On Tue, 28 Jun 2022 22:09:35 +0000 Yosry Ahmed <yosryahmed@xxxxxxxxxx> 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@xxxxxxxxxx
>
> This stat will be used by subsequent patches to count KVM mmu
> memory usage.
Nits and triviata:
> --- a/Documentation/filesystems/proc.rst
> +++ b/Documentation/filesystems/proc.rst
> @@ -977,6 +977,7 @@ Example output. You may not have all of these fields.
> SUnreclaim: 142336 kB
> KernelStack: 11168 kB
> PageTables: 20540 kB
> + SecPageTables: 0 kB
> NFS_Unstable: 0 kB
> Bounce: 0 kB
> WritebackTmp: 0 kB
> @@ -1085,6 +1086,9 @@ KernelStack
> Memory consumed by the kernel stacks of all tasks
> PageTables
> Memory consumed by userspace page tables
> +SecPageTables
> + Memory consumed by secondary page tables, this currently
> + currently includes KVM mmu allocations on x86 and arm64.
Something happened to the whitespace there.
> + "Node %d SecPageTables: %8lu kB\n"
> ...
> + nid, K(node_page_state(pgdat, NR_SECONDARY_PAGETABLE)),
The use of "sec" in the user-facing changes and "secondary" in the
programmer-facing changes is irksome. Can we be consistent? I'd
prefer "secondary" throughout.
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