linux-kernel - Re: [PATCH 3/4] mm: khugepaged: move mm to list tail when MADV_COLD/MADV

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Message-ID: <20251221021044.2r5fhepiyyhvuo7h@master>
Date: Sun, 21 Dec 2025 02:10:44 +0000
From: Wei Yang <richard.weiyang@...il.com>
To: "David Hildenbrand (Red Hat)" <david@...nel.org>
Cc: Vernon Yang <vernon2gm@...il.com>, akpm@...ux-foundation.org,
	lorenzo.stoakes@...cle.com, ziy@...dia.com, baohua@...nel.org,
	lance.yang@...ux.dev, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, Vernon Yang <yanglincheng@...inos.cn>
Subject: Re: [PATCH 3/4] mm: khugepaged: move mm to list tail when
 MADV_COLD/MADV_FREE

On Fri, Dec 19, 2025 at 09:58:17AM +0100, David Hildenbrand (Red Hat) wrote:
>On 12/19/25 06:29, Vernon Yang wrote:
>> On Thu, Dec 18, 2025 at 10:31:58AM +0100, David Hildenbrand (Red Hat) wrote:
>> > On 12/15/25 10:04, Vernon Yang wrote:
>> > > For example, create three task: hot1 -> cold -> hot2. After all three
>> > > task are created, each allocate memory 128MB. the hot1/hot2 task
>> > > continuously access 128 MB memory, while the cold task only accesses
>> > > its memory briefly andthen call madvise(MADV_COLD). However, khugepaged
>> > > still prioritizes scanning the cold task and only scans the hot2 task
>> > > after completing the scan of the cold task.
>> > > 
>> > > So if the user has explicitly informed us via MADV_COLD/FREE that this
>> > > memory is cold or will be freed, it is appropriate for khugepaged to
>> > > scan it only at the latest possible moment, thereby avoiding unnecessary
>> > > scan and collapse operations to reducing CPU wastage.
>> > > 
>> > > Here are the performance test results:
>> > > (Throughput bigger is better, other smaller is better)
>> > > 
>> > > Testing on x86_64 machine:
>> > > 
>> > > | task hot2           | without patch | with patch    |  delta  |
>> > > |---------------------|---------------|---------------|---------|
>> > > | total accesses time |  3.14 sec     |  2.92 sec     | -7.01%  |
>> > > | cycles per access   |  4.91         |  2.07         | -57.84% |
>> > > | Throughput          |  104.38 M/sec |  112.12 M/sec | +7.42%  |
>> > > | dTLB-load-misses    |  288966432    |  1292908      | -99.55% |
>> > > 
>> > > Testing on qemu-system-x86_64 -enable-kvm:
>> > > 
>> > > | task hot2           | without patch | with patch    |  delta  |
>> > > |---------------------|---------------|---------------|---------|
>> > > | total accesses time |  3.35 sec     |  2.96 sec     | -11.64% |
>> > > | cycles per access   |  7.23         |  2.12         | -70.68% |
>> > > | Throughput          |  97.88 M/sec  |  110.76 M/sec | +13.16% |
>> > > | dTLB-load-misses    |  237406497    |  3189194      | -98.66% |
>> > 
>> > Again, I also don't like that because you make assumptions on a full process
>> > based on some part of it's address space.
>> > 
>> > E.g., if a library issues a MADV_COLD on some part of the memory the library
>> > manages, why should the remaining part of the process suffer as well?
>> 
>> Yes, you make a good point, thanks!
>> 
>> > This seems to be an heuristic focused on some specific workloads, no?
>> 
>> Right.
>> 
>> Could we use the VM_NOHUGEPAGE flag to indicate that this region should
>> not be collapsed, so that khugepaged can simply skip this VMA during
>> scanning? This way, it won't affect the remaining part of the task's
>> memory regions.
>
>I thought we would skip these regions already properly in khugeapged, or
>maybe I misunderstood your question.
>

I think we should, but seems we didn't do this for anonymous memory during
khugepaged.

We check the vma with thp_vma_allowable_order() during scan.

  * For anonymous memory during khugepaged, if we always enable 2M collapse,
    we will scan this vma. Even VM_NOHUGEPAGE is set.

  * For other cases, it looks good since __thp_vma_allowable_order() will skip
    this vma with vma_thp_disabled().

>-- 
>Cheers
>
>David

-- 
Wei Yang
Help you, Help me