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

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Message-ID: <c2f129c5-7a59-46b7-8fd6-0bb052e597ba@lucifer.local>
Date: Mon, 15 Sep 2025 12:13:38 +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 01:10:22PM +0200, David Hildenbrand wrote:
> On 15.09.25 12:59, Lorenzo Stoakes wrote:
> > On Mon, Sep 15, 2025 at 12:52:53PM +0200, David Hildenbrand wrote:
> > > On 15.09.25 12:48, Lorenzo Stoakes wrote:
> > > > On Mon, Sep 15, 2025 at 12:44:34PM +0200, David Hildenbrand wrote:
> > > > > > > > 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?
> > > > >
> > > > > Well, the easy case if someone updates eagerness, then we simply et it to
> > > > > whatever magic value we compute and document.
> > > > >
> > > > > The other direction is more problematic, likely we'll simply warn and do
> > > > > something reasonable (map it to whatever eagerness scale is closest or
> > > > > simply indicate it as "-1" -- user intervened or sth like that)
> > > >
> > > > I don't love the idea of a -1 situation, as that's going to create some
> > > > confusion.
> > >
> > > swapiness also has a "max" parameter, so we could just say "override" /"
> > > disabled" / whatever?
> >
> > I don't love the user being able to override this though, let's just nuke their
> > ability to set this pleeeease.
> >
> > Because if they can override it, then we have to do some deeply nasty scaling
> > for mTHP again.
>
> There are ways to have it working internally, just using a different "scale"
> instead of the 100 -> 50 -> 25 etc.

Right. I mean with the exponential scale we could just algorithimically figure
out what the eagerness should be.

>
> I am afraid we cannot change the parameter to ignore other values because of
> the interaction with the shrinker that easily .... we might be able to
> detracted at wait a bunch of kernel releases probably.

:(

BTW 'Detracted at wait'? :P You mean we might be able to remove after a few
releases?

>
> --
> Cheers
>
> David / dhildenb
>

Cheers, Lorenzo