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Date: Fri, 10 Jan 2025 15:01:01 -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 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.
I should probably add some threshold to scan_pmd so we set the bitmap
if at least half of the region is full or scale it on max_ptes_none
like I do in scan_bitmap.

Thanks for your review-- Have a good weekend!
>
> diff --git a/mytests/mthp.c b/mytests/mthp.c
> new file mode 100644
> index 000000000000..e3029dbcf035
> --- /dev/null
> +++ b/mytests/mthp.c
> @@ -0,0 +1,45 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +/*
> + *
> + * Author: Dev Jain <dev.jain@....com>
> + *
> + * Program to test khugepaged mTHP collapse
> + */
> +
> +#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);
> +       size_t chunk_size = (1UL << 25);
> +
> +       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;
> +       }
> +
> +       /* Fill first pte in every 64K interval */
> +       for (int i = 0; i < chunk_size; i += mthp_size)
> +               ptr[i] = i;
> +
> +       if (madvise(ptr, chunk_size, MADV_HUGEPAGE)) {
> +               perror("madvise");
> +               return 1;
> +       }
> +       sleep(100);
> +       return 0;
> +}
> --
> 2.30.2
>
> Set enabled = madvise, hugepages-2048k/enabled = hugepages-64k/enabled =
> inherit. Run the program in the background, then run tools/mm/thpmaps.
> You will see PMD collapse correctly, but when you echo never into
> hugepages-2048k/enabled and test this again, you won't see contpte 64K
> collapse. With my series, you will see something like
>
> anon-cont-pte-aligned-64kB : 32768 kB (100%).
>

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