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Date: Mon, 13 Jan 2025 16:00:45 -0700
From: Nico Pache <npache@...hat.com>
To: Dev Jain <dev.jain@....com>
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Subject: Re: [RFC 00/11] khugepaged: mTHP support

On Sun, Jan 12, 2025 at 7:12 AM Dev Jain <dev.jain@....com> wrote:
>
>
>
> On 11/01/25 3:31 am, Nico Pache wrote:
> > On Thu, Jan 9, 2025 at 9:56 PM Dev Jain <dev.jain@....com> wrote:
> >>
> >>
> >>
> >> On 10/01/25 7:57 am, Nico Pache wrote:
> >>> On Wed, Jan 8, 2025 at 11:22 PM Dev Jain <dev.jain@....com> wrote:
> >>>>
> >>>>
> >>>> On 09/01/25 5:01 am, Nico Pache wrote:
> >>>>> The following series provides khugepaged and madvise collapse with the
> >>>>> capability to collapse regions to mTHPs.
> >>>>>
> >>>>> To achieve this we generalize the khugepaged functions to no longer depend
> >>>>> on PMD_ORDER. Then during the PMD scan, we keep track of chunks of pages
> >>>>> (defined by MTHP_MIN_ORDER) that are fully utilized. This info is tracked
> >>>>> using a bitmap. 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. max_ptes_none is mapped to a 0-100 range to
> >>>>> determine how full a mTHP order needs to be before collapsing it.
> >>>>>
> >>>>> Some design choices to note:
> >>>>>     - bitmap structures are allocated dynamically because on some arch's
> >>>>>        (like PowerPC) the value of MTHP_BITMAP_SIZE cannot be computed at
> >>>>>        compile time leading to warnings.
> >>>>>     - The recursion is masked through a stack structure.
> >>>>>     - A MTHP_MIN_ORDER was added to compress the bitmap, and ensure it was
> >>>>>        64bit on x86. This provides some optimization on the bitmap operations.
> >>>>>        if other arches/configs that have larger than 512 PTEs per PMD want to
> >>>>>        compress their bitmap further we can change this value per arch.
> >>>>>
> >>>>> Patch 1-2:  Some refactoring to combine madvise_collapse and khugepaged
> >>>>> Patch 3:    A minor "fix"/optimization
> >>>>> Patch 4:    Refactor/rename hpage_collapse
> >>>>> Patch 5-7:  Generalize khugepaged functions for arbitrary orders
> >>>>> Patch 8-11: The mTHP patches
> >>>>>
> >>>>> This series acts as an alternative to Dev Jain's approach [1]. The two
> >>>>> series differ in a few ways:
> >>>>>      - My approach uses a bitmap to store the state of the linear scan_pmd to
> >>>>>        then determine potential mTHP batches. Devs incorporates his directly
> >>>>>        into the scan, and will try each available order.
> >>>>>      - Dev is attempting to optimize the locking, while my approach keeps the
> >>>>>        locking changes to a minimum. I believe his changes are not safe for
> >>>>>        uffd.
> >>>>>      - Dev's changes only work for khugepaged not madvise_collapse (although
> >>>>>        i think that was by choice and it could easily support madvise)
> >>>>>      - Dev scales all khugepaged sysfs tunables by order, while im removing
> >>>>>        the restriction of max_ptes_none and converting it to a scale to
> >>>>>        determine a (m)THP threshold.
> >>>>>      - Dev turns on khugepaged if any order is available while mine still
> >>>>>        only runs if PMDs are enabled. I like Dev's approach and will most
> >>>>>        likely do the same in my PATCH posting.
> >>>>>      - mTHPs need their ref count updated to 1<<order, which Dev is missing.
> >>>>>
> >>>>> Patch 11 was inspired by one of Dev's changes.
> >>>>>
> >>>>> [1] https://lore.kernel.org/lkml/20241216165105.56185-1-dev.jain@arm.com/
> >>>>>
> >>>>> Nico Pache (11):
> >>>>>      introduce khugepaged_collapse_single_pmd to collapse a single pmd
> >>>>>      khugepaged: refactor madvise_collapse and khugepaged_scan_mm_slot
> >>>>>      khugepaged: Don't allocate khugepaged mm_slot early
> >>>>>      khugepaged: rename hpage_collapse_* to khugepaged_*
> >>>>>      khugepaged: generalize hugepage_vma_revalidate for mTHP support
> >>>>>      khugepaged: generalize alloc_charge_folio for mTHP support
> >>>>>      khugepaged: generalize __collapse_huge_page_* for mTHP support
> >>>>>      khugepaged: introduce khugepaged_scan_bitmap for mTHP support
> >>>>>      khugepaged: add mTHP support
> >>>>>      khugepaged: remove max_ptes_none restriction on the pmd scan
> >>>>>      khugepaged: skip collapsing mTHP to smaller orders
> >>>>>
> >>>>>     include/linux/khugepaged.h |   4 +-
> >>>>>     mm/huge_memory.c           |   3 +-
> >>>>>     mm/khugepaged.c            | 436 +++++++++++++++++++++++++------------
> >>>>>     3 files changed, 306 insertions(+), 137 deletions(-)
> >>>>
> >>>> Before I take a proper look at your series, can you please include any testing
> >>>> you may have done?
> >>>
> >>> I Built these changes for the following arches: x86_64, arm64,
> >>> arm64-64k, ppc64le, s390x
> >>>
> >>> x86 testing:
> >>> - Selftests mm
> >>> - some stress-ng tests
> >>> - compile kernel
> >>> - I did some tests with my defer [1] set on top. This pushes all the
> >>> work to khugepaged, which removes the noise of all the PF allocations.
> >>>
> >>> I recently got an ARM64 machine and did some simple sanity tests (on
> >>> both 4k and 64k) like selftests, stress-ng, and playing around with
> >>> the tunables, etc.
> >>>
> >>> I will also be running all the builds through our CI, and perf testing
> >>> environments before posting.
> >>>
> >>> [1] https://lore.kernel.org/lkml/20240729222727.64319-1-npache@redhat.com/
> >>>
> >>>>
> >>>
> >> I tested your series with the program I was using and it is not working;
> >> can you please confirm it.
> >
> > Yes, this is expected because you are not fully filling any 32K chunk
> > (MIN_MTHP_ORDER) so no bit is ever set.
>
> That is weird, because if this is the case, then PMD-collapse should
> have also failed, but that succeeded. Do you have some userspace program
> I can test with?
Not exactly, if max_ptes_none is still 511, the old behavior is kept.

I modified your program to set the first 8 pages (32k chunk) in every
64k region.

#include <unistd.h>
#include <sys/ioctl.h>
#include <string.h>
#include <stdint.h>
#include <stdlib.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/mman.h>
#include <sys/time.h>
#include <sys/random.h>
#include <assert.h>

int main(int argc, char *argv[])
{
    char *ptr;
    unsigned long mthp_size = (1UL << 16); // 64 KB chunk size
    size_t chunk_size = (1UL << 25);       // 32 MB total size

    // mmap() to allocate memory at a specific address (1 GB address)
    ptr = mmap((void *)(1UL << 30), chunk_size, PROT_READ | PROT_WRITE,
               MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
    if (((unsigned long)ptr) != (1UL << 30)) {
        printf("mmap did not work on required address\n");
        return 1;
    }

    // Touch the first 8 pages in every 64 KB chunk
    for (int i = 0; i < chunk_size; i += mthp_size) {
        // Touch the first 8 pages within the 64 KB chunk (8 * 4 KB = 32 KB)
        for (int j = 0; j < 8; ++j) {
            ptr[i + j * 4096] = i + j * 4096;  // Touch the first byte
of each page
        }
    }

    // Use madvise() to advise the kernel to use huge pages for this memory
    if (madvise(ptr, chunk_size, MADV_HUGEPAGE)) {
        perror("madvise");
        return 1;
    }

    sleep(100); // Sleep to allow time for the kernel to process the advice
    return 0;
}

There's some rounding errors in how I compute the threshold_bits... I
think I will adopt how you do the max_ptes_none shifting for better
accuracy. Currently if you run this with max_ptes_none=255 (or even
lower values like 200...) it will still collapse to a 64k chunk when
in reality it should only do 32k because only half the bitmap is set
for this order, and 255 < 50% of 512.

I'm adding a threshold to the bitmap_set, and doing better scaling
like you do. My next version should handle the example code better.

>

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