Re: [PATCH v4 1/4] mm: add NR_SECONDARY_PAGETABLE to count secondary page table uses.

From: Shakeel Butt <shakeelb@google.com>
To: Sean Christopherson <seanjc@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>,
	Yosry Ahmed <yosryahmed@google.com>,
	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>
Subject: Re: [PATCH v4 1/4] mm: add NR_SECONDARY_PAGETABLE to count secondary page table uses.
Date: Fri, 13 May 2022 10:13:54 -0700	[thread overview]
Message-ID: <CALvZod6nERq4j=L0V+pc-rd5+QKi4yb_23tWV-1MF53xL5KE6Q@mail.gmail.com> (raw)
In-Reply-To: <Yn6DeEGLyR4Q0cDp@google.com>

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.