[Haifeng Xu]

On 2023/3/8 17:13, David Hildenbrand wrote:
> On 08.03.23 10:03, Haifeng Xu wrote:
>>
>>
>> On 2023/3/7 10:48, Matthew Wilcox wrote:
>>> On Tue, Mar 07, 2023 at 10:36:55AM +0800, Haifeng Xu wrote:
>>>> On 2023/3/6 21:49, David Hildenbrand wrote:
>>>>> On 06.03.23 03:49, Haifeng Xu wrote:
>>>>>> mem_cgroup_oom_synchronize() has checked whether current memcg_in_oom is
>>>>>> set or not, so remove the check in handle_mm_fault().
>>>>>
>>>>> "mem_cgroup_oom_synchronize() will returned immediately if memcg_in_oom is not set, so remove the check from handle_mm_fault()".
>>>>>
>>>>> However, that requires now always an indirect function call -- do we care about dropping that optimization?
>>>>>
>>>>>
>>>>
>>>> If memcg_in_oom is set, we will check it twice, one is from handle_mm_fault(), the other is from mem_cgroup_oom_synchronize(). That seems a bit redundant.
>>>>
>>>> if memcg_in_oom is not set, mem_cgroup_oom_synchronize() returns directly. Though it's an indirect function call, but the time spent can be negligible
>>>> compare to the whole mm user falut preocess. And that won't cause stack overflow error.
>>>
>>> I suggest you measure it.
>>
>> test steps:
>> 1) Run command: ./mmap_anon_test(global alloc, so the memcg_in_oom is not set)
>> 2) Calculate the quotient of cost time and page-fault counts, run 10 rounds and average the results.
>>
>> The test result shows that whether using indirect function call or not, the time spent in user fault
>> is almost the same, about 2.3ms.
> 
> I guess most of the benchmark time is consumed by allocating fresh pages in your test (also, why exactly do you use MAP_SHARED?).
> 
> Is 2.3ms the total time for writing to that 1GiB of memory or how did you derive that number? Posting both results would be cleaner (with more digits ;) ).
> 

Hi Daivd, the details of test result were posted last week. Do you have any suggestions or more concerns about this change?

Thanks.