On Thu, Jan 9, 2025 at 5:40 AM Vlastimil Babka <vbabka@xxxxxxx> 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@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/20241226170710.1159679-1-surenb@xxxxxxxxxx/ > > [5] https://lore.kernel.org/all/20250107030415.721474-1-surenb@xxxxxxxxxx/ > > [6] https://lore.kernel.org/all/20241226200335.1250078-1-surenb@xxxxxxxxxx/ > > > > 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(-) > > >
