Re: [PATCH v7 0/7] Swap-out mTHP without splitting

[Yosry Ahmed <yosryahmed@google.com>]

On Mon, Apr 8, 2024 at 11:40 AM Ryan Roberts <ryan.roberts@arm.com> wrote:
>
> Hi All,
>
> This series adds support for swapping out multi-size THP (mTHP) without needing
> to first split the large folio via split_huge_page_to_list_to_order(). It
> closely follows the approach already used to swap-out PMD-sized THP.
>
> There are a couple of reasons for swapping out mTHP without splitting:
>
>   - Performance: It is expensive to split a large folio and under extreme memory
>     pressure some workloads regressed performance when using 64K mTHP vs 4K
>     small folios because of this extra cost in the swap-out path. This series
>     not only eliminates the regression but makes it faster to swap out 64K mTHP
>     vs 4K small folios.
>
>   - Memory fragmentation avoidance: If we can avoid splitting a large folio
>     memory is less likely to become fragmented, making it easier to re-allocate
>     a large folio in future.
>
>   - Performance: Enables a separate series [7] to swap-in whole mTHPs, which
>     means we won't lose the TLB-efficiency benefits of mTHP once the memory has
>     been through a swap cycle.
>
> I've done what I thought was the smallest change possible, and as a result, this
> approach is only employed when the swap is backed by a non-rotating block device
> (just as PMD-sized THP is supported today). Discussion against the RFC concluded
> that this is sufficient.
>
>
> Performance Testing
> ===================
>
> I've run some swap performance tests on Ampere Altra VM (arm64) with 8 CPUs. The
> VM is set up with a 35G block ram device as the swap device and the test is run
> from inside a memcg limited to 40G memory. I've then run `usemem` from
> vm-scalability with 70 processes, each allocating and writing 1G of memory. I've
> repeated everything 6 times and taken the mean performance improvement relative
> to 4K page baseline:
>
> | alloc size |                baseline |           + this series |
> |            | mm-unstable (~v6.9-rc1) |                         |
> |:-----------|------------------------:|------------------------:|
> | 4K Page    |                    0.0% |                    1.3% |
> | 64K THP    |                  -13.6% |                   46.3% |
> | 2M THP     |                   91.4% |                   89.6% |
>
> So with this change, the 64K swap performance goes from a 14% regression to a
> 46% improvement. While 2M shows a small regression I'm confident that this is
> just noise.
>
> ---
> The series applies against mm-unstable (as of 2024-04-08) after dropping v6 of
> this series from it. The performance numbers are from v5. Since the delta is
> very small I don't anticipate any performance changes. I'm optimistically hoping
> this is the final version.
>
>
> Changes since v6 [6]
> ====================
>
>   - patch #1
>     - swap_page_trans_huge_swapped() takes order instead of nr_pages (per Chris)
>   - patch #2
>     - Fix bug in swap_pte_batch() to consider swp pte bits (per David)
>     - Improved docs for clear_not_present_full_ptes() (per David)
>     - Improved docs for free_swap_and_cache_nr() (per David)
>   - patch #5
>     - Split out change to get_swap_pages() interface into own patch (per David)
>   - patch #6 (was patch #5)
>     - Improved readability of shrink_folio_list() with longer lines (per David)
>
>
> Changes since v5 [5]
> ====================
>
>   - patch #2
>     - Don't bother trying to reclaim swap if none of the entries' refs have gone
>       to 0 in free_swap_and_cache_nr() (per Huang, Ying)
>   - patch #5
>     - Only update THP_SWPOUT_FALLBACK counters for pmd-mappable folios (per
>       Barry Song)
>   - patch #6
>     - Fix bug in madvise_cold_or_pageout_pte_range(): don't continue without ptl
>       (reported by Barry [8], sysbot [9])
>
>
> Changes since v4 [4]
> ====================
>
>   - patch #3:
>     - Added R-B from Huang, Ying - thanks!
>   - patch #4:
>     - get_swap_pages() now takes order instead of nr_pages (per Huang, Ying)
>     - Removed WARN_ON_ONCE() from get_swap_pages()
>     - Reworded comment for scan_swap_map_try_ssd_cluster() (per Huang, Ying)
>     - Unified VM_WARN_ON()s in scan_swap_map_slots() to scan: (per Huang, Ying)
>     - Removed redundant "order == 0" check (per Huang, Ying)
>   - patch #5:
>     - Marked list_empty() check with data_race() (per David)
>     - Added R-B from Barry and David - thanks!
>   - patch #6:
>     - Implemented mkold_ptes() generic helper (pre David)
>     - Enhanced folio_pte_batch() to report any_young (per David)
>     - madvise_cold_or_pageout_pte_range() sets old in batch (per David)
>     - Added R-B from Barry - thanks!
>
>
> Changes since v3 [3]
> ====================
>
>  - Renamed SWAP_NEXT_NULL -> SWAP_NEXT_INVALID (per Huang, Ying)
>  - Simplified max offset calculation (per Huang, Ying)
>  - Reinstated struct percpu_cluster to contain per-cluster, per-order `next`
>    offset (per Huang, Ying)
>  - Removed swap_alloc_large() and merged its functionality into
>    scan_swap_map_slots() (per Huang, Ying)
>  - Avoid extra cost of folio ref and lock due to removal of CLUSTER_FLAG_HUGE
>    by freeing swap entries in batches (see patch 2) (per DavidH)
>  - vmscan splits folio if its partially mapped (per Barry Song, DavidH)
>  - Avoid splitting in MADV_PAGEOUT path (per Barry Song)
>  - Dropped "mm: swap: Simplify ssd behavior when scanner steals entry" patch
>    since it's not actually a problem for THP as I first thought.
>
>
> Changes since v2 [2]
> ====================
>
>  - Reuse scan_swap_map_try_ssd_cluster() between order-0 and order > 0
>    allocation. This required some refactoring to make everything work nicely
>    (new patches 2 and 3).
>  - Fix bug where nr_swap_pages would say there are pages available but the
>    scanner would not be able to allocate them because they were reserved for the
>    per-cpu allocator. We now allow stealing of order-0 entries from the high
>    order per-cpu clusters (in addition to exisiting stealing from order-0
>    per-cpu clusters).
>
>
> Changes since v1 [1]
> ====================
>
>  - patch 1:
>     - Use cluster_set_count() instead of cluster_set_count_flag() in
>       swap_alloc_cluster() since we no longer have any flag to set. I was unable
>       to kill cluster_set_count_flag() as proposed against v1 as other call
>       sites depend explicitly setting flags to 0.
>  - patch 2:
>     - Moved large_next[] array into percpu_cluster to make it per-cpu
>       (recommended by Huang, Ying).
>     - large_next[] array is dynamically allocated because PMD_ORDER is not
>       compile-time constant for powerpc (fixes build error).
>
>
> [1] https://lore.kernel.org/linux-mm/20231010142111.3997780-1-ryan.roberts@arm.com/
> [2] https://lore.kernel.org/linux-mm/20231017161302.2518826-1-ryan.roberts@arm.com/
> [3] https://lore.kernel.org/linux-mm/20231025144546.577640-1-ryan.roberts@arm.com/
> [4] https://lore.kernel.org/linux-mm/20240311150058.1122862-1-ryan.roberts@arm.com/
> [5] https://lore.kernel.org/linux-mm/20240327144537.4165578-1-ryan.roberts@arm.com/
> [6] https://lore.kernel.org/linux-mm/20240403114032.1162100-1-ryan.roberts@arm.com/
> [7] https://lore.kernel.org/linux-mm/20240304081348.197341-1-21cnbao@gmail.com/
> [8] https://lore.kernel.org/linux-mm/CAGsJ_4yMOow27WDvN2q=E4HAtDd2PJ=OQ5Pj9DG+6FLWwNuXUw@mail.gmail.com/
> [9] https://lore.kernel.org/linux-mm/579d5127-c763-4001-9625-4563a9316ac3@redhat.com/
>
> Thanks,
> Ryan
>
> Ryan Roberts (7):
>   mm: swap: Remove CLUSTER_FLAG_HUGE from swap_cluster_info:flags
>   mm: swap: free_swap_and_cache_nr() as batched free_swap_and_cache()
>   mm: swap: Simplify struct percpu_cluster
>   mm: swap: Update get_swap_pages() to take folio order
>   mm: swap: Allow storage of all mTHP orders
>   mm: vmscan: Avoid split during shrink_folio_list()
>   mm: madvise: Avoid split during MADV_PAGEOUT and MADV_COLD

+Zi Yan

While looking at the page splitting code, I noticed that
split_huge_page_to_list_to_order() will refuse to split a folio in the
swapcache to any order higher than 0. It has the following check:

if (new_order) {
        /* Only swapping a whole PMD-mapped folio is supported */
        if (folio_test_swapcache(folio))
                return -EINVAL;
        ...
}

I am guessing with this series this may no longer be applicable?

>
>  include/linux/pgtable.h |  59 ++++++++
>  include/linux/swap.h    |  35 +++--
>  mm/huge_memory.c        |   3 -
>  mm/internal.h           |  75 +++++++++-
>  mm/madvise.c            |  99 +++++++-----
>  mm/memory.c             |  17 ++-
>  mm/swap_slots.c         |   6 +-
>  mm/swapfile.c           | 325 +++++++++++++++++++++++-----------------
>  mm/vmscan.c             |  20 +--
>  9 files changed, 422 insertions(+), 217 deletions(-)
>
> --
> 2.25.1
>
>