Re: [PATCH v2 0/7] Improve gfn-to-memslot performance during page faults

On 05/08/21 00:28, David Matlack wrote:
> This series improves the performance of gfn-to-memslot lookups during
> page faults. Ben Gardon originally identified this performance gap and
> sufficiently addressed it in Google's kernel by reading the memslot once
> at the beginning of the page fault and passing around the pointer.
> 
> This series takes an alternative approach by introducing a per-vCPU
> cache of the least recently used memslot index. This avoids needing to
> binary search the existing memslots multiple times during a page fault.
> Unlike passing around the pointer, the cache has an additional benefit
> in that it speeds up gfn-to-memslot lookups *across* faults and during
> spte prefetching where the gfn changes.
> 
> This difference can be seen clearly when looking at the performance of
> fast_page_fault when multiple slots are in play:
> 
> Metric                        | Baseline     | Pass*    | Cache**
> ----------------------------- | ------------ | -------- | ----------
> Iteration 2 dirty memory time | 2.8s         | 1.6s     | 0.30s
> 
> * Pass: Lookup the memslot once per fault and pass it around.
> ** Cache: Cache the last used slot per vCPU (i.e. this series).
> 
> (Collected via ./dirty_log_perf_test -v64 -x64)
> 
> I plan to also send a follow-up series with a version of Ben's patches
> to pass the pointer to the memslot through the page fault handling code
> rather than looking it up multiple times. Even when applied on top of
> the cache series it has some performance improvements by avoiding a few
> extra memory accesses (mainly kvm->memslots[as_id] and
> slots->used_slots). But it will be a judgement call whether or not it's
> worth the code churn and complexity.

Queued, thanks.

Paolo

> v2:
>   * Rename lru to last_used [Paolo]
>   * Tree-wide replace search_memslots with __gfn_to_memslot [Paolo]
>   * Avoid speculation when accessesing slots->memslots [Paolo]
>   * Refactor tdp_set_spte_atomic to leverage vcpu->last_used_slot [Paolo]
>   * Add Paolo's Reviewed-by tags
>   * Fix build failures in mmu_audit.c [kernel test robot]
> 
> v1: https://lore.kernel.org/kvm/[email protected]/
> 
> David Matlack (7):
>    KVM: Rename lru_slot to last_used_slot
>    KVM: Move last_used_slot logic out of search_memslots
>    KVM: Cache the last used slot index per vCPU
>    KVM: x86/mmu: Leverage vcpu->last_used_slot in
>      tdp_mmu_map_handle_target_level
>    KVM: x86/mmu: Leverage vcpu->last_used_slot for rmap_add and
>      rmap_recycle
>    KVM: x86/mmu: Rename __gfn_to_rmap to gfn_to_rmap
>    KVM: selftests: Support multiple slots in dirty_log_perf_test
> 
>   arch/powerpc/kvm/book3s_64_vio.c              |  2 +-
>   arch/powerpc/kvm/book3s_64_vio_hv.c           |  2 +-
>   arch/s390/kvm/kvm-s390.c                      |  4 +-
>   arch/x86/kvm/mmu/mmu.c                        | 54 +++++++------
>   arch/x86/kvm/mmu/mmu_audit.c                  |  4 +-
>   arch/x86/kvm/mmu/tdp_mmu.c                    | 42 +++++++---
>   include/linux/kvm_host.h                      | 80 +++++++++++++++----
>   .../selftests/kvm/access_tracking_perf_test.c |  2 +-
>   .../selftests/kvm/demand_paging_test.c        |  2 +-
>   .../selftests/kvm/dirty_log_perf_test.c       | 76 +++++++++++++++---
>   .../selftests/kvm/include/perf_test_util.h    |  2 +-
>   .../selftests/kvm/lib/perf_test_util.c        | 20 +++--
>   .../kvm/memslot_modification_stress_test.c    |  2 +-
>   virt/kvm/kvm_main.c                           | 26 +++++-
>   14 files changed, 238 insertions(+), 80 deletions(-)
>