Message-ID: <444e5e05-2dc6-4201-b872-7eb8238c612c@redhat.com>
Date: Mon, 15 Sep 2025 12:39:11 +0200
From: David Hildenbrand <david@...hat.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: Kiryl Shutsemau <kas@...nel.org>, Nico Pache <npache@...hat.com>,
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Subject: Re: [PATCH v11 00/15] khugepaged: mTHP support

On 15.09.25 12:35, Lorenzo Stoakes wrote:
> On Mon, Sep 15, 2025 at 12:22:07PM +0200, David Hildenbrand wrote:
>> On 15.09.25 11:22, Kiryl Shutsemau wrote:
>>> On Fri, Sep 12, 2025 at 05:31:51PM -0600, Nico Pache wrote:
>>>> On Fri, Sep 12, 2025 at 6:25 AM David Hildenbrand <david@...hat.com> wrote:
>>>>>
>>>>> On 12.09.25 14:19, Kiryl Shutsemau wrote:
>>>>>> On Thu, Sep 11, 2025 at 09:27:55PM -0600, Nico Pache wrote:
>>>>>>> The following series provides khugepaged with the capability to collapse
>>>>>>> anonymous memory regions to mTHPs.
>>>>>>>
>>>>>>> To achieve this we generalize the khugepaged functions to no longer depend
>>>>>>> on PMD_ORDER. Then during the PMD scan, we use a bitmap to track individual
>>>>>>> pages that are occupied (!none/zero). After the PMD scan is done, we do
>>>>>>> binary recursion on the bitmap to find the optimal mTHP sizes for the PMD
>>>>>>> range. The restriction on max_ptes_none is removed during the scan, to make
>>>>>>> sure we account for the whole PMD range. When no mTHP size is enabled, the
>>>>>>> legacy behavior of khugepaged is maintained. max_ptes_none will be scaled
>>>>>>> by the attempted collapse order to determine how full a mTHP must be to be
>>>>>>> eligible for the collapse to occur. If a mTHP collapse is attempted, but
>>>>>>> contains swapped out, or shared pages, we don't perform the collapse. It is
>>>>>>> now also possible to collapse to mTHPs without requiring the PMD THP size
>>>>>>> to be enabled.
>>>>>>>
>>>>>>> When enabling (m)THP sizes, if max_ptes_none >= HPAGE_PMD_NR/2 (255 on
>>>>>>> 4K page size), it will be automatically capped to HPAGE_PMD_NR/2 - 1 for
>>>>>>> mTHP collapses to prevent collapse "creep" behavior. This prevents
>>>>>>> constantly promoting mTHPs to the next available size, which would occur
>>>>>>> because a collapse introduces more non-zero pages that would satisfy the
>>>>>>> promotion condition on subsequent scans.
>>>>>>
>>>>>> Hm. Maybe instead of capping at HPAGE_PMD_NR/2 - 1 we can count
>>>>>> all-zeros 4k as none_or_zero? It mirrors the logic of shrinker.
>>>>>>
>>>>>
>>>>> I am all for not adding any more ugliness on top of all the ugliness we
>>>>> added in the past.
>>>>>
>>>>> I will soon propose deprecating that parameter in favor of something
>>>>> that makes a bit more sense.
>>>>>
>>>>> In essence, we'll likely have an "eagerness" parameter that ranges from
>>>>> 0 to 10. 10 is essentially "always collapse" and 0 "never collapse if
>>>>> not all is populated".
>>>> Hi David,
>>>>
>>>> Do you have any reason for 0-10, I'm guessing these will map to
>>>> different max_ptes_none values.
>>>> I suggest 0-5, mapping to 0,32,64,128,255,511
>>>
>>> That's too x86-64 specific.
>>>
>>> And the whole idea is not to map to directly, but give kernel wiggle
>>> room to play.
>>
>> Initially we will start out simple and map it directly. But yeah, the idea
>> is to give us some more room later.
> 
> I think it's less 'wiggle room' and more us being able to _abstract_ what this
> measurement means while reserving the right to adjust this.
> 
> But maybe we are saying the same thing in different ways.
> 
>>
>> I had something logarithmic in mind which would roughly be (ignoring the the
>> weird -1 for simplicity and expressing it as "used" instead of none-or-zero)
>>
>> 0 -> ~100% used (~0% none)
> 
> So equivalent to 511 today?
> 
>> 1 -> ~50% used (~50% none)
>> 2 -> ~25% used (~75% none)
>> 3 -> ~12.5% used (~87.5% none)
>> 4 -> ~11.25% used (~88,75% none)
>> ...
>> 10 -> ~0% used (~100% none)
> 
> So equivalent to 0 today?

Yes.

> 
> And with a logarithmic weighting towards values closer to "0% used"?
> 
> This seems sensible given the only reports we've had of non-0/511 uses here are
> in that range...
> 
> But ofc this interpretation should be something we determine + treated as an
> implementation detail that we can modify later.
> 
>>
>> Mapping that to actual THP sizes (#pages in a thp) on an arch will be easy.
> 
> And at different mTHP levels too right?

Yes exactly.

-- 
Cheers

David / dhildenb

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