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 <[email protected]> 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
