Date: Mon, 18 Mar 2024 18:31:10 +0800
From: Baolin Wang <baolin.wang@...ux.alibaba.com>
To: David Hildenbrand <david@...hat.com>, "Huang, Ying" <ying.huang@...el.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: [RFC PATCH v2] mm: support multi-size THP numa balancing



On 2024/3/18 18:15, David Hildenbrand wrote:
> On 18.03.24 11:13, Baolin Wang wrote:
>>
>>
>> On 2024/3/18 17:48, David Hildenbrand wrote:
>>> On 18.03.24 10:42, Baolin Wang wrote:
>>>>
>>>>
>>>> On 2024/3/18 14:16, 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. Thus let's support the
>>>>>> exclusive
>>>>>> mTHP numa balancing firstly.
>>>>>>
>>>>>> 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_estimated_sharers()
>>>>>> 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.
>>>>>>
>>>>>> Allow migrating mTHP:
>>>>>> As mentioned in the previous thread[1], large folios are more
>>>>>> susceptible
>>>>>> to false sharing issues, leading to pages ping-pong back and forth
>>>>>> during
>>>>>> numa balancing, which is currently hard to resolve. Therefore, as a
>>>>>> start to
>>>>>> support mTHP numa balancing, only exclusive mappings are allowed to
>>>>>> perform
>>>>>> numa migration to avoid the false sharing issues with large folios.
>>>>>> Similarly,
>>>>>> use the estimated mapcount to skip shared mappings, which seems can
>>>>>> work
>>>>>> in most cases (?), and we've used folio_estimated_sharers() to skip
>>>>>> shared
>>>>>> mappings in migrate_misplaced_folio() for numa balancing, seems no 
>>>>>> real
>>>>>> complaints.
>>>>>
>>>>> IIUC, folio_estimated_sharers() cannot identify multi-thread
>>>>> applications.  If some mTHP is shared by multiple threads in one
>>>>
>>>> Right.
>>>>
>>>
>>> Wasn't this "false sharing" previously raised/described by Mel in this
>>> context?
>>
>> Yes, I got confused with the process's false sharing.
>>
>>>>> process, how to deal with that?
>>>>
>>>> IMHO, seems the should_numa_migrate_memory() already did something to
>>>> help?
>>>>
>>>> ......
>>>>      if (!cpupid_pid_unset(last_cpupid) &&
>>>>                  cpupid_to_nid(last_cpupid) != dst_nid)
>>>>          return false;
>>>>
>>>>      /* Always allow migrate on private faults */
>>>>      if (cpupid_match_pid(p, last_cpupid))
>>>>          return true;
>>>> ......
>>>>
>>>> If the node of the CPU that accessed the mTHP last time is different
>>>> from this time, which means there is some contention for that mTHP 
>>>> among
>>>> threads. So it will not allow migration.
>>>>
>>>> If the contention for the mTHP among threads is light or the accessing
>>>> is relatively stable, then we can allow migration?
>>>>
>>>>> For example, I think that we should avoid to migrate on the first 
>>>>> fault
>>>>> for mTHP in should_numa_migrate_memory().
>>>>>
>>>>> More thoughts?  Can we add a field in struct folio for mTHP to count
>>>>> hint page faults from the same node?
>>>>
>>>> IIUC, we do not need add a new field for folio, seems we can reuse
>>>> ->_flags_2a field. But how to use it? If there are multiple consecutive
>>>> NUMA faults from the same node, then allow migration?
>>>
>>> _flags_2a cannot be used. You could place something after _deferred_list
>>
>> Could you be more explicit? I didn't see _flags_2 currently being used,
>> did I miss something?
> 
> Yes, that we use it implicitly via page->flags on subpages (for example, 
> some flags are still per-subpage and not per-folio).

Yes, thanks for reminding:)

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