Eager Page Splitting improves the performance of dirty-logging (used in live migrations) when guest memory is backed by huge-pages. It's an optimization used in Google Cloud since 2016 on x86, and for the last couple of months on ARM. Background and motivation ========================= Dirty logging is typically used for live-migration iterative copying. KVM implements dirty-logging at the PAGE_SIZE granularity (will refer to 4K pages from now on). It does it by faulting on write-protected 4K pages. Therefore, enabling dirty-logging on a huge-page requires breaking it into 4K pages in the first place. KVM does this breaking on fault, and because it's in the critical path it only maps the 4K page that faulted; every other 4K page is left unmapped. This is not great for performance on ARM for a couple of reasons: - Splitting on fault can halt vcpus for milliseconds in some implementations. Splitting a block PTE requires using a broadcasted TLB invalidation (TLBI) for every huge-page (due to the break-before-make requirement). Note that x86 doesn't need this. We observed some implementations that take millliseconds to complete broadcasted TLBIs when done in parallel from multiple vcpus. And that's exactly what happens when doing it on fault: multiple vcpus fault at the same time triggering TLBIs in parallel. - Read intensive guest workloads end up paying for dirty-logging. Only mapping the faulting 4K page means that all the other pages that were part of the huge-page will now be unmapped. The effect is that any access, including reads, now has to fault. Eager Page Splitting (on ARM) ============================= Eager Page Splitting fixes the above two issues by eagerly splitting huge-pages when enabling dirty logging. The goal is to avoid doing it while faulting on write-protected pages. This is what the TDP MMU does for x86 [0], except that x86 does it for different reasons: to avoid grabbing the MMU lock on fault. Note that taking care of write-protection faults still requires grabbing the MMU lock on ARM, but not on x86 (with the fast_page_fault path). An additional benefit of eagerly splitting huge-pages is that it can be done in a controlled way (e.g., via an IOCTL). This series provides two knobs for doing it, just like its x86 counterpart: when enabling dirty logging, and when using the KVM_CLEAR_DIRTY_LOG ioctl. The benefit of doing it on KVM_CLEAR_DIRTY_LOG is that this ioctl takes ranges, and not complete memslots like when enabling dirty logging. This means that the cost of splitting (mainly broadcasted TLBIs) can be throttled: split a range, wait for a bit, split another range, etc. The benefits of this approach were presented by Oliver Upton at KVM Forum 2022 [1]. Implementation ============== Patches 3-4 add a pgtable utility function for splitting huge block PTEs: kvm_pgtable_stage2_split(). Patches 5-9 add support for eagerly splitting huge-pages when enabling dirty-logging and when using the KVM_CLEAR_DIRTY_LOG ioctl. Note that this is just like what x86 does, and the code is actually based on it. And finally, patch 9: KVM: arm64: Use local TLBI on permission relaxation adds support for using local TLBIs instead of broadcasts when doing permission relaxation. This last patch is key to achieving good performance during dirty-logging, as eagerly breaking huge-pages replaces mapping new pages with permission relaxation. Got this patch (indirectly) from Marc Z. and took the liberty of adding a commit message. Note: this applies on top of 6.2-rc6. Performance evaluation ====================== The performance benefits were tested using the dirty_log_perf_test selftest with 2M huge-pages. The first test uses a write-only sequential workload where the stride is 2M instead of 4K [2]. The idea with this experiment is to emulate a random access pattern writing a different huge-page at every access. Observe that the benefit increases with the number of vcpus: up to 5.76x for 152 vcpus. This table shows the guest dirtying time when using the CLEAR ioctl (and KVM_CAP_MANUAL_DIRTY_LOG_PROTECT2): /dirty_log_perf_test_sparse -s anonymous_hugetlb_2mb -b 1G -v $i -i 3 -m 2 +-------+----------+------------------+ | vCPUs | 6.2-rc3 | 6.2-rc3 + series | | | (ms) | (ms) | +-------+----------+------------------+ | 1 | 2.63 | 1.66 | | 2 | 2.95 | 1.70 | | 4 | 3.21 | 1.71 | | 8 | 4.97 | 1.78 | | 16 | 9.51 | 1.82 | | 32 | 20.15 | 3.03 | | 64 | 40.09 | 5.80 | | 128 | 80.08 | 12.24 | | 152 | 109.81 | 15.14 | +-------+----------+------------------+ This secondv test measures the benefit of eager page splitting on read intensive workloads (1 write for every 10 reads). As in the other test, the benefit increases with the number of vcpus, up to 8.82x for 152 vcpus. This table shows the guest dirtying time when using the CLEAR ioctl (and KVM_CAP_MANUAL_DIRTY_LOG_PROTECT2): ./dirty_log_perf_test -s anonymous_hugetlb_2mb -b 1G -v $i -i 3 -m 2 -w 10 +-------+----------+------------------+ | vCPUs | 6.2-rc3 | 6.2-rc3 + series | | | (sec) | (sec) | +-------+----------+------------------+ | 1 | 0.65 | 0.07 | | 2 | 0.70 | 0.08 | | 4 | 0.71 | 0.08 | | 8 | 0.72 | 0.08 | | 16 | 0.76 | 0.08 | | 32 | 1.61 | 0.14 | | 64 | 3.46 | 0.30 | | 128 | 5.49 | 0.64 | | 152 | 6.44 | 0.63 | +-------+----------+------------------+ Changes from v1: https://lore.kernel.org/kvmarm/20230113035000.480021-1-ricarkol@xxxxxxxxxx/ - added a capability to set the eager splitting chunk size. This indirectly sets the number of pages in the cache. It also allows for opting out of this feature. (Oliver, Marc) - changed kvm_pgtable_stage2_split() to split 1g huge-pages using either 513 or 1 at a time (with a cache of 1). (Oliver, Marc) - added force_pte arg to kvm_pgtable_stage2_create_removed(). - renamed free_removed to free_unlinked. (Ben and Oliver) - added KVM_PGTABLE_WALK ctx->flags for skipping BBM and CMO, instead of KVM_PGTABLE_WALK_REMOVED. (Oliver) Changes from the RFC: https://lore.kernel.org/kvmarm/20221112081714.2169495-1-ricarkol@xxxxxxxxxx/ - dropped the changes to split on POST visits. No visible perf benefit. - changed the kvm_pgtable_stage2_free_removed() implementation to reuse the stage2 mapper. - dropped the FEAT_BBM changes and optimization. Will send this on a different series. Thanks, Ricardo [0] https://lore.kernel.org/kvm/20220119230739.2234394-1-dmatlack@xxxxxxxxxx/ [1] https://kvmforum2022.sched.com/event/15jJq/kvmarm-at-scale-improvements-to-the-mmu-in-the-face-of-hardware-growing-pains-oliver-upton-google [2] https://github.com/ricarkol/linux/commit/f78e9102b2bff4fb7f30bee810d7d611a537b46d [3] https://lore.kernel.org/kvmarm/20221107215644.1895162-1-oliver.upton@xxxxxxxxx/ Marc Zyngier (1): KVM: arm64: Use local TLBI on permission relaxation Ricardo Koller (11): KVM: arm64: Add KVM_PGTABLE_WALK ctx->flags for skipping BBM and CMO KVM: arm64: Rename free_unlinked to free_removed KVM: arm64: Add helper for creating unlinked stage2 subtrees KVM: arm64: Add kvm_pgtable_stage2_split() KVM: arm64: Refactor kvm_arch_commit_memory_region() KVM: arm64: Add kvm_uninit_stage2_mmu() KVM: arm64: Export kvm_are_all_memslots_empty() KVM: arm64: Add KVM_CAP_ARM_EAGER_SPLIT_CHUNK_SIZE KVM: arm64: Split huge pages when dirty logging is enabled KVM: arm64: Open-code kvm_mmu_write_protect_pt_masked() KVM: arm64: Split huge pages during KVM_CLEAR_DIRTY_LOG Documentation/virt/kvm/api.rst | 26 ++++ arch/arm64/include/asm/kvm_asm.h | 4 + arch/arm64/include/asm/kvm_host.h | 18 +++ arch/arm64/include/asm/kvm_mmu.h | 1 + arch/arm64/include/asm/kvm_pgtable.h | 85 +++++++++++- arch/arm64/kvm/arm.c | 22 +++ arch/arm64/kvm/hyp/nvhe/hyp-main.c | 10 ++ arch/arm64/kvm/hyp/nvhe/mem_protect.c | 6 +- arch/arm64/kvm/hyp/nvhe/tlb.c | 54 ++++++++ arch/arm64/kvm/hyp/pgtable.c | 187 +++++++++++++++++++++++--- arch/arm64/kvm/hyp/vhe/tlb.c | 32 +++++ arch/arm64/kvm/mmu.c | 183 ++++++++++++++++++++----- include/linux/kvm_host.h | 2 + include/uapi/linux/kvm.h | 1 + virt/kvm/kvm_main.c | 2 +- 15 files changed, 579 insertions(+), 54 deletions(-) -- 2.39.1.519.gcb327c4b5f-goog
