Messages in this thread |  | | Subject | Re: [PATCH v2 0/9] KVM: X86: Some light optimizations on rmap logic | From | Paolo Bonzini <> | Date | Mon, 26 Jul 2021 15:05:52 +0200 |
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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
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