[PATCH v3 0/7] mm/kvm: Improve parallelism for access bit harvesting

James Houghton
2024-04-01T23:29:57Z

This patchset adds a fast path in KVM to test and clear access bits on
sptes without taking the mmu_lock. It also adds support for using a
bitmap to (1) test the access bits for many sptes in a single call to
mmu_notifier_test_young, and to (2) clear the access bits for many ptes
in a single call to mmu_notifier_clear_young.

With Yu's permission, I'm now working on getting this series into a
mergeable state.

I'm posting this as an RFC because I'm not sure if the arm64 bits are
correct, and I haven't done complete performance testing. I want to do
broader experimentation to see how much this improves VM performance in
a cloud environment, but I want to be sure that the code is mergeable
first.

Yu has posted other performance results[1], [2]. This v3 shouldn't
significantly change the x86 results, but the arm64 results may have
changed.

The most important changes since v2[3]:

- Split the test_clear_young MMU notifier back into test_young and
clear_young. I did this because the bitmap passed in has a distinct
meaning for each of them, and I felt that this was cleaner.

- The return value of test_young / clear_young now indicates if the
bitmap was used.

- Removed the custom spte walker to implement the lockless path. This
was important for arm64 to be functionally correct (thanks Oliver),
and it avoids a lot of problems brought up in review of v2 (for
example[4]).

- Add kvm_arch_prepare_bitmap_age and kvm_arch_finish_bitmap_age to
allow for arm64 to implement its bitmap-based aging to grab the MMU
lock for reading while allowing x86 to be lockless.

- The powerpc changes have been dropped.

- The logic to inform architectures how to use the bitmap has been
cleaned up (kvm_should_clear_young has been split into
kvm_gfn_should_age and kvm_gfn_record_young) (thanks Nicolas).

There were some smaller changes too:
- Added test_clear_young_metadata (thanks Sean).
- MMU_NOTIFIER_RANGE_LOCKLESS has been renamed to
MMU_NOTIFIER_YOUNG_FAST, to indicate to the caller that passing a
bitmap for MGLRU look-around is likely to be beneficial.
- Cleaned up comments that describe the changes to
mmu_notifier_test_young / mmu_notifier_clear_young (thanks Nicolas).

[1]: https://lore.kernel.org/all/20230609005943.43041-1-yuzhao@google.com/
[2]: https://lore.kernel.org/all/20230609005935.42390-1-yuzhao@google.com/
[3]: https://lore.kernel.org/kvmarm/20230526234435.662652-1-yuzhao@google.com/
[4]: https://lore.kernel.org/all/ZItX64Bbx5vdjo9M@google.com/

James Houghton (5):
mm: Add a bitmap into mmu_notifier_{clear,test}_young
KVM: Move MMU notifier function declarations
KVM: Add basic bitmap support into kvm_mmu_notifier_test/clear_young
KVM: x86: Participate in bitmap-based PTE aging
KVM: arm64: Participate in bitmap-based PTE aging

Yu Zhao (2):
KVM: x86: Move tdp_mmu_enabled and shadow_accessed_mask
mm: multi-gen LRU: use mmu_notifier_test_clear_young()

Documentation/admin-guide/mm/multigen_lru.rst | 6 +-
arch/arm64/include/asm/kvm_host.h | 5 +
arch/arm64/include/asm/kvm_pgtable.h | 4 +-
arch/arm64/kvm/hyp/pgtable.c | 21 +-
arch/arm64/kvm/mmu.c | 23 ++-
arch/x86/include/asm/kvm_host.h | 20 ++
arch/x86/kvm/mmu.h | 6 -
arch/x86/kvm/mmu/mmu.c | 16 +-
arch/x86/kvm/mmu/spte.h | 1 -
arch/x86/kvm/mmu/tdp_mmu.c | 10 +-
include/linux/kvm_host.h | 101 ++++++++--
include/linux/mmu_notifier.h | 93 ++++++++-
include/linux/mmzone.h | 6 +-
include/trace/events/kvm.h | 13 +-
mm/mmu_notifier.c | 20 +-
mm/rmap.c | 9 +-
mm/vmscan.c | 183 ++++++++++++++----
virt/kvm/kvm_main.c | 100 +++++++---
18 files changed, 509 insertions(+), 128 deletions(-)

base-commit: 0cef2c0a2a356137b170c3cb46cb9c1dd2ca3e6b
--
2.44.0.478.gd926399ef9-goog