lkml.org 
[lkml]   [2021]   [Jun]   [25]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    From
    Subject[PATCH v2 0/9] KVM: X86: Some light optimizations on rmap logic
    Date
    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(-)

    --
    2.31.1


    \
     
     \ /
      Last update: 2021-06-25 17:32    [W:4.935 / U:0.008 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site