[v9,00/17] reimplement per-vma lock as a refcount

Message

Suren Baghdasaryan Jan. 11, 2025, 4:25 a.m. UTC

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 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/[e-mail]/
[2] https://lore.kernel.org/all/ZsQyI%2F087V34JoIt@xsang-OptiPlex-9020/
[3] https://lore.kernel.org/all/[e-mail]/
[4] https://lore.kernel.org/all/[e-mail]/
[5] https://lore.kernel.org/all/[e-mail]/

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(-)

Comments

Matthew Wilcox Jan. 11, 2025, 4:52 a.m. UTC | #1

On Fri, Jan 10, 2025 at 08:25:47PM -0800, Suren Baghdasaryan wrote:
> - Added static check for no-limit case in __refcount_add_not_zero_limited,
> per David Laight

Ugh, no, don't listen to David.

Suren Baghdasaryan Jan. 11, 2025, 9:45 a.m. UTC | #2

On Fri, Jan 10, 2025 at 8:52 PM Matthew Wilcox <[e-mail]> wrote:
>
> On Fri, Jan 10, 2025 at 08:25:47PM -0800, Suren Baghdasaryan wrote:
> > - Added static check for no-limit case in __refcount_add_not_zero_limited,
> > per David Laight
>
> Ugh, no, don't listen to David.

I thought his suggestion to add a check which can be verified at
compile time made sense. Could you please explain why that's a bad
idea? I'm really curious.