From mboxrd@z Thu Jan 1 00:00:00 1970 From: Marc Zyngier Subject: Re: [PATCH v6 1/4] mm: add NR_SECONDARY_PAGETABLE to count secondary page table uses. Date: Wed, 29 Jun 2022 11:29:50 +0100 Message-ID: <8735fn3gpt.wl-maz@kernel.org> References: <20220628220938.3657876-1-yosryahmed@google.com> <20220628220938.3657876-2-yosryahmed@google.com> Mime-Version: 1.0 (generated by SEMI-EPG 1.14.7 - "Harue") Return-path: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1656498593; bh=6t4Qww5gQL2dPbX3lWrf1eT1qNVh0NYZw5CcQii/4dM=; h=Date:From:To:Cc:Subject:In-Reply-To:References:From; b=nvDqmPQ6eBKyTUKtozYUlsMzXrump4i5EVvFFIkTkP4/jSoJzd2ro8PLYBFW362Mj 09dLnzVJ1RBJIqoRvakCpzH7jvr2a3OYquFglJCocMOwPRxfXkZIO4Jj7d4RAWO1vY LDKGc4+cPyW1yZ159pyhUr3Gxa+30RByCwS+FI8+9+n+xUuM1z5uAQ2eAJqgPaXuFM uyQ175SDqmHxSNcDppdDbIFGv20UX1/gQbmiLW0m4f92bY0OOCprHBsNzQgLjQg3+x LoqmwaPmDGkJLxdhgkeXsgvBwGSJJeJLo+yQupwk03grJMn5nSqh302TLrLAqd/ihr UfpW+K4y5g1pw== In-Reply-To: <20220628220938.3657876-2-yosryahmed@google.com> List-ID: Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Yosry Ahmed Cc: Tejun Heo , Johannes Weiner , Zefan Li , James Morse , Alexandru Elisei , Suzuki K Poulose , Paolo Bonzini , Sean Christopherson , Vitaly Kuznetsov , Wanpeng Li , Jim Mattson , Joerg Roedel , Andrew Morton , Michal Hocko , Roman Gushchin , Shakeel Butt , Oliver Upton , Huang@google.com, Shaoqin , cgroups@vger.kernel.org, linux-kernel@vger.kernel.org, On Tue, 28 Jun 2022 23:09:35 +0100, Yosry Ahmed 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 > Acked-by: Shakeel Butt Acked-by: Marc Zyngier M. -- Without deviation from the norm, progress is not possible.