Re: [RFC 00/11] khugepaged: mTHP support

From: Dev Jain
Date: Thu Jan 09 2025 - 23:56:46 EST



On 10/01/25 7:57 am, Nico Pache wrote:
On Wed, Jan 8, 2025 at 11:22 PM Dev Jain <dev.jain@xxxxxxx> 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@xxxxxxx/

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@xxxxxxxxxx/



I tested your series with the program I was using and it is not working; can you please confirm it.

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@xxxxxxx>
+ *
+ * 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%).