On Mon, Jun 14, 2021 at 11:54:59AM +0200, Paolo Bonzini wrote:
> On 12/06/21 01:56, David Matlack wrote:
> > This patch series adds support for the TDP MMU in the fast_page_fault
> > path, which enables certain write-protection and access tracking faults
> > to be handled without taking the KVM MMU lock. This series brings the
> > performance of these faults up to par with the legacy MMU.
>
> Hi David,
>
> I have one very basic question: is the speedup due to lock contention, or to
> cacheline bouncing, or something else altogether? In other words, what do
> the profiles look like before vs. after these patches?
The speed up comes from a combination of:
- Less time spent in kvm_vcpu_gfn_to_memslot.
- Less lock contention on the MMU lock in read mode.
Before:
Overhead Symbol
- 45.59% [k] kvm_vcpu_gfn_to_memslot
- 45.57% kvm_vcpu_gfn_to_memslot
- 29.25% kvm_page_track_is_active
+ 15.90% direct_page_fault
+ 13.35% mmu_need_write_protect
+ 9.10% kvm_mmu_hugepage_adjust
+ 7.20% try_async_pf
+ 18.16% [k] _raw_read_lock
+ 10.57% [k] direct_page_fault
+ 8.77% [k] handle_changed_spte_dirty_log
+ 4.65% [k] mark_page_dirty_in_slot
1.62% [.] run_test
+ 1.35% [k] x86_virt_spec_ctrl
+ 1.18% [k] try_grab_compound_head
[...]
After:
Overhead Symbol
+ 26.23% [k] x86_virt_spec_ctrl
+ 15.93% [k] vmx_vmexit
+ 6.33% [k] vmx_vcpu_run
+ 4.31% [k] vcpu_enter_guest
+ 3.71% [k] tdp_iter_next
+ 3.47% [k] __vmx_vcpu_run
+ 2.92% [k] kvm_vcpu_gfn_to_memslot
+ 2.71% [k] vcpu_run
+ 2.71% [k] fast_page_fault
+ 2.51% [k] kvm_vcpu_mark_page_dirty
(Both profiles were captured during "Iteration 2 dirty memory" of
dirty_log_perf_test.)
Related to the kvm_vcpu_gfn_to_memslot overhead: I actually have a set of
patches from Ben I am planning to send soon that will reduce the number of
redundant gfn-to-memslot lookups in the page fault path.
>
> Thanks,
>
> Paolo
>
