linux-kernel - Re: [PATCH] mm: remove redundant check in handle_mm

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Message-ID: <562e9cc3-d0aa-23e9-bd19-266b5aef2ae7@redhat.com>
Date:   Wed, 8 Mar 2023 10:13:15 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Haifeng Xu <haifeng.xu@...pee.com>,
        Matthew Wilcox <willy@...radead.org>
Cc:     akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: remove redundant check in handle_mm_fault

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 ;) ).

-- 
Thanks,

David / dhildenb