linux-kernel - Re: [PATCH v11 00/15] khugepaged: mTHP support

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Message-ID: <4508810d-7645-4f57-a0c5-2ee9f44206fc@lucifer.local>
Date: Mon, 15 Sep 2025 11:40:27 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: David Hildenbrand <david@...hat.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 Mon, Sep 15, 2025 at 11:35:53AM +0100, Lorenzo Stoakes wrote:
 > On Mon, Sep 15, 2025 at 12:22:07PM +0200, David Hildenbrand wrote:
> > 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?
>
> 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?
>

Another point here, since we have to keep:

/sys/kernel/mm/transparent_hugepage/khugepaged/max_ptes_none

Around, and users will try to set values there, presumably we will now add:

/sys/kernel/mm/transparent_hugepage/khugepaged/eagerness

How will we map <-> the two tunables?