Re: [PATCH] mm: Free per cpu pages async to shorten program exit time

[David Hildenbrand <david@redhat.com>]

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.

-- 
Thanks,

David / dhildenb