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Date:   Wed, 29 Jun 2022 11:29:50 +0100
Message-ID: <8735fn3gpt.wl-maz@kernel.org>
From:   Marc Zyngier <maz@kernel.org>
To:     Yosry Ahmed <yosryahmed@google.com>
Cc:     Tejun Heo <tj@kernel.org>, Johannes Weiner <hannes@cmpxchg.org>,
        Zefan Li <lizefan.x@bytedance.com>,
        James Morse <james.morse@arm.com>,
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        Paolo Bonzini <pbonzini@redhat.com>,
        Sean Christopherson <seanjc@google.com>,
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        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>,
        Shakeel Butt <shakeelb@google.com>,
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        Shaoqin <shaoqin.huang@intel.com>, cgroups@vger.kernel.org,
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        linux-mm@kvack.org
Subject: Re: [PATCH v6 1/4] mm: add NR_SECONDARY_PAGETABLE to count secondary page table uses.
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On Tue, 28 Jun 2022 23:09:35 +0100,
Yosry Ahmed <yosryahmed@google.com> wrote:
> 
> We keep track of several kernel memory stats (total kernel memory, page
> tables, stack, vmalloc, etc) on multiple levels (global, per-node,
> per-memcg, etc). These stats give insights to users to how much memory
> is used by the kernel and for what purposes.
> 
> Currently, memory used by kvm mmu is not accounted in any of those
> kernel memory stats. This patch series accounts the memory pages
> used by KVM for page tables in those stats in a new
> NR_SECONDARY_PAGETABLE stat. This stat can be later extended to account
> for other types of secondary pages tables (e.g. iommu page tables).
> 
> KVM has a decent number of large allocations that aren't for page
> tables, but for most of them, the number/size of those allocations
> scales linearly with either the number of vCPUs or the amount of memory
> assigned to the VM. KVM's secondary page table allocations do not scale
> linearly, especially when nested virtualization is in use.
> 
> From a KVM perspective, NR_SECONDARY_PAGETABLE will scale with KVM's
> per-VM pages_{4k,2m,1g} stats unless the guest is doing something
> bizarre (e.g. accessing only 4kb chunks of 2mb pages so that KVM is
> forced to allocate a large number of page tables even though the guest
> isn't accessing that much memory). However, someone would need to either
> understand how KVM works to make that connection, or know (or be told) to
> go look at KVM's stats if they're running VMs to better decipher the stats.
> 
> Furthermore, having NR_PAGETABLE side-by-side with NR_SECONDARY_PAGETABLE
> is informative. For example, when backing a VM with THP vs. HugeTLB,
> NR_SECONDARY_PAGETABLE is roughly the same, but NR_PAGETABLE is an order
> of magnitude higher with THP. So having this stat will at the very least
> prove to be useful for understanding tradeoffs between VM backing types,
> and likely even steer folks towards potential optimizations.
> 
> The original discussion with more details about the rationale:
> https://lore.kernel.org/all/87ilqoi77b.wl-maz@kernel.org
> 
> This stat will be used by subsequent patches to count KVM mmu
> memory usage.
> 
> Signed-off-by: Yosry Ahmed <yosryahmed@google.com>
> Acked-by: Shakeel Butt <shakeelb@google.com>

Acked-by: Marc Zyngier <maz@kernel.org>

	M.

-- 
Without deviation from the norm, progress is not possible.