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Subject: Re: [PATCH] mm: Free per cpu pages async to shorten program exit time
To:     Claudio Imbrenda <imbrenda@linux.ibm.com>
Cc:     ultrachin@163.com, akpm@linux-foundation.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, brookxu.cn@gmail.com,
        chen xiaoguang <xiaoggchen@tencent.com>,
        zeng jingxiang <linuszeng@tencent.com>,
        lu yihui <yihuilu@tencent.com>
References: <20211008063933.331989-1-ultrachin@163.com>
 <d71e6021-777b-3ca9-b08f-64fe7ff51e08@redhat.com>
 <20211008105205.07d2f205@p-imbrenda>
 <1db11d75-d2d8-ef71-471a-ddad5c90a733@redhat.com>
 <20211008112235.1d37f2db@p-imbrenda>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
Message-ID: <a279f2a0-7ede-88b6-f5e5-3d13699d18b4@redhat.com>
Date:   Fri, 8 Oct 2021 11:24:14 +0200
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On 08.10.21 11:22, Claudio Imbrenda wrote:
> On Fri, 8 Oct 2021 11:15:25 +0200
> David Hildenbrand <david@redhat.com> wrote:
> 
>> On 08.10.21 10:52, Claudio Imbrenda wrote:
>>> On Fri, 8 Oct 2021 10:17:50 +0200
>>> David Hildenbrand <david@redhat.com> wrote:
>>>    
>>>> On 08.10.21 08:39, ultrachin@163.com wrote:
>>>>> From: chen xiaoguang <xiaoggchen@tencent.com>
>>>>>
>>>>> The exit time is long when program allocated big memory and
>>>>> the most time consuming part is free memory which takes 99.9%
>>>>> of the total exit time. By using async free we can save 25% of
>>>>> exit time.
>>>>>
>>>>> Signed-off-by: chen xiaoguang <xiaoggchen@tencent.com>
>>>>> Signed-off-by: zeng jingxiang <linuszeng@tencent.com>
>>>>> Signed-off-by: lu yihui <yihuilu@tencent.com>
>>>>
>>>> I recently discussed with Claudio if it would be possible to tear down
>>>> the process MM deferred, because for some use cases (secure/encrypted
>>>> virtualization, very large mmaps) tearing down the page tables is
>>>> already the much more expensive operation.
>>>>
>>>> There is mmdrop_async(), and I wondered if one could reuse that concept
>>>> when tearing down a process -- I didn't look into feasibility, however,
>>>> so it's just some very rough idea.
>>>
>>> I have done some experiments by unconditionally replacing mmdrop with
>>> mmdrop_async in exit.c and nothing broke, and exit time of large
>>> processes was almost instant (with the actual cleanup being performed in
>>> background)
>>>
>>> my approach is probably simpler/cleaner:
>>>
>>> diff --git a/include/asm-generic/mmu_context.h b/include/asm-generic/mmu_context.h
>>> index 91727065bacb..900931a6a105 100644
>>> --- a/include/asm-generic/mmu_context.h
>>> +++ b/include/asm-generic/mmu_context.h
>>> @@ -73,4 +73,8 @@ static inline void deactivate_mm(struct task_struct *tsk,
>>>    }
>>>    #endif
>>>    
>>> +#ifndef arch_exit_mm_mmput
>>> +#define arch_exit_mm_mmput mmput
>>> +#endif
>>> +
>>>    #endif /* __ASM_GENERIC_MMU_CONTEXT_H */
>>> diff --git a/kernel/exit.c b/kernel/exit.c
>>> index 9a89e7f36acb..604cb9c740fa 100644
>>> --- a/kernel/exit.c
>>> +++ b/kernel/exit.c
>>> @@ -498,7 +498,7 @@ static void exit_mm(void)
>>>           task_unlock(current);
>>>           mmap_read_unlock(mm);
>>>           mm_update_next_owner(mm);
>>> -       mmput(mm);
>>> +       arch_exit_mm_mmput(mm);
>>>           if (test_thread_flag(TIF_MEMDIE))
>>>                   exit_oom_victim();
>>>    }
>>>
>>> these are the minimal changes to common code, then each architecture can
>>> define their own arch_exit_mm_mmput as they see fit (for example, to free
>>> asynchronously only for certain classes of mm, like big ones, VMs, or so).
>>>
>>> Another option is to simply always replace mmput with mmput_async, which I
>>> expect will raise more eyebrows.
>>
>> Thanks Claudio.
>>
>> I guess we'd use some heuristic to keep the eyebrows down. Having
>> something like
>>
>> if (should_mput_async_on_exit(mm))
>> 	mmput_async(mm);
>> else
>> 	mmput(mm);
>>
>> whereby the heuristic can optionally consult the arch/config-knobs/...
>> doesn't sound too wrong to me if it works.
>>
> 
> yes, that is one of the possible solutions I had thought of.
> 
> although probably the small patch I posted above is even less intrusive
> and should hopefully raise even fewer eyebrows, while also leaving the
> door open to arch-specific code to do more than just mmput_async, if
> needed.

More flexibility might raise more eyebrows. :)

-- 
Thanks,

David / dhildenb