I noticed paging64/32_walk_addr_generic and gfn_to_memslot are very hot in kvm
modules:
# perf report
# Events: 38K cycles
#
# Overhead Command Shared Object Symbol
# ........ ............... .................. ......
#
6.73% qemu-system-x86 [kernel.kallsyms] [k] native_read_tsc
4.51% qemu-system-x86 [kernel.kallsyms] [k] delay_tsc
3.93% qemu-system-x86 [kvm] [k] paging64_walk_addr_generic
3.91% qemu-system-x86 [kernel.kallsyms] [k] arch_local_irq_restore
3.28% qemu-system-x86 [kvm_intel] [k] vmx_vcpu_run
3.12% qemu-system-x86 [kernel.kallsyms] [k] lock_acquire
3.10% qemu-system-x86 [kernel.kallsyms] [k] paravirt_read_tsc
2.70% qemu-system-x86 [kernel.kallsyms] [k] copy_user_generic_string
2.30% qemu-system-x86 [kernel.kallsyms] [k] check_flags
2.15% qemu-system-x86 [kernel.kallsyms] [k] do_raw_spin_lock
2.03% qemu-system-x86 [kvm] [k] gfn_to_memslot
The aim of this patchset is to reduce the overload of these two functions, it
does:
- sort memslots by ->base_gfn and use binary search to search the right slot
which is implemented by patch 1 ~ patch 7
- cache guest page number to frame number to reduce *_walk_addr_generic called,
and patch 7 does it.
After the patchset:
# Events: 36K cycles
#
# Overhead Command Shared Object Symbol
# ........ ............... .................. ......
#
5.64% qemu-system-x86 [kernel.kallsyms] [k] native_read_tsc
4.27% qemu-system-x86 [kvm_intel] [k] vmx_vcpu_run
3.85% qemu-system-x86 [kernel.kallsyms] [k] delay_tsc
3.32% qemu-system-x86 [kernel.kallsyms] [k] arch_local_irq_restore
2.68% qemu-system-x86 [kernel.kallsyms] [k] paravirt_read_tsc
2.41% qemu-system-x86 [kernel.kallsyms] [k] lock_acquire
2.25% qemu-system-x86 [kernel.kallsyms] [k] copy_user_generic_string
2.14% qemu-system-x86 [kvm] [k] kvm_mmu_pte_write
1.82% qemu-system-x86 [kernel.kallsyms] [k] check_flags
1.80% qemu-system-x86 [kvm] [k] paging64_walk_addr_generic
[......]
[......]
0.81% qemu-system-x86 [kvm] [k] gfn_to_memslot
There is the Unixbecn(the parameter is -c 10 fsdisk) test report:
Before patchset:
2090.5
1882.5
1845.2
After vtlb patch:
2237.4
2151.8
2118.0
After memslots sorted patches:
2392.5
2485.9
2416.1
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