Re: [PATCH v8 00/16] move per-vma lock into vm_area_struct

[Suren Baghdasaryan <surenb@google.com>]

On Wed, Jan 8, 2025 at 6:30 PM Suren Baghdasaryan <surenb@google.com> wrote:
>
> Back when per-vma locks were introduces, vm_lock was moved out of
> vm_area_struct in [1] because of the performance regression caused by
> false cacheline sharing. Recent investigation [2] revealed that the
> regressions is limited to a rather old Broadwell microarchitecture and
> even there it can be mitigated by disabling adjacent cacheline
> prefetching, see [3].
> Splitting single logical structure into multiple ones leads to more
> complicated management, extra pointer dereferences and overall less
> maintainable code. When that split-away part is a lock, it complicates
> things even further. With no performance benefits, there are no reasons
> for this split. Merging the vm_lock back into vm_area_struct also allows
> vm_area_struct to use SLAB_TYPESAFE_BY_RCU later in this patchset.
> This patchset:
> 1. moves vm_lock back into vm_area_struct, aligning it at the cacheline
> boundary and changing the cache to be cacheline-aligned to minimize
> cacheline sharing;
> 2. changes vm_area_struct initialization to mark new vma as detached until
> it is inserted into vma tree;
> 3. replaces vm_lock and vma->detached flag with a reference counter;
> 4. changes vm_area_struct cache to SLAB_TYPESAFE_BY_RCU to allow for their
> reuse and to minimize call_rcu() calls.
>
> Pagefault microbenchmarks show performance improvement:
> Hmean     faults/cpu-1    507926.5547 (   0.00%)   506519.3692 *  -0.28%*
> Hmean     faults/cpu-4    479119.7051 (   0.00%)   481333.6802 *   0.46%*
> Hmean     faults/cpu-7    452880.2961 (   0.00%)   455845.6211 *   0.65%*
> Hmean     faults/cpu-12   347639.1021 (   0.00%)   352004.2254 *   1.26%*
> Hmean     faults/cpu-21   200061.2238 (   0.00%)   229597.0317 *  14.76%*
> Hmean     faults/cpu-30   145251.2001 (   0.00%)   164202.5067 *  13.05%*
> Hmean     faults/cpu-48   106848.4434 (   0.00%)   120641.5504 *  12.91%*
> Hmean     faults/cpu-56    92472.3835 (   0.00%)   103464.7916 *  11.89%*
> Hmean     faults/sec-1    507566.1468 (   0.00%)   506139.0811 *  -0.28%*
> Hmean     faults/sec-4   1880478.2402 (   0.00%)  1886795.6329 *   0.34%*
> Hmean     faults/sec-7   3106394.3438 (   0.00%)  3140550.7485 *   1.10%*
> Hmean     faults/sec-12  4061358.4795 (   0.00%)  4112477.0206 *   1.26%*
> Hmean     faults/sec-21  3988619.1169 (   0.00%)  4577747.1436 *  14.77%*
> Hmean     faults/sec-30  3909839.5449 (   0.00%)  4311052.2787 *  10.26%*
> Hmean     faults/sec-48  4761108.4691 (   0.00%)  5283790.5026 *  10.98%*
> Hmean     faults/sec-56  4885561.4590 (   0.00%)  5415839.4045 *  10.85%*
>
> Changes since v7 [4]:
> - Removed additional parameter for vma_iter_store() and introduced
> vma_iter_store_attached() instead, per Vlastimil Babka and
> Liam R. Howlett
> - Fixed coding style nits, per Vlastimil Babka
> - Added Reviewed-bys and Acked-bys, per Vlastimil Babka
> - Added Reviewed-bys and Acked-bys, per Liam R. Howlett
> - Added Acked-by, per Davidlohr Bueso
> - Removed unnecessary patch changeing nommu.c
> - Folded a fixup patch [5] into the patch it was fixing
> - Changed calculation in __refcount_add_not_zero_limited() to avoid
> overflow, to change the limit to be inclusive and to use INT_MAX to
> indicate no limits, per Vlastimil Babka and Matthew Wilcox
> - Folded a fixup patch [6] into the patch it was fixing
> - Added vm_refcnt rules summary in the changelog, per Liam R. Howlett
> - Changed writers to not increment vm_refcnt and adjusted VMA_REF_LIMIT
> to not reserve one count for a writer, per Liam R. Howlett
> - Changed vma_refcount_put() to wake up writers only when the last reader
> is leaving, per Liam R. Howlett
> - Fixed rwsem_acquire_read() parameters when read-locking a vma to match
> the way down_read_trylock() does lockdep, per Vlastimil Babka
> - Folded vma_lockdep_init() into vma_lock_init() for simplicity
> - Brought back vma_copy() to keep vm_refcount at 0 during reuse,
> per Vlastimil Babka
>
> What I did not include in this patchset:
> - Liam's suggestion to change dump_vma() output since it's unclear to me
> how it should look like. The patch is for debug only and not critical for
> the rest of the series, we can change the output later or even drop it if
> necessary.
>
> [1] https://lore.kernel.org/all/20230227173632.3292573-34-surenb@google.com/
> [2] https://lore.kernel.org/all/ZsQyI%2F087V34JoIt@xsang-OptiPlex-9020/
> [3] https://lore.kernel.org/all/CAJuCfpEisU8Lfe96AYJDZ+OM4NoPmnw9bP53cT_kbfP_pR+-2g@mail.gmail.com/
> [4] https://lore.kernel.org/all/20241226170710.1159679-1-surenb@google.com/
> [5] https://lore.kernel.org/all/20250107030415.721474-1-surenb@google.com/
> [6] https://lore.kernel.org/all/20241226200335.1250078-1-surenb@google.com/
>
> Patchset applies over mm-unstable after reverting v7
> (current SHA range: 588f0086398e - fb2270654630)

^^^ Please note that to apply this patchset over mm-unstable you
should revert the previous version. Thanks!

>
> Suren Baghdasaryan (16):
>   mm: introduce vma_start_read_locked{_nested} helpers
>   mm: move per-vma lock into vm_area_struct
>   mm: mark vma as detached until it's added into vma tree
>   mm: introduce vma_iter_store_attached() to use with attached vmas
>   mm: mark vmas detached upon exit
>   types: move struct rcuwait into types.h
>   mm: allow vma_start_read_locked/vma_start_read_locked_nested to fail
>   mm: move mmap_init_lock() out of the header file
>   mm: uninline the main body of vma_start_write()
>   refcount: introduce __refcount_{add|inc}_not_zero_limited
>   mm: replace vm_lock and detached flag with a reference count
>   mm/debug: print vm_refcnt state when dumping the vma
>   mm: remove extra vma_numab_state_init() call
>   mm: prepare lock_vma_under_rcu() for vma reuse possibility
>   mm: make vma cache SLAB_TYPESAFE_BY_RCU
>   docs/mm: document latest changes to vm_lock
>
>  Documentation/mm/process_addrs.rst |  44 +++++----
>  include/linux/mm.h                 | 152 ++++++++++++++++++++++-------
>  include/linux/mm_types.h           |  36 ++++---
>  include/linux/mmap_lock.h          |   6 --
>  include/linux/rcuwait.h            |  13 +--
>  include/linux/refcount.h           |  20 +++-
>  include/linux/slab.h               |   6 --
>  include/linux/types.h              |  12 +++
>  kernel/fork.c                      | 128 +++++++++++-------------
>  mm/debug.c                         |  12 +++
>  mm/init-mm.c                       |   1 +
>  mm/memory.c                        |  94 +++++++++++++++---
>  mm/mmap.c                          |   3 +-
>  mm/userfaultfd.c                   |  32 +++---
>  mm/vma.c                           |  23 ++---
>  mm/vma.h                           |  15 ++-
>  tools/testing/vma/linux/atomic.h   |   5 +
>  tools/testing/vma/vma_internal.h   |  93 ++++++++----------
>  18 files changed, 435 insertions(+), 260 deletions(-)
>
> --
> 2.47.1.613.gc27f4b7a9f-goog
>