Message-ID: <562e9cc3-d0aa-23e9-bd19-266b5aef2ae7@redhat.com>
Date:   Wed, 8 Mar 2023 10:13:15 +0100
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Subject: Re: [PATCH] mm: remove redundant check in handle_mm_fault
To:     Haifeng Xu <haifeng.xu@shopee.com>,
        Matthew Wilcox <willy@infradead.org>
Cc:     akpm@linux-foundation.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
References: <20230306024959.131468-1-haifeng.xu@shopee.com>
 <df3997ed-a844-597b-fbb1-154caad78543@redhat.com>
 <354360d5-dce6-a11c-ee61-d41e615bfa05@shopee.com>
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 <6df72872-2829-47ab-552c-7ef8a6470e6f@shopee.com>
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From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
In-Reply-To: <6df72872-2829-47ab-552c-7ef8a6470e6f@shopee.com>
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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