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

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

On Mon, Sep 15, 2025 at 01:01:26PM +0100, Kiryl Shutsemau wrote:
> On Mon, Sep 15, 2025 at 01:45:39PM +0200, David Hildenbrand wrote:
> > On 15.09.25 13:35, Lorenzo Stoakes wrote:
> > > On Mon, Sep 15, 2025 at 01:29:22PM +0200, David Hildenbrand wrote:
> > > > On 15.09.25 13:23, Lorenzo Stoakes wrote:
> > > > > On Mon, Sep 15, 2025 at 01:14:32PM +0200, David Hildenbrand wrote:
> > > > > > On 15.09.25 13:02, Lorenzo Stoakes wrote:
> > > > > > > On Mon, Sep 15, 2025 at 12:52:03PM +0200, David Hildenbrand wrote:
> > > > > > > > On 15.09.25 12:43, Lorenzo Stoakes wrote:
> > > > > > > > > On Mon, Sep 15, 2025 at 12:22:07PM +0200, David Hildenbrand wrote:
> > > > > > > > > >
> > > > > > > > > > 0 -> ~100% used (~0% none)
> > > > > > > > > > 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)
> > > > > > > > >
> > > > > > > > > Oh and shouldn't this be inverted?
> > > > > > > > >
> > > > > > > > > 0 eagerness = we eat up all none PTE entries? Isn't that pretty eager? :P
> > > > > > > > > 10 eagerness = we aren't eager to eat up none PTE entries at all?
> > > > > > > > >
> > > > > > > > > Or am I being dumb here?
> > > > > > > >
> > > > > > > > Good question.
> > > > > > > >
> > > > > > > > For swappiness it's: 0 -> no swap (conservative)
> > > > > > > >
> > > > > > > > So intuitively I assumed: 0 -> no pte_none (conservative)
> > > > > > > >
> > > > > > > > You're the native speaker, so you tell me :)
> > > > > > >
> > > > > > > To me this is about 'eagerness to consume empty PTE entries' so 10 is more
> > > > > > > eager, 0 is not eager at all, i.e. inversion of what you suggest :)
> > > > > >
> > > > > > Just so we are on the same page: it is about "eagerness to collapse", right?
> > > > > >
> > > > > > Wouldn't a 0 mean "I am not eager, I will not waste any memory, I am very
> > > > > > careful and bail out on any pte_none" vs. 10 meaning "I am very eager, I
> > > > > > will collapse no matter what I find in the page table, waste as much memory
> > > > > > as I want"?
> > > > >
> > > > > Yeah, this is my understanding of your scale, or is my understanding also
> > > > > inverted? :)
> > > > >
> > > > > Right now it's:
> > > > >
> > > > > eagerness max_ptes_none
> > > > >
> > > > > 0 -> 511
> > > > > ...
> > > > > 10 -> 0
> > > > >
> > > > > Right?
> > > >
> > > > Just so we are on the same page, this is what I had:
> > > >
> > > > 0 -> ~100% used (~0% none)
> > > >
> > > > So "0" -> 0 pte_none or 512 used.
> > > >
> > > > (note the used vs. none)
> > >
> > > OK right so we're talking about the same thing, I guess?
> > >
> > > I was confused partly becuase of the scale, becuase weren't people setting
> > > this parameter to low values in practice?
> > >
> > > And now we make it so we have equivalent of:
> > >
> > > 0 -> 0
> > > 1 -> 256
> > > 2 -> 384
> >
> > Ah, there is the problem, that's not what I had in mind.
> >
> > 0 -> ~100% used (~0% none)
> > ...
> > 8 -> ~87,5% used (~12.5% none)
> > 9 -> ~75% used (~25% none)
> > 9 -> ~50% used (~50% none)
> > 10 -> ~0% used (~100% none)
> >
> > Hopefully I didn't mess it up again.
>
> I think this kind of table is fine for initial implementation of the
> knob, but we don't want to document it to userspace like this.
> I think we want to be strategically ambiguous on what the knob does
> exactly, so kernel could evolve the meaning of the knob over time.
>
> We don't want to repeat the problem we have with max_ptes_none which too
> prescriptive and got additional meaning with introduction of shrinker.
>
> As kernel evolves, we want ability to adjust the meaning and keep the
> knob useful.

I mean, having said this exact thing several times in the thread obviously
I agree... FWIW...

To repeat, I think it should be an abstraction that we entirely control and
whose meaning we can vary over time.