linux-kernel - Re: [Patch v3 9/9] KVM: x86/mmu: Reduce default cache size in KVM from 40 to PT64_ROOT_MAX

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Message-ID: <CANgfPd9xu5yw+k4WfCQKVsouM5mKtMNG_Kd7_tokkJm=ha4JTA@mail.gmail.com>
Date:   Tue, 27 Dec 2022 11:52:25 -0800
From:   Ben Gardon <bgardon@...gle.com>
To:     Vipin Sharma <vipinsh@...gle.com>
Cc:     seanjc@...gle.com, pbonzini@...hat.com, dmatlack@...gle.com,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [Patch v3 9/9] KVM: x86/mmu: Reduce default cache size in KVM
 from 40 to PT64_ROOT_MAX_LEVEL

On Wed, Dec 21, 2022 at 6:35 PM Vipin Sharma <vipinsh@...gle.com> wrote:
>
> KVM_ARCH_NR_OBJS_PER_MEMORY_CACHE is set to 40 without any specific
> reason. Reduce default size to PT64_ROOT_MAX_LEVEL, which is currently
> 5.
>
> Change mmu_pte_list_desc_cache size to what is needed as it is more than
> 5 but way less than 40.

Why do you say more than 5? At least to resolve a page fault we'll
never need more than 4 pages on a system with 5 level paging since the
root is already allocated.

>
> Tested by running dirty_log_perf_test on both tdp and shadow MMU with 48
> vcpu and 2GB/vcpu size on a 2 NUMA node machine. No impact on
> performance noticed.
>
> Ran perf on dirty_log_perf_test and found kvm_mmu_get_free_page() calls
> reduced by ~3300 which is near to 48 (vcpus) * 2 (nodes) * 35 (cache
> size).
>
> Signed-off-by: Vipin Sharma <vipinsh@...gle.com>
> ---
>  arch/x86/include/asm/kvm_types.h | 2 +-
>  arch/x86/kvm/mmu/mmu.c           | 7 ++++---
>  2 files changed, 5 insertions(+), 4 deletions(-)
>
> diff --git a/arch/x86/include/asm/kvm_types.h b/arch/x86/include/asm/kvm_types.h
> index 08f1b57d3b62..752dab218a62 100644
> --- a/arch/x86/include/asm/kvm_types.h
> +++ b/arch/x86/include/asm/kvm_types.h
> @@ -2,6 +2,6 @@
>  #ifndef _ASM_X86_KVM_TYPES_H
>  #define _ASM_X86_KVM_TYPES_H
>
> -#define KVM_ARCH_NR_OBJS_PER_MEMORY_CACHE 40
> +#define KVM_ARCH_NR_OBJS_PER_MEMORY_CACHE PT64_ROOT_MAX_LEVEL

Please add a comment explaining why this value was chosen.

>
>  #endif /* _ASM_X86_KVM_TYPES_H */
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 7454bfc49a51..f89d933ff380 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -677,11 +677,12 @@ static int mmu_topup_sp_memory_cache(struct kvm_mmu_memory_cache *cache,
>
>  static int mmu_topup_memory_caches(struct kvm_vcpu *vcpu, bool maybe_indirect)
>  {
> -       int r, nid;
> +       int r, nid, desc_capacity;
>
>         /* 1 rmap, 1 parent PTE per level, and the prefetched rmaps. */
> -       r = kvm_mmu_topup_memory_cache(&vcpu->arch.mmu_pte_list_desc_cache,
> -                                      1 + PT64_ROOT_MAX_LEVEL + PTE_PREFETCH_NUM);
> +       desc_capacity = 1 + PT64_ROOT_MAX_LEVEL + PTE_PREFETCH_NUM;
> +       r = __kvm_mmu_topup_memory_cache(&vcpu->arch.mmu_pte_list_desc_cache,
> +                                        desc_capacity, desc_capacity);
>         if (r)
>                 return r;
>
> --
> 2.39.0.314.g84b9a713c41-goog
>