linux-kernel - Re: [PATCH] KVM: VMX: Optimize flushing the PML buffer

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Date:   Thu, 4 Feb 2021 16:02:22 -0800
From:   Ben Gardon <bgardon@...gle.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, kvm <kvm@...r.kernel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Peter Shier <pshier@...gle.com>,
        Junaid Shahid <junaids@...gle.com>,
        Jim Mattson <jmattson@...gle.com>,
        Makarand Sonare <makarandsonare@...gle.com>,
        Kai Huang <kai.huang@...el.com>
Subject: Re: [PATCH] KVM: VMX: Optimize flushing the PML buffer

On Thu, Feb 4, 2021 at 2:51 PM Peter Xu <peterx@...hat.com> wrote:
>
> Hi, Ben,
>
> On Thu, Feb 04, 2021 at 02:19:59PM -0800, Ben Gardon wrote:
> > The average time for each run demonstrated a strange bimodal distribution,
> > with clusters around 2 seconds and 2.5 seconds. This may have been a
> > result of vCPU migration between NUMA nodes.
>
> Have you thought about using numactl or similar technique to verify your idea
> (force both vcpu threads binding, and memory allocations)?
>
> From the numbers it already shows improvements indeed, but just curious since
> you raised this up. :)

Frustratingly, the test machines I have don't have numactl installed
but I've been meaning to add cpu pinning to the selftests perf tests
anyway, so maybe this is a good reason to do it.

>
> > @@ -5707,13 +5708,18 @@ static void vmx_flush_pml_buffer(struct kvm_vcpu *vcpu)
> >       else
> >               pml_idx++;
> >
> > +     memslots = kvm_vcpu_memslots(vcpu);
> > +
> >       pml_buf = page_address(vmx->pml_pg);
> >       for (; pml_idx < PML_ENTITY_NUM; pml_idx++) {
> > +             struct kvm_memory_slot *memslot;
> >               u64 gpa;
> >
> >               gpa = pml_buf[pml_idx];
> >               WARN_ON(gpa & (PAGE_SIZE - 1));
> > -             kvm_vcpu_mark_page_dirty(vcpu, gpa >> PAGE_SHIFT);
> > +
> > +             memslot = __gfn_to_memslot(memslots, gpa >> PAGE_SHIFT);
> > +             mark_page_dirty_in_slot(vcpu->kvm, memslot, gpa >> PAGE_SHIFT);
>
> Since at it: make "gpa >> PAGE_SHIFT" a temp var too?

That's a good idea, I'll try it.

>
> Thanks,
>
> --
> Peter Xu
>