linux-kernel - Re: [PATCH 2/2] mm: support multi-size THP numa balancing

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Date: Wed, 27 Mar 2024 09:47:24 +0100
From: David Hildenbrand <david@...hat.com>
To: "Huang, Ying" <ying.huang@...el.com>,
 Baolin Wang <baolin.wang@...ux.alibaba.com>
Cc: akpm@...ux-foundation.org, mgorman@...hsingularity.net,
 wangkefeng.wang@...wei.com, jhubbard@...dia.com, 21cnbao@...il.com,
 ryan.roberts@....com, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] mm: support multi-size THP numa balancing

On 27.03.24 09:21, Huang, Ying wrote:
> Baolin Wang <baolin.wang@...ux.alibaba.com> writes:
> 
>> On 2024/3/27 10:04, Huang, Ying wrote:
>>> Baolin Wang <baolin.wang@...ux.alibaba.com> writes:
>>>
>>>> Now the anonymous page allocation already supports multi-size THP (mTHP),
>>>> but the numa balancing still prohibits mTHP migration even though it is an
>>>> exclusive mapping, which is unreasonable.
>>>>
>>>> Allow scanning mTHP:
>>>> Commit 859d4adc3415 ("mm: numa: do not trap faults on shared data section
>>>> pages") skips shared CoW pages' NUMA page migration to avoid shared data
>>>> segment migration. In addition, commit 80d47f5de5e3 ("mm: don't try to
>>>> NUMA-migrate COW pages that have other uses") change to use page_count()
>>>> to avoid GUP pages migration, that will also skip the mTHP numa scaning.
>>>> Theoretically, we can use folio_maybe_dma_pinned() to detect the GUP
>>>> issue, although there is still a GUP race, the issue seems to have been
>>>> resolved by commit 80d47f5de5e3. Meanwhile, use the folio_likely_mapped_shared()
>>>> to skip shared CoW pages though this is not a precise sharers count. To
>>>> check if the folio is shared, ideally we want to make sure every page is
>>>> mapped to the same process, but doing that seems expensive and using
>>>> the estimated mapcount seems can work when running autonuma benchmark.
>>> Because now we can deal with shared mTHP, it appears even possible
>>> to
>>> remove folio_likely_mapped_shared() check?
>>
>> IMO, the issue solved by commit 859d4adc3415 is about shared CoW
>> mapping, and I prefer to measure it in another patch:)
> 
> I mean we can deal with shared mTHP (by multiple threads or multiple
> processes) with this patch.  Right?

It's independent of the folio order. We don't want to mess with shared COW pages, see

commit 859d4adc3415a64ccb8b0c50dc4e3a888dcb5805
Author: Henry Willard <henry.willard@...cle.com>
Date:   Wed Jan 31 16:21:07 2018 -0800

     mm: numa: do not trap faults on shared data section pages.
     
     Workloads consisting of a large number of processes running the same
     program with a very large shared data segment may experience performance
     problems when numa balancing attempts to migrate the shared cow pages.
     This manifests itself with many processes or tasks in
     TASK_UNINTERRUPTIBLE state waiting for the shared pages to be migrated.
..

that introduced this handling.

-- 
Cheers,

David / dhildenb