On 11/02/2025 00:30, 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 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 will be scaled by the attempted collapse > order to determine how full a THP must be to be eligible. If a mTHP collapse > is attempted, but contains swapped out, or shared pages, we dont perform the > collapse. > > With the default max_ptes_none=511, the code should keep its most of its > original behavior. To exercise mTHP collapse we need to set max_ptes_none<=255. > With max_ptes_none > HPAGE_PMD_NR/2 you will experience collapse "creep" and nit: I think you mean "max_ptes_none >= HPAGE_PMD_NR/2" (greater or *equal*)? This is making my head hurt, but I *think* I agree with you that if max_ptes_none is less than half of the number of ptes in a pmd, then creep doesn't happen. To make sure I've understood; - to collapse to 16K, you would need >=3 out of 4 PTEs to be present - to collapse to 32K, you would need >=5 out of 8 PTEs to be present - to collapse to 64K, you would need >=9 out of 16 PTEs to be present - ... So if we start with 3 present PTEs in a 16K area, we collapse to 16K and now have 4 PTEs in a 32K area which is insufficient to collapse to 32K. Sounds good to me! > constantly promote mTHPs to the next available size. > > Patch 1: Some refactoring to combine madvise_collapse and khugepaged > Patch 2: Refactor/rename hpage_collapse > Patch 3-5: Generalize khugepaged functions for arbitrary orders > Patch 6-9: The mTHP patches > > --------- > Testing > --------- > - Built for x86_64, aarch64, ppc64le, and s390x > - selftests mm > - I created a test script that I used to push khugepaged to its limits while > monitoring a number of stats and tracepoints. The code is available > here[1] (Run in legacy mode for these changes and set mthp sizes to inherit) > The summary from my testings was that there was no significant regression > noticed through this test. In some cases my changes had better collapse > latencies, and was able to scan more pages in the same amount of time/work, > but for the most part the results were consistant. > - redis testing. I tested these changes along with my defer changes > (see followup post for more details). > - some basic testing on 64k page size. > - lots of general use. These changes have been running in my VM for some time. > > Changes since V1 [2]: > - Minor bug fixes discovered during review and testing > - removed dynamic allocations for bitmaps, and made them stack based > - Adjusted bitmap offset from u8 to u16 to support 64k pagesize. > - Updated trace events to include collapsing order info. > - Scaled max_ptes_none by order rather than scaling to a 0-100 scale. > - No longer require a chunk to be fully utilized before setting the bit. Use > the same max_ptes_none scaling principle to achieve this. > - Skip mTHP collapse that requires swapin or shared handling. This helps prevent > some of the "creep" that was discovered in v1. > > [1] - https://gitlab.com/npache/khugepaged_mthp_test > [2] - https://lore.kernel.org/lkml/20250108233128.14484-1-npache@xxxxxxxxxx/ > > Nico Pache (9): > introduce khugepaged_collapse_single_pmd to unify khugepaged and > madvise_collapse > 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: improve tracepoints for mTHP orders > khugepaged: skip collapsing mTHP to smaller orders > > include/linux/khugepaged.h | 4 + > include/trace/events/huge_memory.h | 34 ++- > mm/khugepaged.c | 422 +++++++++++++++++++---------- > 3 files changed, 306 insertions(+), 154 deletions(-) >
