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

[Zi Yan <ziy@nvidia.com>]

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On 3 Jun 2024, at 14:18, Yosry Ahmed wrote:

> 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?

Yes, you can remove it but please make sure the swapcache code below is right[1].

[1] https://elixir.bootlin.com/linux/v6.10-rc2/source/mm/huge_memory.c#L2868

Best Regards,
Yan, Zi

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