Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
MIME-Version: 1.0
References: <20220429201131.3397875-1-yosryahmed@google.com>
 <20220429201131.3397875-2-yosryahmed@google.com> <87ilqoi77b.wl-maz@kernel.org>
 <CAJD7tkY7JF25XXUFq2mGroetMkfo-2zGOaQC94pjZE3D42+oaw@mail.gmail.com>
 <Yn2TGJ4vZ/fst+CY@cmpxchg.org> <Yn2YYl98Vhh/UL0w@google.com>
 <Yn5+OtZSSUZZgTQj@cmpxchg.org> <Yn6DeEGLyR4Q0cDp@google.com> <CALvZod6nERq4j=L0V+pc-rd5+QKi4yb_23tWV-1MF53xL5KE6Q@mail.gmail.com>
In-Reply-To: <CALvZod6nERq4j=L0V+pc-rd5+QKi4yb_23tWV-1MF53xL5KE6Q@mail.gmail.com>
From:   Yosry Ahmed <yosryahmed@google.com>
Date:   Thu, 19 May 2022 18:56:54 -0700
Message-ID: <CAJD7tka-5+XRkthNV4qCg8woPCpjcwynQoRBame-3GP1L8y+WQ@mail.gmail.com>
Subject: Re: [PATCH v4 1/4] mm: add NR_SECONDARY_PAGETABLE to count secondary
 page table uses.
To:     Shakeel Butt <shakeelb@google.com>
Cc:     Sean Christopherson <seanjc@google.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Marc Zyngier <maz@kernel.org>, Tejun Heo <tj@kernel.org>,
        Zefan Li <lizefan.x@bytedance.com>,
        James Morse <james.morse@arm.com>,
        Alexandru Elisei <alexandru.elisei@arm.com>,
        Suzuki K Poulose <suzuki.poulose@arm.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Michal Hocko <mhocko@kernel.org>,
        Roman Gushchin <roman.gushchin@linux.dev>,
        Oliver Upton <oupton@google.com>,
        Cgroups <cgroups@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-arm-kernel@lists.infradead.org, kvmarm@lists.cs.columbia.edu,
        kvm@vger.kernel.org, Linux-MM <linux-mm@kvack.org>
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

On Fri, May 13, 2022 at 10:14 AM Shakeel Butt <shakeelb@google.com> wrote:
>
> On Fri, May 13, 2022 at 9:12 AM Sean Christopherson <seanjc@google.com> wrote:
> >
> [...]
> >
> > It was mostly an honest question, I too am trying to understand what userspace
> > wants to do with this information.  I was/am also trying to understand the benefits
> > of doing the tracking through page_state and not a dedicated KVM stat.  E.g. KVM
> > already has specific stats for the number of leaf pages mapped into a VM, why not
> > do the same for non-leaf pages?
>
> Let me answer why a more general stat is useful and the potential
> userspace reaction:
>
> For a memory type which is significant enough, it is useful to expose
> it in the general interfaces, so that the general data/stat collection
> infra can collect them instead of having workload dependent stat
> collectors. In addition, not necessarily that stat has to have a
> userspace reaction in an online fashion. We do collect stats for
> offline analysis which greatly influence the priority order of
> optimization workitems.
>
> Next the question is do we really need a separate stat item
> (secondary_pagetable instead of just plain pagetable) exposed in the
> stable API? To me secondary_pagetable is general (not kvm specific)
> enough and can be significant, so having a separate dedicated stat
> should be ok. Though I am ok with lump it with pagetable stat for now
> but we do want it to be accounted somewhere.

Any thoughts on this? Johannes?