Re: [PATCH v9 00/17] reimplement per-vma lock as a refcount

Shivank Garg
2025-01-28T05:27:15Z

Hi Suren,

I tested the patch-series on AMD EPYC Zen 5 system
(2-socket, 64-core per socket with SMT Enabled, 4 NUMA nodes)
using mmtests's PFT (config-workload-pft-threads) benchmark on
mm-unstable.

On 1/11/2025 9:55 AM, Suren Baghdasaryan 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. regroups vm_area_struct members to fit them into 3 cachelines;
> 5. 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%*

Results:
mm-unstable-vanilla mm-unstable-v9-pervma-lock
Hmean faults/cpu-1 2018530.3023 ( 0.00%) 2051456.8039 ( 1.63%)
Hmean faults/cpu-4 718869.0234 ( 0.00%) 720985.6986 ( 0.29%)
Hmean faults/cpu-7 377965.1187 ( 0.00%) 368305.2802 ( -2.56%)
Hmean faults/cpu-12 215502.5334 ( 0.00%) 212764.0061 ( -1.27%)
Hmean faults/cpu-21 149946.1873 ( 0.00%) 150688.2173 ( 0.49%)
Hmean faults/cpu-30 142379.7863 ( 0.00%) 143677.5012 ( 0.91%)
Hmean faults/cpu-48 139625.5003 ( 0.00%) 156217.1383 * 11.88%*
Hmean faults/cpu-79 119093.6132 ( 0.00%) 140501.1787 * 17.98%*
Hmean faults/cpu-110 102446.6879 ( 0.00%) 114128.7155 ( 11.40%)
Hmean faults/cpu-128 96640.4022 ( 0.00%) 109474.8875 ( 13.28%)
Hmean faults/sec-1 2018197.4666 ( 0.00%) 2051119.1375 ( 1.63%)
Hmean faults/sec-4 2853494.9619 ( 0.00%) 2865639.8350 ( 0.43%)
Hmean faults/sec-7 2631049.4283 ( 0.00%) 2564037.1696 ( -2.55%)
Hmean faults/sec-12 2570378.4204 ( 0.00%) 2540353.2525 ( -1.17%)
Hmean faults/sec-21 3018640.3933 ( 0.00%) 3014396.9773 ( -0.14%)
Hmean faults/sec-30 4150723.9209 ( 0.00%) 4167550.4070 ( 0.41%)
Hmean faults/sec-48 6459327.6559 ( 0.00%) 7217660.4385 * 11.74%*
Hmean faults/sec-79 8977397.1421 ( 0.00%) 10695351.6214 * 19.14%*
Hmean faults/sec-110 10590055.2262 ( 0.00%) 12309035.9250 ( 16.23%)
Hmean faults/sec-128 11448246.6485 ( 0.00%) 13554648.3823 ( 18.40%)

Please add my:
Tested-by: Shivank Garg <shivankg@amd.com>

I would be happy to test future versions if needed.

Thanks,
Shivank

>
> Changes since v8 [4]:
> - Change subject for the cover letter, per Vlastimil Babka
> - Added Reviewed-by and Acked-by, per Vlastimil Babka
> - Added static check for no-limit case in __refcount_add_not_zero_limited,
> per David Laight
> - Fixed vma_refcount_put() to call rwsem_release() unconditionally,
> per Hillf Danton and Vlastimil Babka
> - Use a copy of vma->vm_mm in vma_refcount_put() in case vma is freed from
> under us, per Vlastimil Babka
> - Removed extra rcu_read_lock()/rcu_read_unlock() in vma_end_read(),
> per Vlastimil Babka
> - Changed __vma_enter_locked() parameter to centralize refcount logic,
> per Vlastimil Babka
> - Amended description in vm_lock replacement patch explaining the effects
> of the patch on vm_area_struct size, per Vlastimil Babka
> - Added vm_area_struct member regrouping patch [5] into the series
> - Renamed vma_copy() into vm_area_init_from(), per Liam R. Howlett
> - Added a comment for vm_area_struct to update vm_area_init_from() when
> adding new members, per Vlastimil Babka
> - Updated a comment about unstable src->shared.rb when copying a vma in
> vm_area_init_from(), per Vlastimil Babka
>
> [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/20250109023025.2242447-1-surenb@google.com/
> [5] https://lore.kernel.org/all/20241111205506.3404479-5-surenb@google.com/
>
> Patchset applies over mm-unstable after reverting v8
> (current SHA range: 235b5129cb7b - 9e6b24c58985)
>
> Suren Baghdasaryan (17):
> 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: move lesser used vma_area_struct members into the last cacheline
> 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 | 156 ++++++++++++++++++++++-------
> include/linux/mm_types.h | 75 +++++++-------
> include/linux/mmap_lock.h | 6 --
> include/linux/rcuwait.h | 13 +--
> include/linux/refcount.h | 24 ++++-
> include/linux/slab.h | 6 --
> include/linux/types.h | 12 +++
> kernel/fork.c | 129 +++++++++++-------------
> mm/debug.c | 12 +++
> mm/init-mm.c | 1 +
> mm/memory.c | 97 ++++++++++++++++--
> 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, 465 insertions(+), 281 deletions(-)
>