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

[Suren Baghdasaryan <surenb@google.com>]

On Thu, Jan 9, 2025 at 5:40 AM Vlastimil Babka <vbabka@suse.cz> wrote:
>
> Btw the subject became rather incomplete given all the series does :)

Missed this one. What do you think is worth mentioning here? It's
true, the patchset does many small things but I wanted to outline the
main conceptual changes. Please LMK if you think there are more
changes big enough to be mentioned here.

>
> On 1/9/25 3:30 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. 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%*
>
> Given how patch 2 discusses memory growth due to moving the lock, should
> also patch 11 discuss how the replacement with refcount reduces the
> memory footprint? And/or the cover letter could summarize the impact of
> the whole series in that aspect? Perhaps the refcount doesn't reduce
> anything as it's smaller but sits alone in the cacheline? Could it be
> grouped with some non-hot fields instead as a followup, so could we get
> to <=192 (non-debug) size without impacting performance?
>
> > 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)
> >
> > 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(-)
> >
>