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

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Date:   Fri, 8 Oct 2021 14:38:15 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     David Hildenbrand <david@...hat.com>, ultrachin@....com,
        akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Cc:     brookxu.cn@...il.com, chen xiaoguang <xiaoggchen@...cent.com>,
        zeng jingxiang <linuszeng@...cent.com>,
        lu yihui <yihuilu@...cent.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>
Subject: Re: [PATCH] mm: Free per cpu pages async to shorten program exit time

On 10/8/21 10:17, David Hildenbrand wrote:
> On 08.10.21 08:39, ultrachin@....com wrote:
>> From: chen xiaoguang <xiaoggchen@...cent.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@...cent.com>
>> Signed-off-by: zeng jingxiang <linuszeng@...cent.com>
>> Signed-off-by: lu yihui <yihuilu@...cent.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.

OK, but what exactly is the benefit here? The cpu time will have to be spent
in any case, but we move it to a context that's not accounted to the exiting
process. Is that good? Also if it's a large process and restarts
immediately, allocating all the memory back again, it might not be available
as it's still being freed in the background, leading to a risk of OOM?

> 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.
>