linux-kernel - Re: [PATCH v4 3/3] KVM: x86/mmu: Add detailed page size stats

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Message-ID: <CAL715WK+uSx3MjRH7NcMsBELz9-2jmWhJs0c3+_atto0iv8UOg@mail.gmail.com>
Date:   Thu, 12 Aug 2021 10:44:26 -0700
From:   Mingwei Zhang <mizhang@...gle.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     Jim Mattson <jmattson@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Joerg Roedel <joro@...tes.org>, kvm <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Ben Gardon <bgardon@...gle.com>,
        David Matlack <dmatlack@...gle.com>,
        Jing Zhang <jingzhangos@...gle.com>
Subject: Re: [PATCH v4 3/3] KVM: x86/mmu: Add detailed page size stats

Hi Peter,


On Wed, Aug 11, 2021 at 6:12 AM Peter Xu <peterx@...hat.com> wrote:
>
> On Tue, Aug 10, 2021 at 06:06:51PM -0700, Mingwei Zhang wrote:
> > Regarding the pursuit for accuracy, I think there might be several
> > reasons. One of the most critical reasons that I know is that we need
> > to ensure dirty logging works correctly, i.e., when dirty logging is
> > enabled, all huge pages (both 2MB and 1GB) _are_ gone. Hope that
> > clarifies a little bit?
>
> It's just for statistics, right?  I mean dirty log should be working even
> without this change.

That's true. What I meant was that the accurate stats might be able to
help verifying a property of dirty logging as a side benefit. Sorry
for the confusion.

>
> But I didn't read closely last night, so we want to have "how many huge pages
> we're mapping", not "how many we've mapped in the history".  Yes that makes
> sense to be accurate.  I should have looked more carefully, sorry.
>
> PS: it turns out atomic is not that expensive as I thought even on a 200 core
> system, which takes 7ns (but for sure it's still expensive than normal memory
> ops, and bus locking); I thought it'll be bigger as on a 40 core system I got
> 15ns which is 2x of my laptop of 8 cores, but it didn't really grow but shrink.

Thanks for the information about atomic access!
>
> Thanks,
>
> --
> Peter Xu
>