linux-kernel - Re: [PATCH v2 0/9] KVM: X86: Some light optimizations on rmap logic

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Message-ID: <6400f7db-3194-ac9b-3116-44d1201564eb@redhat.com>
Date:   Mon, 26 Jul 2021 15:05:52 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Peter Xu <peterx@...hat.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Maxim Levitsky <mlevitsk@...hat.com>
Subject: Re: [PATCH v2 0/9] KVM: X86: Some light optimizations on rmap logic

On 25/06/21 17:32, Peter Xu wrote:
> v2:
> - Rebased to kvm-queue since I found quite a few conflicts already
> - Add an example into patch commit message of "KVM: X86: Introduce
>    mmu_rmaps_stat per-vm debugfs file"
> - Cleanup more places in patch "KVM: X86: Optimize pte_list_desc with per-array
>    counter" and squashed
> 
> All things started from patch 1, which introduced a new statistic to keep "max
> rmap entry count per vm".  At that time I was just curious about how many rmap
> is there normally for a guest, and it surprised me a bit.
> 
> For TDP mappings it's all fine as mostly rmap of a page is either 0 or 1
> depending on faulted or not.  It turns out with EPT=N there seems to be a huge
> number of pages that can have tens or hundreds of rmap entries even for an idle
> guest.  Then I continued with the rest.
> 
> To understand better on "how much of those pages", I did patch 2-6 which
> introduced the idea of per-arch per-vm debugfs nodes, and added a debug file to
> do statistics for rmap, which is similar to kvm_arch_create_vcpu_debugfs() but
> for vm not vcpu.
> 
> I did notice this should be the clean approach as I also see other archs
> randomly create some per-vm debugfs nodes there:
> 
> ---8<---
> *** arch/arm64/kvm/vgic/vgic-debug.c:
> vgic_debug_init[274]           debugfs_create_file("vgic-state", 0444, kvm->debugfs_dentry, kvm,
> 
> *** arch/powerpc/kvm/book3s_64_mmu_hv.c:
> kvmppc_mmu_debugfs_init[2115]  debugfs_create_file("htab", 0400, kvm->arch.debugfs_dir, kvm,
> 
> *** arch/powerpc/kvm/book3s_64_mmu_radix.c:
> kvmhv_radix_debugfs_init[1434] debugfs_create_file("radix", 0400, kvm->arch.debugfs_dir, kvm,
> 
> *** arch/powerpc/kvm/book3s_hv.c:
> debugfs_vcpu_init[2395]        debugfs_create_file("timings", 0444, vcpu->arch.debugfs_dir, vcpu,
> 
> *** arch/powerpc/kvm/book3s_xics.c:
> xics_debugfs_init[1027]        xics->dentry = debugfs_create_file(name, 0444, powerpc_debugfs_root,
> 
> *** arch/powerpc/kvm/book3s_xive.c:
> xive_debugfs_init[2236]        xive->dentry = debugfs_create_file(name, S_IRUGO, powerpc_debugfs_root,
> 
> *** arch/powerpc/kvm/timing.c:
> kvmppc_create_vcpu_debugfs[214] debugfs_file = debugfs_create_file(dbg_fname, 0666, kvm_debugfs_dir,
> ---8<---
> 
> PPC even has its own per-vm dir for that.  I think if patch 2-6 can be
> considered to be accepted then the next thing to consider is to merge all these
> usages to be under the same existing per-vm dentry with their per-arch hooks
> introduced.
> 
> The last 3 patches (patch 7-9) are a few optimizations of existing rmap logic.
> The major test case I used is rmap_fork [1], however it's not really the ideal
> one to show their effect for sure as that test I wrote covers both
> rmap_add/remove, while I don't have good idea on optimizing rmap_remove without
> changing the array structure or adding much overhead (e.g. sort the array, or
> making a tree-like structure somehow to replace the array list).  However it
> already shows some benefit with those changes, so I post them out.
> 
> Applying patch 7-8 will bring a summary of 38% perf boost when I fork 500
> childs with the test I used.  Didn't run perf test on patch 9.  More in the
> commit log.
> 
> Please review, thanks.
> 
> [1] https://github.com/xzpeter/clibs/commit/825436f825453de2ea5aaee4bdb1c92281efe5b3
> 
> Peter Xu (9):
>    KVM: X86: Add per-vm stat for max rmap list size
>    KVM: Introduce kvm_get_kvm_safe()
>    KVM: Allow to have arch-specific per-vm debugfs files
>    KVM: X86: Introduce pte_list_count() helper
>    KVM: X86: Introduce kvm_mmu_slot_lpages() helpers
>    KVM: X86: Introduce mmu_rmaps_stat per-vm debugfs file
>    KVM: X86: MMU: Tune PTE_LIST_EXT to be bigger
>    KVM: X86: Optimize pte_list_desc with per-array counter
>    KVM: X86: Optimize zapping rmap
> 
>   arch/x86/include/asm/kvm_host.h |   1 +
>   arch/x86/kvm/mmu/mmu.c          |  97 +++++++++++++++++------
>   arch/x86/kvm/mmu/mmu_internal.h |   1 +
>   arch/x86/kvm/x86.c              | 131 +++++++++++++++++++++++++++++++-
>   include/linux/kvm_host.h        |   2 +
>   virt/kvm/kvm_main.c             |  37 +++++++--
>   6 files changed, 235 insertions(+), 34 deletions(-)
> 

Looks good, thanks.  I queued it, but for now I have left out the 
statistics part; I would like to check the histogram patches too first.

Paolo