On Fri, Sep 12, 2025 at 11:53 AM David Hildenbrand <david@xxxxxxxxxx> wrote: > > On 12.09.25 17:51, Lorenzo Stoakes wrote: > > On Fri, Sep 12, 2025 at 05:45:26PM +0200, David Hildenbrand wrote: > >> On 12.09.25 17:41, Kiryl Shutsemau wrote: > >>> On Fri, Sep 12, 2025 at 04:56:47PM +0200, David Hildenbrand wrote: > >>>> On 12.09.25 16:35, Kiryl Shutsemau wrote: > >>>>> On Fri, Sep 12, 2025 at 04:28:09PM +0200, David Hildenbrand wrote: > >>>>>> On 12.09.25 15:47, David Hildenbrand 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. > >>>>>>> > >>>>>>> BTW, I thought further about this and I agree: if we count zero-filled > >>>>>>> pages towards none_or_zero one we can avoid the "creep" problem. > >>>>>>> > >>>>>>> The scanning-for-zero part is rather nasty, though. > >>>>>> > >>>>>> Aaand, thinking again from the other direction, this would mean that just > >>>>>> because pages became zero after some time that we would no longer collapse > >>>>>> because none_or_zero would then be higher. Hm .... > >>>>>> > >>>>>> How I hate all of this so very very much :) > >>>>> > >>>>> This is not new. Shrinker has the same problem: it cannot distinguish > >>>>> between hot 4k that happened to be zero from the 4k that is there just > >>>>> because of we faulted in 2M a time. > >>>> > >>>> Right. And so far that problem is isolated to the shrinker. > >>>> > >>>> To me so far "none_or_zero" really meant "will I consume more memory when > >>>> collapsing". That's not true for zero-filled pages, obviously. > >>> > >>> Well, KSM can reclaim these zero-filled memory until we collapse it. > >> > >> KSM is used so rarely (for good reasons) that I would never ever build an > >> argument based on its existence :P > >> > >> But yes: during the very first shrinker discussion I raised that KSM can do > >> the same thing. Obviously that was not good enough. > >> > >> -- > >> Cheers > >> > >> David / dhildenb > >> > > > > With all this stuff said, do we have an actual plan for what we intend to do > > _now_? > > Oh no, no I have to use my brain and it's Friday evening. > > > > > As Nico has implemented a basic solution here that we all seem to agree is not > > what we want. > > > > Without needing special new hardware or major reworks, what would this parameter > > look like? > > > > What would the heuristics be? What about the eagerness scales? > > > > I'm but a simple kernel developer, > > :) > > and interested in simple pragmatic stuff :) > > do you have a plan right now David? > > Ehm, if you ask me that way ... > > > > > Maybe we can start with something simple like a rough percentage per eagerness > > entry that then gets scaled based on utilisation? > > ... I think we should probably: > > 1) Start with something very simple for mTHP that doesn't lock us into any particular direction. > > 2) Add an "eagerness" parameter with fixed scale and use that for mTHP as well I think the best design is to map to different max_ptes_none values, 0-5: 0,32,64,128,255,511 > > 3) Improve that "eagerness" algorithm using a dynamic scale or #whatever > > 4) Solve world peace and world hunger > > 5) Connect it all to memory pressure / reclaim / shrinker / heuristics / hw hotness / #whatever > > > I maintain my initial position that just using > > max_ptes_none == 511 -> collapse mTHP always > max_ptes_none != 511 -> collapse mTHP only if we all PTEs are non-none/zero I think we should implement the eagerness toggle, and map it to different max_pte_none values like I described above. This fits nicely in the current collapse_max_ptes_none() function. If we go with just 0/511 without the eagerness changes, we will be removing configurability, only to reintroduce it again. When we can leave the configurability from the start. > > As a starting point is probably simple and best, and likely leaves room for any > changes later. > > > Of course, we could do what Nico is proposing here, as 1) and change it all later. I dont think this is much different than the eagerness approach; it just compresses the max_ptes_none from 0-512 to 0-5/10. I will wait for your RFC for the next version. Does your implementation/thoughts align with what I describe above? > > It's just when it comes to documenting all that stuff in patch #15 that I feel like > "alright, we shouldn't be doing it longterm like that, so let's not make anybody > depend on any weird behavior here by over-domenting it". > > I mean > > " > +To prevent "creeping" behavior where collapses continuously promote to larger > +orders, if max_ptes_none >= HPAGE_PMD_NR/2 (255 on 4K page size), it is > +capped to HPAGE_PMD_NR/2 - 1 for mTHP collapses. This is due to the fact > +that introducing more than half of the pages to be non-zero it will always > +satisfy the eligibility check on the next scan and the region will be collapse. > " > > Is just way, way to detailed. > > I would just say "The kernel might decide to use a more conservative approach > when collapsing smaller THPs" etc. Sounds good I can make it more ambiguous! Cheers. -- Nico > > > Thoughts? > > -- > Cheers > > David / dhildenb >
