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%*
Changes since v9 [4]:
PATCH [4/18]
- Change VM_BUG_ON_VMA() to WARN_ON_ONCE() in vma_assert_{attached|detached},
per Lorenzo Stoakes
- Rename vma_iter_store() into vma_iter_store_new(), per Lorenzo Stoakes
- Expand changelog, per Lorenzo Stoakes
- Update vma tests to check for vma detached state correctness,
per Lorenzo Stoakes
PATCH [5/18]
- Add Reviewed-by, per Lorenzo Stoakes
PATCH [6/18]
- Add Acked-by, per Lorenzo Stoakes
PATCH [7/18]
- Refactor the code, per Lorenzo Stoakes
- Remove Vlastimil's Acked-by since code is changed
PATCH [8/18]
- Drop inline for mmap_init_lock(), per Lorenzo Stoakes
- Add Reviewed-by, per Lorenzo Stoakes
PATCH [9/18]
- Add Reviewed-by, per Lorenzo Stoakes
PATCH [10/18]
- New patch to add refcount_add_not_zero_acquire/refcount_set_release
- Add Acked-by #slab, per Vlastimil Babka
PATCH [11/18]
- Change refcount limit to be used with xxx_acquire functions
PATCH [12/18]
- Use __refcount_inc_not_zero_limited_acquire() in vma_start_read(),
per Hillf Danton
- Refactor vma_assert_locked() to avoid vm_refcnt read when CONFIG_DEBUG_VM=n,
per Mateusz Guzik
- Update changelog, per Wei Yang
- Change vma_start_read() to return EAGAIN if vma got isolated and changed
lock_vma_under_rcu() back to detect this condition, per Wei Yang
- Change VM_BUG_ON_VMA() to WARN_ON_ONCE() when checking vma detached state,
per Lorenzo Stoakes
- Remove Vlastimil's Reviewed-by since code is changed
PATCH [13/18]
- Update vm_area_struct for tests, per Lorenzo Stoakes
- Add Reviewed-by, per Lorenzo Stoakes
PATCH [14/18]
- Minimized duplicate code, per Lorenzo Stoakes
PATCH [15/18]
- Add Reviewed-by, per Lorenzo Stoakes
PATCH [17/18]
- Use refcount_set_release() in vma_mark_attached(), per Will Deacon
PATCH [18/18]
- Updated documenation, per Lorenzo Stoakes
- Add Reviewed-by, per Lorenzo Stoakes
[1] https://lore.kernel.org/all/20230227173632.3292573-34-surenb@xxxxxxxxxx/
[2] https://lore.kernel.org/all/ZsQyI%2F087V34JoIt@xsang-OptiPlex-9020/
[3] https://lore.kernel.org/all/CAJuCfpEisU8Lfe96AYJDZ+OM4NoPmnw9bP53cT_kbfP_pR+-2g@xxxxxxxxxxxxxx/
[4] https://lore.kernel.org/all/20250111042604.3230628-1-surenb@xxxxxxxxxx/
Patchset applies over mm-unstable
Suren Baghdasaryan (18):
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: provide ops for cases when object's memory can be reused
refcount: introduce __refcount_{add|inc}_not_zero_limited_acquire
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/RCU/whatisRCU.rst | 10 +
Documentation/core-api/refcount-vs-atomic.rst | 37 +++-
Documentation/mm/process_addrs.rst | 44 +++--
include/linux/mm.h | 176 ++++++++++++++----
include/linux/mm_types.h | 75 ++++----
include/linux/mmap_lock.h | 6 -
include/linux/rcuwait.h | 13 +-
include/linux/refcount.h | 125 +++++++++++++
include/linux/slab.h | 15 +-
include/linux/types.h | 12 ++
kernel/fork.c | 129 ++++++-------
mm/debug.c | 6 +
mm/init-mm.c | 1 +
mm/memory.c | 106 ++++++++++-
mm/mmap.c | 3 +-
mm/nommu.c | 4 +-
mm/userfaultfd.c | 38 ++--
mm/vma.c | 27 ++-
mm/vma.h | 15 +-
tools/include/linux/refcount.h | 5 +
tools/testing/vma/linux/atomic.h | 6 +
tools/testing/vma/vma.c | 42 ++++-
tools/testing/vma/vma_internal.h | 127 ++++++-------
23 files changed, 702 insertions(+), 320 deletions(-)
base-commit: 47aa60e930fe7fc2a945e4406e3ad1dfa73bb47c
--
2.48.1.601.g30ceb7b040-goog
