Re: [PATCH v2 0/3] Optimize S2 page splitting

[Leonardo Bras <leo.bras@arm.com>]

On Thu, Jun 18, 2026 at 02:14:41PM +0100, Leonardo Bras wrote:
> While playing with dirty-bit tracking, I decided to take a look on how page
> splitting works. Found out all entries are walked, even though we can infer,
> for instance that:
> - If a level-3 entry is walked, it means the parent level-2 entry is split
> - If a split just succeeded in an table entry, it means all children nodes
>   are already split
> 
> This patches' idea is to introduce new walking flags to skip pagetable
> levels 0-3.
> 
> The idea of skipping child nodes was also tested, but it was marginally
> slower than just skipping levels, so it was discarted.
> 
> Optimization measured on two scenarios involving eager-splitting on a
> VM with 1 memslot of 16GB:
> - Scenario 1: No manual protect, whole memslot split at dirty-track enable
>   (KVM_SET_USER_MEMORY_REGION2 ioctl with KVM_MEM_LOG_DIRTY_PAGES)
>   - Split happens only once, whole region
>   - Evalutes improved batch performance of splitting
> - Scenario 2: Manual protect, split happens during every dirty-bit clean
>   (KVM_CLEAR_DIRTY_LOG ioctl), average for 2 iterations.
>   - Split called multiple times, for smaller 64-page sections.
>   - Evaluate improved performance for multiple calls
> 
> Scenario 1, improvement on dirty-track enable ioctl for the memslot:
> - Memory was already split (4k pages):  -44.01% runtime (stdev 2.80%)
> - THP backed memory:                    -24.66% runtime (stdev 1.21%)
> - 16x1GB hugetlb memory:                -24.78% runtime (stdev 0.85%)
> 
> Scenario 2, improvement on dirty-log clean ioctl for the memslot:
> - Memory was already split (4k pages):  -38.98% runtime (stdev 1.91%)
> - THP backed memory:                    -25.49% runtime (stdev 0.65%)
> - 16x1GB hugetlb memory:                -24.24% runtime (stdev 0.65%)
> 
> For collecting above numbers, the following script was ran in both vanilla
> and patched kernels, with kernel parameter 'default_hugepagesz=1G', on an
> TX2 with 32GB RAM.
> 
> --- dirty_test.sh
> #!/bin/bash
> filename=$(uname -r |cut -d'-' -f 4-)
> 
> run_test(){
>   uname -a
>   cat /proc/cmdline
> 
>   #prepare
>   sudo bash -c 'echo 64 > /proc/sys/vm/nr_hugepages'
> 
>   ./dirty_log_perf_test -g -b 64G
>   ./dirty_log_perf_test -g -b 64G -s anonymous_thp
>   ./dirty_log_perf_test -g -b 64G -s shared_hugetlb
> 
>   ./dirty_log_perf_test -b 64G
>   ./dirty_log_perf_test -b 64G -s anonymous_thp
>   ./dirty_log_perf_test -b 64G -s shared_hugetlb
> }
> 
> run_test 2>&1 | tee ${filename}
> ---

s/64G/16G/ on above script

> 
> Above dirty_log_perf_test command is the standard kvm selftest found in the
> kernel tree. It tested the following guest modes:
> Testing guest mode: PA-bits:40,  VA-bits:48,  4K pages
> Testing guest mode: PA-bits:40,  VA-bits:48, 64K pages
> Testing guest mode: PA-bits:36,  VA-bits:48,  4K pages
> Testing guest mode: PA-bits:36,  VA-bits:48, 64K pages
> 
> Performance numbers from above modes were used to calculate average and
> stdev showed in the optimization results.
> 
> Changes since v1:
> - Fixed inverted flag verification priority (Sashiko)
> - Fixed incorrectly skipping POST call if level was skipped (Sashiko), and to that
> - New pre-patch that changes goto-out -> return to avoid re-testing walk_continue 
> v1 Link: https://lore.kernel.org/lkml/20260610202112.2695205-2-leo.bras@arm.com/
> 
> Changes since RFC:
> - Changed approach from return value to walk flags (Will Deacon)
> - Discarted skip_child approach (Oliver Upton)
> - Measured in real hardware, and from userspace perspective (Marc Zyngier)
> - Better explanation of what and how numbers were collected
> RFC Link: https://lore.kernel.org/all/20260515195904.2466381-1-leo.bras@arm.com/
> 
> Thanks!
> Leo
> 
> Leonardo Bras (3):
>   KVM: arm64: Avoid re-testing walk_continue
>   KVM: arm64: Introduce KVM_PGTABLE_WALK_SKIP_LEVEL* walk flags
>   KVM: arm64: Make stage2_split_walker() skip unnecessary walks
> 
>  arch/arm64/include/asm/kvm_pgtable.h | 13 +++++++++++++
>  arch/arm64/kvm/hyp/pgtable.c         | 28 +++++++++++++++++++++-------
>  2 files changed, 34 insertions(+), 7 deletions(-)
> 
> 
> base-commit: 66affa37cfac0aec061cc4bcf4a065b0c52f7e19
> -- 
> 2.54.0
>