linux-kernel - Re: [RFC PATCH v0 2/2] mm: sched: Batch-migrate misplaced pages

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Message-ID: <dbc7c66b-24c9-49f4-8988-a7eec1280ca8@redhat.com>
Date: Mon, 26 May 2025 11:29:09 +0200
From: David Hildenbrand <david@...hat.com>
To: Zi Yan <ziy@...dia.com>
Cc: Bharata B Rao <bharata@....com>, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org, Jonathan.Cameron@...wei.com, dave.hansen@...el.com,
 gourry@...rry.net, hannes@...xchg.org, mgorman@...hsingularity.net,
 mingo@...hat.com, peterz@...radead.org, raghavendra.kt@....com,
 riel@...riel.com, rientjes@...gle.com, sj@...nel.org, weixugc@...gle.com,
 willy@...radead.org, ying.huang@...ux.alibaba.com, dave@...olabs.net,
 nifan.cxl@...il.com, joshua.hahnjy@...il.com, xuezhengchu@...wei.com,
 yiannis@...corp.com, akpm@...ux-foundation.org
Subject: Re: [RFC PATCH v0 2/2] mm: sched: Batch-migrate misplaced pages

On 22.05.25 19:30, Zi Yan wrote:
> On 22 May 2025, at 13:21, David Hildenbrand wrote:
> 
>> On 22.05.25 18:38, Zi Yan wrote:
>>> On 22 May 2025, at 12:26, David Hildenbrand wrote:
>>>
>>>> On 22.05.25 18:24, Zi Yan wrote:
>>>>> On 22 May 2025, at 12:11, David Hildenbrand wrote:
>>>>>
>>>>>> On 21.05.25 10:02, Bharata B Rao wrote:
>>>>>>> Currently the folios identified as misplaced by the NUMA
>>>>>>> balancing sub-system are migrated one by one from the NUMA
>>>>>>> hint fault handler as and when they are identified as
>>>>>>> misplaced.
>>>>>>>
>>>>>>> Instead of such singe folio migrations, batch them and
>>>>>>> migrate them at once.
>>>>>>>
>>>>>>> Identified misplaced folios are isolated and stored in
>>>>>>> a per-task list. A new task_work is queued from task tick
>>>>>>> handler to migrate them in batches. Migration is done
>>>>>>> periodically or if pending number of isolated foios exceeds
>>>>>>> a threshold.
>>>>>>
>>>>>> That means that these pages are effectively unmovable for other purposes (CMA, compaction, long-term pinning, whatever) until that list was drained.
>>>>>>
>>>>>> Bad.
>>>>>
>>>>> Probably we can mark these pages and when others want to migrate the page,
>>>>> get_new_page() just looks at the page's target node and get a new page from
>>>>> the target node.
>>>>
>>>> How do you envision that working when CMA needs to migrate this exact page to a different location?
>>>>
>>>> It cannot isolate it for migration because ... it's already isolated ... so it will give up.
>>>>
>>>> Marking might not be easy I assume ...
>>>
>>> I guess you mean we do not have any extra bit to indicate this page is isolated,
>>> but it can be migrated. My point is that if this page is going to be migrated
>>> due to other reasons, like CMA, compaction, why not migrate it to the target
>>> node instead of moving it around within the same node.
>>
>> I think we'd have to identify that
>>
>> a) This page is isolate for migration (could be isolated for other
>>     reasons)
>>
>> b) The one responsible for the isolation is numa code (could be someone
>>     else)
>>
>> c) We're allowed to grab that page from that list (IOW sync against
>>     others, and especially also against), to essentially "steal" the
>>     isolated page.
> 
> Right. c) sounds like adding more contention to the candidate list.
> I wonder if we can just mark the page as migration candidate (using
> a page flag or something else), then migrate it whenever CMA,
> compaction, long-term pinning and more look at the page.

I mean, all these will migrate the page either way, no need to add 
another flag for that.

I guess what you mean, indicating that the migration destination should 
be on a different node than the current one.

Well, and for the NUMA scanner (below) to find which pages to migrate.

... to be this raises some questions: like, if we don't migrate 
immediately, could that information ("migrate this page") actually now 
be wrong? I guess a way to obtain the destination node would suffice: if 
the destination node matches, no need to migrate from that NUMA scanner.

In addition,
> periodically, the migration task would do a PFN scanning and migrate
> any migration candidate. I remember Willy did some experiments showing
> that PFN scanning is very fast.

PFN scanning can be faster than walking lists, but I suspect it depends 
on how many pages there really are to be migrated ... and some other 
factors :)

-- 
Cheers,

David / dhildenb